# Battery Balancing / BMS Holy War



## Ziggythewiz (May 16, 2010)

*Since there's no winning this debate, I figure we can at least keep score.*

*Let us know how you manage your pack, as well as your reasons for doing so. Let's try to stick to non-repetitive arguments (at least for the first few pages) so those trying to learn something aren't eternally lost to the black hole. *

*Quick intro for noobs* (potential converts): 
To prevent damage to your expensive lithium battery pack it needs to be managed. This can be automated by a trustworthy and capable Battery Management System (BMS) or manually using top or bottom balancing and some basic monitoring. 

The ideal pack will have very similar cells to facilitate balancing (same size, manufacturer, batch, quality). No matter how alike they are, no two cells are identical, much less 30-100 cells in series as found in typical EVs. 

Types of* BMS* Poll and info here.

*Top balancing* seeks to match all cells near 100% SOC so they finish charging together, providing better protection while charging at high SOC but greater variance while discharging at low SOC.

*Bottom balancing* seeks to match all cells near 0% SOC so they finish discharging together, providing better protection while discharging at low SOC but greater variance while charging at high SOC.

Basic monitoring should include the use of a Digital Volt Meter (DVM), Amp Hour (AH) Counter, and ideally some form of simple split pack voltage comparator to detect a cell anomaly.


*Some historical perspective:*
Note: Lots of learning has occurred since then.


tomofreno said:


> This debate has recurred at several times in the last 3 or 4 years. The first "discussions" were in late 2009. A later one was started by Electriccar I think. Only the earliest "pioneers" like Jukka Jarvin and Cedric Lynch were using lithium cells in vehicles much before 2009, but they and others were using them in 2004. I recall them dividing them into pre-2005 and post 2005, with quality much worse in the pre group. There was also much discussion on the old Yahoo Groups Thundersky group (now removed) which Cedric participated in a bit. It also had Jack's early efforts at a crude BMS using a Darlington pair to shunt 2-3A, which got very hot resulting in him railing against the danger of fire with a BMS. Here are some links for late 2009/early 2010 in this site:
> 
> Lots of discussion of top v bottom balance by JRP3, Jack, and Dimitri in this thread of 11/22/09 which also references another one of same date, and has Jack’s “ragged top” graph of cell V:
> http://www.diyelectriccar.com/forums/showthread.php/bottom-balancing-thread-saferi-more-usefuli-38662.html
> ...


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## Ziggythewiz (May 16, 2010)

My pack is top balanced with no BMS.

I see a BMS as a large expense that is unnecessary with proper care and feeding.
70 or 80% DOD is recomended to prolong cell life, while charging is nearly always to 90-100% SOC. It's easier to stay away from the bottom than it is to stay away from the top, so better protection on the top end is more useful.
Potential overdischarge would happen when you are in the car, able to notice and respond to any warnings, limp triggers, or disable features you've installed. Potential overcharge would happen when the car is unattended, usually when you are having dinner, watching the game, or sleeping. That makes it more important to protect from overcharging.
Top balancing is easier
Cells (at least CALBs) usually arrive at ~60% SOC. It's faster to charge them to nearly 100% than to drain them to nearly 0% just to charge them again.
There are more purpose built battery chargers available than dischargers.


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## vmrod (Jul 2, 2010)

BMS (Lithiumate Lite)
Top Balance Cells
Valet Mode input to Zilla controller during BMS warning
HV Contactor open for BMS fault (not main contactor, but an additional contactor)
Disable charger for HCV or BMS indicates complete charge

Note:
Batteries not purchased yet, but coming very, very soon. (Test pack wired in place now) All other equipment either purchased and/or somewhat wired up.


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## Yabert (Feb 7, 2010)

I use Mini bms (top balance) since more than a years now with great result. It saved my cells at least two times during deep discharge (rare event in my case).

But if I have to restart a simple conversion (48v-72v) I will probably try bottom balancing without BMS because I know now that can work properly.


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## Siwastaja (Aug 1, 2012)

Poll = a great idea.

And a... balanced starting post!

Of course a BMS. It's "active security", and while passive is typically better and more reliable, li-ion does not have passive security (such as a physical mechanism to naturally change overcharge into heat without damaging the cells) -- and pretending it does doesn't change the fact. So, either use an automated system, or really know what you are doing. For very tight budget, I can imagine going without a BMS.


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## frodus (Apr 12, 2008)

While I don't own a DIY build anymore, the build I was working on had a Lithiumate Pro integrated to work with a Curtis AC controller and DeltaQ charger. 

My current vehicle, Brammo Empulse R, has a full BMS designed and built by Brammo. It throttles back if a cell is too cold, too low, etc. and displays faults on the screen.

Interestingly though, I'm using 40 of my cells leftover from my DIY project to build a 12V 100Ah pack + inverter for portable power/emergency charging @120V L1 rates. That will not have a BMS due to the voltage being so low. The differential isn't so extreme, so a pack monitor like the JLD404 will work. To charge, I'm using 4 3.7V DC-DC converters to charge each group of cells individually... so they're automatically balanced each charge.

IMHO basic monitoring with throttle cutback/charger cutoff is bare minimum.


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## Elithion (Oct 6, 2009)

Ziggythewiz said:


> *Quick intro for noobs* ....


I commend you for your ability to describe each option so concisely and accurately. Thanks.
(I added to your "reputation"







for this post.)


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## dladd (Jun 1, 2011)

I top balanced, then installed MiniBMS to hopefully keep them top balanced (there's a little balancing going on at the top of charge with the shunts). Mostly the MiniBMS is there to cut off the charger in the event of high voltage, and to warn me of low voltage cells when driving. My setup is really more of a *B*attery *M*onitoring *S*ystem as opposed to *M*anagement.

On the charge side, my charger is set to charge to an average of 3.5vpc. The charger runs through a SSR that is controlled by the MiniBMS. If any single cell reaches 3.6v, AC power is immediately cut from the charger. In over a year and 10k miles, this has happened exactly zero times.

On the discharge side, my controller is set to limit pack voltage drop, then the MiniBMS will sound an audible alarm if any single cell drops below 2.7 (or so) volts. No further action is taken by the BMS, it's up to the driver to back off or stop driving if a cell is alarming. I get this alarm a lot, especially when it's cold out. I have 3-4 cells that routinely sag to 2.3v when the rest of the pack stays up at 2.8v. So I'm not really protected on the low side, but I am warned. 

Mostly I use an amp counter for range. I have the EVDisplay, and it is set up so that zero ah remaining is actually 100ah used from my pack. I have 130ah cells that range in capacity from 119 - 140ah in actual testing, so at 0% I'm really at ~15% SOC on my weakest cells. I've never once brought them past zero. In fact in the year plus I've been driving, the lowest I've seen is 4ah remaining (which is 96ah used from my 130ah pack).

I am much more concerned about the charge side, since that happens unattended in my garage while I sleep. That is why I decided to use a BMS with a high voltage cutoff that will shut down the charger if any single cell runs away.


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## Tyn245GL (Oct 12, 2009)

I use the Elithion Lithiumate Pro BMS on my 90 cell CALB SE130AHA series pack. The system has 9 battery banks.
In addition to Lithiumate Pro's core function: managing and monitoring the pack, I also use most of the charge and discharge control and safety functions that are built-in in the Lithiumate Pro:
- Plus, Minus and Precharge contactors switched via vehicle key signal via BMS
- CAN Bus integration with my Brusa NLG513 charger: current monitoring, variable CCL and cut-off at High Limit all via CAN Bus
- CAN Bus integration with the MES TIM600 inverter: DCL variable current limiting, native torque reduction at low SOC and cut-off at bottom SOC, and CCL variable current limiting and torque limiting during regen at top SOC 
- regen switched off at High Limit and low temperature
- CAN Bus integration with Lithiumate Pro's HVFE - High Voltage Front End which provides the additional safety test functions described below:
- Plus, Minus and Precharge contactors switched via vehicle key signal via BMS, only after safety tests have been performed, such as:
* battery voltage present
* contactor health checks
* isolation test
* successful precharge

For daily driver info I use MetricMind's Evision2 digital instrument which shows:
- Pack Voltage, Amps, Temperature, Split Pack Balance, SOC
- Actual Kw, Wh/km, Remaining Range (DTE), speed, Ah used (coulomb count)
- Charge indicator, Time Till Full (TTF)
- aux battery voltage (12V)
At the moment Evision2 and LithiumatePro work independently. Both have their own current, voltage and temp sensors.
I wish some time there will be an integration between the two systems. Both are very valuable but the combination would be state of the art for me.


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## Yukon_Shane (Jul 15, 2010)

I top balance and use a mini-BMS. I allow the BMS to top balance my cells but truthfully I'm not terrible concerned about them going out of balance, my guess is they'll stay pretty much in balance. The reason I use a BMS is because I like the idea of activiley monitoring my cells. I want to know if any given cell falls below or above the threshhold I've set for them. 

It seems like a very simple and straighforward way to ensure that nothing strange is going on during charging or discharging.


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## ndplume (May 31, 2010)

52S1P GBS 100AH
Lithiumate Pro, 4 banks
Top Balanced EVERY time I fully charge.

Over Charge protected by BMS control of charger, secondary by Voltage cut off of charger.

Low Cell signal sounds tone (primary) and flashes strobe under passenger dash board. Not connected to my controller. I depend on driver to not kill cells at low end based on warning signals.

Cell boards are used to keep cells warm for charging in winter (About 15F capable) in my un-insulated boxes. (Dallas = 3 weeks of winter)

Torque via android bluetooth to see low/high cells, monitor temperature.

Properly installed and configured, I consider my BMS as my best chance to get entitlement life from my pack. I sleep easy with it watching my pack. Lithium Cells are considered more cost effective, However Only if you get 2,000+ charge cycles. 

( The improved ride performance of Lithium over lead because of the diet is lagniappe ! )


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## PStechPaul (May 1, 2012)

Although I don't (yet) have a lithium pack or a roadworthy EV, I voted for BMS, because that is what I plan to use, even if I go for a 300V SLA pack for my tractor project. I believe in the concept of the BMS and I think every pack should have one, although the exact type and capabilities are open to discussion and personal choice. To me, the challenge of designing and building and testing a BMS (and an EV) is my main motivation, and the actual end product, not so much.

My ideas are expressed in detail in the BMS Design Guidelines thread, but essentially I like the idea of a full time one per cell system which can at least partially prevent overcharge or depletion and cell reversal, and ideally remove (isolate) a compromised cell from the pack, and also provide real time data to a main controller and/or charger for warning, display, and shut-down.


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## Ampster (Oct 6, 2012)

Cell monitoring and top balancing by Emus BMS. 
In the first phase of my VW build I tried monitoring my pack with cell logs and had no active cell balancing. I really liked the data about how each cell was doing. Despite the low voltage audible warnings my controller went into limp mode once because I miscalculated the energy needed for one trip. No cell damage was done because the Curtiss LVC was set high for my pack of 32 cells.
On phase two of my build I wanted a Android app to view my individual cell health and the ability to set the BMS parameters. Right now I use a programable volt meter (JLD 404) to control my charger but plan on wiring up the bells and whistles from the Emus to give me redundency. Currently I charge my Winston's to 3.4 volts and begin shunting at 3.35 volts. I will experiment with turning off shunting by setting the shunt voltage higher than the HVC of the charger. I like that flexibility and in that mode I guess you could say I will have a top balanced pack with an expensive monitoring system.


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## azdeltawye (Dec 30, 2008)

Battery pack: 45 cell series string of TS 180Ah prismatic LiFePO4

Balance methodology: Top balanced
BMS: first generation mini-BMS (similar to Volt Blocher) with 10W shunt resistor and optically coupled LVC & HVC 4-wire bus to control unit.
Cell Undervoltage condition: Dash warning indication & throttle reduced 50% (valet mode) after time delay.
Pack Undervoltage condition: Controller reduces battery current until undervoltage condition clears.
Cell Overvoltage condition: Battery charger output reduced to 2.0 A & activate shunt resistor on cell BMS board.
Pack Overvoltage condition: Independent Overvoltage relay will trip lockout relay & cut AC power to battery charger.
Total E-miles driven as of 3/15/13: 30,102
Vehicle in service date: 5/2/10


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## dougingraham (Jul 26, 2011)

My pack is 52 cells of the GBS 100AH cells.

Bottom balanced which makes me the BMS.

I chose to do this because when it goes flat at the bottom there will be no damage to the cells. My Soliton will stop me from running the pack to death with its low voltage settings. Cutoff at charge is something everyone worries about but it becomes completely unimportant once you realize that it is better to not completely charge the cells anyway. On the first charge you watch for the cell that gets to 3.5 volts first and then measure the pack voltage at that point and set the charger to CV at that pack voltage.

I plan to put in a half pack comparator which will indicate a single cell failure.


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## onegreenev (May 18, 2012)

Below in red is my addition to the previous quotes. 



> Top balancing seeks to match all cells near 100% SOC and low currents so they finish charging together, providing better protection while charging at high SOC but greater variance while discharging at low SOC and high current levels.






> Bottom balancing seeks to match all cells near 0% SOC so they finish discharging together, providing better protection while discharging at low SOC and high current levels, but greater variance while charging at high SOC and low current levels.





> Basic monitoring should include the use of a Digital Volt Meter (DVM), Amp Hour (AH) Counter, and ideally some form of simple split pack voltage comparator to detect a cell anomaly.



Bottom balance. Charger set to cut off current at 3.65 volts nominal and no cell reaches over 3.8 volts during the charge at low current levels. Elcon 3KW 120 volt charger with 10 different voltage levels. Mine was set up like that from Elcon in Sacramento. I can set my charger to 3.5 volts per cell nominal if needed. 
Controller: Synkromotive is set to 2.4 volts cut back and 1.8 volts sag shut off under current load. Will be set higher with higher voltage pack. 

Volt meter, Amp meter and will add in a JLD AH meter and considering the split pack comparator. I believe in monitoring the pack but not needed on a single cell daily basis.

Pete


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## EVfun (Mar 14, 2010)

I'd say top balance and learn when to call a tow truck.



> Top balancing seeks to match all cells near 100% SOC so they finish charging together when your not there to watch them, providing better protection while charging at high SOC but greater variance while discharging at low SOC when your right foot is in control.


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## onegreenev (May 18, 2012)

I say bottom balance and know your batteries will be fine when you need to call a tow truck. It will happen.


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## EVfun (Mar 14, 2010)

I say top balance and know your batteries will be fine if the charger timer fails (you know -- when you are not there to notice.)


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## onegreenev (May 18, 2012)

Well poop, it could happen. Less likely to happen vs driving your car that last bit. You know, when you let your teenager loose with your car. It will happen. 

I trust my Elcon. I trust my Controller.


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## Siwastaja (Aug 1, 2012)

I thought that, to begin with, "no BMS" was for those who are careful, know what they are doing, can stick to some rules, and are the only drivers of the car. But due to high cost and some people still having distrust in electrical systems, it appears that a lot of people are choosing this route.

But I think I understand the idea of bottom balancing pretty well. You can greatly reduce the risk of severe abuse while driving when you, for some reason, cannot control yourself or others driving the car. Instead, you run a higher risk to slowly damage your cells, but that may or may not happen during years and at least probably not suddenly, and even if it happens, only a few cells suffer, those that were lowest in capacity to begin with. For some people, this may appear to be a good tradeoff. (Also, you may be able to prevent this from happening by monitoring the cells while charging every now and then and adjusting the charger stop voltage.)


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## Ziggythewiz (May 16, 2010)

Siwastaja said:


> greatly reduce the risk of severe abuse while driving when you, for some reason, cannot control yourself or others driving the car.


Most controllers have low voltage limiting. Some are adjustable for both limp and halt modes. Also, it's not hard to use a JLD404 or other smart meter to protect the pack when you can't. You could even switch in a variety of limp/halt modes depending on the trustworthiness of the driver.


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## tomofreno (Mar 3, 2009)

dougingraham said:


> ...Cutoff at charge is something everyone worries about but it becomes completely unimportant once you realize that it is better to not completely charge the cells anyway...


 It is curious to me that you feel no need to completely charge them, but evidently do plan to run them down close to full discharge since you feel you need to protect against over discharge. The data and manufacturers recommendations I have seen show that cell life decreases with repeated discharges to lower SOC and specifies ending charging at something like 3.6V and 0.05C in CV mode. I have seen reports that PHEVs operate in the middle range of SOC, avoiding both higher and lower regions for longer cell life, but I've not seen anything recommending operation at the lower range of SOC. Why the preference for operating over the bottom X% or so of SOC rather than the top X% or middle that these recommendations seem to indicate?


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## onegreenev (May 18, 2012)

You want to protect on both ends. Stay off the top and bottom. Bottom balancing better protects the cells at a low SOC when stupid gets in the mix. It not a matter of IF but WHEN. The top is easy to stay off of because once charged and you head off down the road you are even further off the top. You never drive with a full pack but you always drive towards empty. Stay off the top and bottom.


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## Ziggythewiz (May 16, 2010)

onegreenev said:


> Bottom balancing better protects the cells at a low SOC when stupid gets in the mix. It not a matter of IF but WHEN.


Just leave stupid out of your mix and it never needs to happen. Why do you think stupid is a requirement?



onegreenev said:


> You never drive with a full pack but you always drive towards empty.


Yet you're never charging with an empty pack but you always charge towards full. Funny how that works...


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## onegreenev (May 18, 2012)

Because in the real world where real people live stupid is pretty rampant. 

Well, do you charge at stupid high currents like when you drive? I think not.


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## onegreenev (May 18, 2012)

Your pack is at some state of empty more than its ever at full.


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## EVfun (Mar 14, 2010)

EVfun said:


> I say top balance and know your batteries will be fine if the charger timer fails (you know -- when you are not there to notice.)





onegreenev said:


> Well poop, it could happen. Less likely to happen vs driving your car that last bit. You know, when you let your teenager loose with your car. It will happen.


Oh, I have had that happen, but didn't loose a single cell. With the low level of faith you seem to have in the electronics of a BMS I'm surprised you have such faith in the electronics of your charger. An amp hour counter with a low lift lockout function (E-meter, Clean Power Auto EV Display, ZEVA fuel gauge Driver Plus, JDL404, ect.) could easily handle contactor cut-off at a minimum SOC. I haven't felt the need since only my wife and I will be driving our EV.


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## Ziggythewiz (May 16, 2010)

onegreenev said:


> Your pack is at some state of empty more than its ever at full.


Yet your pack is at some state of full more than it's ever at empty. Funny how that works...

Do you even think about ANYTHING you say?


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## Siwastaja (Aug 1, 2012)

Well, there have always been people who are logical and try to base what they say on facts.... And then there are people who like to follow their "feelings" on technical matters. They also tend to give non-technical blanket statements that sound nice, are easy to learn and follow, but may be complete nonsense (like shown previously). Again, no offense, just saying what needs to be said.

It's needless to say which one works better in large scale, but even non-technical people may succeed in technical matters in some cases.

But there's more to it; those who believe in their false theories tend not to share their failures, or at least cannot admit the failure was due to their false theory, usually turning the matters upside down to show that the failure indeed _proved_ their theories. These things can go pretty wild sometimes.

This makes claims without solid basis look more attractive; people who prefer them also prefer to claim that they "just work", which may be untrue, while technically oriented people tend to report any problems because they usually want to learn from their mistakes. This always makes bogus technology look more attractive.

This is not directly related to the balancing choice, because you can run either bottom or top balancing in a way that works better than doing the another one the wrong way. This topic already shows that some bottom balancers know exactly what they are doing. But then again, because they do, they could as well run a top-balanced system; it doesn't matter so much.


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## onegreenev (May 18, 2012)

Ziggythewiz said:


> Yet your pack is at some state of full more than it's ever at empty. Funny how that works...
> 
> Do you even think about ANYTHING you say?



Always! You?

So you could say that your pack is sitting at a level less than full far longer than its ever at a state of full.


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## onegreenev (May 18, 2012)

Ziggythewiz said:


> Yet your pack is at some state of full more than it's ever at empty. Funny how that works...
> 
> Do you even think about ANYTHING you say?



So the statement implies that you are off the top and bottom more than any other time. Some choose to set up with bottom balancing and forgo the complex BMS and rely on the charger/controller to terminate and include some basic monitoring and maybe some sort of valet mode. Some choose the same but top balance and some just use the BMS believing it is the only way. 

Being off the top and bottom is primary as is being able to safely terminate the charge and to shut you down before killing your cells at the bottom . 

A BMS system is not manditory but a way to terminate on the top and bottom is. If you already have a smart charger and controller why over ride them with the BMS. 

You choose what ever you want and you also promote what ever you want. 

You know, I'd much rather have a few circuits in my system that will do what is required vs a pile of them all attached to my cells. 

Yes I can top balance with or without a BMS successfully. 
It's just a choice.


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## onegreenev (May 18, 2012)

EVfun said:


> Oh, I have had that happen, but didn't loose a single cell. With the low level of faith you seem to have in the electronics of a BMS I'm surprised you have such faith in the electronics of your charger. An amp hour counter with a low lift lockout function (E-meter, Clean Power Auto EV Display, ZEVA fuel gauge Driver Plus, JDL404, ect.) could easily handle contactor cut-off at a minimum SOC. I haven't felt the need since only my wife and I will be driving our EV.


I have more faith in my charger and controller than I do my JDL or e-meter. If its so easy and reliable with those it is just as so with my charger and controller.


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## Ziggythewiz (May 16, 2010)

onegreenev said:


> So you could say that your pack is sitting at a level less than full far longer than its ever at a state of full.


Nope. My pack sits at 'full' from ~ 7 PM to 8 AM. ~8:30-3 it's ~70% SOC. By the time I get home I'm ~45% SOC and it goes up from there.

My pack is closer to empty than full ~20 min / day, and most of that is while charging. The other 1,420 min are spent closer to full.


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## onegreenev (May 18, 2012)

Yup. Not talking about the time sitting with no power. Only while actively charging or discharging. The rest is safe idle time.


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## Siwastaja (Aug 1, 2012)

... and only extremes matter in this debate anyway... Of course this is something _most_ of you understand very well.

This is especially "funny" since onegreenev has admitted he overcharges some of his cells to 3.8V on every charge . But I guess the cells just survive from that little abuse, at least for some time. The point of high-current abuse vs. low-current abuse is, anyway, valid.

It's very reassuring to see cells doing just fine in hands of people who don't know how to use them. It's called robustness.

However, the event of charger failing should be thought about. It has happened and you are not there to take action. In addition, chargers are not precision devices, so check the output voltage every now and then.

By the way, BMS controlling a relay that can switch off the charger, when configured correctly, has THREE independent safety mechanisms:
1) BMS can shut down the charger even if it fails to go to CV mode.
2) Charger can run on CV mode even if BMS fails to shut down the charger
3) Top-balancing reduces the risk of a few cells to go much higher. [in the case of BMS failing but charger working in CV mode]

I have to quote this from dougingraham just to show that some bottom balance people know what they are doing, and are doing it just right:

"On the first charge you watch for the cell that gets to 3.5 volts first and then measure the pack voltage at that point and set the charger to CV at that pack voltage."

This should work pretty well given that:
1) The charger doesn't fail on to CC mode.
2) The charger CV setting doesn't drift
3) The cell balance doesn't drift
4) The cell capacities don't drift (balance on the bottom is not enough, because you rely on a _limited unbalance_ you have measured on the charge side once).
5) In case 2,3 or 4 happens (and at least one of them WILL eventually happen), you have enough leeway in charge voltage. Notice the difference; person A really measures 3.8V from the highest cells, which already is overcharging, while person B selects 3.5V as the _highest_ (not average) cell to build in some leeway into his system.

LiFePO4 is known to drift very little, but I'd recommend occasional checkups on the charge side, even if the discharge side stays in balance, because they are two separate things. Because in this scenario, you have just one specific kind of unbalance on the top and need to maintain that.


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## Ziggythewiz (May 16, 2010)

onegreenev said:


> Yup. Not talking about the time sitting with no power. Only while actively charging or discharging. The rest is safe idle time.


Ok, so ~20 min in the lower half, and ~220 in the upper.


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## onegreenev (May 18, 2012)

No 240 minutes between the top and bottom.


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## onegreenev (May 18, 2012)

> This is especially "funny" since onegreenev has admitted he overcharges some of his cells to 3.8V on every charge


Actually 3.78 since every one is counting. I did have one once go over 4 before my pack was properly balanced. Once balanced they do fall below 3.8. Once power is removed they all drop to 3.35


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## Siwastaja (Aug 1, 2012)

I can imagine that in my usage the battery would sit (actively, in a charger) at >90% SoC for about 2-3 hours every DAY, and <20% maybe once a year. Hopefully never. It's very hard to understand how the "just one more mile" attitude would help in anything. Plan the trip beforehand, and if you fail, just tow it home, like you probably have to do ANYWAY if you run flat. (The only exception would be a very slight miscalculation that gets you almost back home. OTOH, in that case, it would be easy to have an extension cord ready and ask someone to give you a little bit charge from their home which wouldn't take much time.)


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## Siwastaja (Aug 1, 2012)

onegreenev said:


> Actually 3.78 since every one is counting. I did have one once go over 4 before my pack was properly balanced. Once balanced they do fall below 3.8. Once power is removed they all drop to 3.35


I think this data shows that your cells are very similar in _capacity_. Did you or the seller do any hand-picking to choose similar cells? 

Did you use all of the cells or did you pick out some? Have you lost any cells due to any reason? Is the one that went over 4 still doing fine?

This data point is important because someone else could have cells that differ more, and would need more leeway in the charge voltage.


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## EVfun (Mar 14, 2010)

onegreenev said:


> Actually 3.78 since every one is counting. I did have one once go over 4 before my pack was properly balanced. Once balanced they do fall below 3.8. Once power is removed they all drop to 3.35


That is a big difference (though very small in amp hours) between your charging and my charging. I would consider anything over 3.60 to be big NO -- something is wrong with cell balance. 

I've actually taken one cell to 3.99 volts on accident because there is some drift between cells from the same batch if they have different use histories (the ones with less cycles drift upward very slowly.) My current rule is to bleed down, as a block, the 6 cells I added back a year after moving my pack to another EV when the first one shows a reading of 3.70 or higher near the end of a charge (or after a charge and before the next drive by turning the charger back on for 15 minutes.) I can see this because every charge brings at least some cells to the voltage knee (otherwise the charger wouldn't know to stop.) I check about once a month. I pull the less used block of 6 down until on the next charge none of them go over 3.50 at the end of charge (and I've gotten good at it, it's about 500 columns.) It does lead me to think that a pack made up of used cells is likely not to stay in any state of balance, top or bottom, for over 200 cycles.

When I put together the pack for my Berry Mini-T buggy it will be 39 cells [1]. It will be the 32 with the most use, the 6 added cells, and 1 of the cells that that was in the original 40 block pack. The 2 cells that have never been used will be kept in that state. They have exactly 1 cycle on them, each from the as received state to discharged, then charged, then pulled down by 30 amp hours. That determined that there is very little possible self discharge after 30 months, and no measurable capacity loss from sitting unused (like the cells I'm using, they both showed a little over 60 amp hours when discharged at 0.2C to 3.00 volts.) I plan to install shunt regulators with 1/2 amp capacity to deal with the slight drift based on different use histories. That will require me to charge to just over 3.6 vpc to hit all the regs (anything mounted at the cell level introduces a new form of drift based on each unit not drawing exactly the same amount of current -- even 0.1 milliamp will screw you up within a year.)

[1] I received 42 Thundersky 60 amp hour cells in early March 2010. 40 when into a Datsun (2010.) Then I moved 32 of them to a Manx style beach buggy (2011.) A year later I added 6 more to that pack (2012.) Another year later and I'm putting 39 of them into a Berry Mini-T beach buggy (2013.)


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## onegreenev (May 18, 2012)

Siwastaja said:


> I think this data shows that your cells are very similar in _capacity_. Did you or the seller do any hand-picking to choose similar cells?
> 
> Did you use all of the cells or did you pick out some? Have you lost any cells due to any reason? Is the one that went over 4 still doing fine?
> 
> This data point is important because someone else could have cells that differ more, and would need more leeway in the charge voltage.



I think folks here either did not read or have forgotten what I have written about my cells.

When I first got my cells 3.65 volts per cell as the end voltage of the pack was used. Many were still using 3.8 volts. It has since been reduced to 3.55 volts. Some still use 3.65 volts. Anyway. My charger is set with 10 voltage settings so I can change the end voltages of my pack as needed. The top 5 are no longer used as they are too high for the current assessment of what your top charge should be. I have actually changed to 3.55 volts but that was using my Synkromotive controller to charge my pack using my DC stationary source charge. If I use my Elcon it is set right now for 3.65 volts. By the time my pack reached the end of charge at the nominal 3.65 volts for the pack I had ONE cell in the bunch that ran up to 3.78 volts. All the others were 3.6.. or below. Once I had my pack properly bottom balanced that remained very consistent for each charge. I did in the beginning check my cells voltages. I did not willy nilly do this bottom balance thing. I needed to know it worked. It does quite well. 

My cells were purchased from a person in Oregon who was building electric quads. He purchase a mass of 100ah Hi-Power cells from China. At the time he was actually moving to Florida to do business as Oregon required too much red tape for him. However he ran into a problem. The cells he purchased came with the factory provided and built BMS system for the cells. He connected everything and began testing and running the quads using these cells and BMS systems on all his packs. The BMS systems failed and they failed to where the ones that failed would drain the cells. In all out of about 200 cells he lost about 80 cells. Some were sitting at near zero and never recovered. Some were lost because they fired up the quads and ran them but quickly realized that the cells had drained enough to cause many in the packs to reverse and swell. Out came the BMS systems and out came the cells. He still had a bunch that were good from all that mess but decided to just sell the whole whacking mess of them for a serious discount. I purchased 160 cells and he gave me like 76 more. Most of the 76 had sat for years at nearly zero volts. Most actually were still quite good. Some were not. Some never recovered. Some of the 160 did not make the cut either. I sold and traded a bunch of the best of the bunch and kept like 80 of them. I have 68 good cells and some marginal ones left. The one cell that had gone to the higher voltage I still have but is no longer in the pack. I am using it along with some others and made two 100ah 12 volt packs for use in my charge/discharge testing of my A123 cells along with my PowerLab 8. Works like a charm. Cells are quite healthy. 

When I first got my cells I was still on the fence about the BMS or not. I did a bunch of research and testing and decided to go the bottom balance route but was impatient and did not do a good job and stuffed them in my electric MG Midget and charged them and went for a few rides. My last ride I before my incident I went a bit further than before but my volt meter was still telling me I was good to go so I kept going. I was only about a mile from home and I did notice that the car was still running OK but felt mushy or sluggish but could not pin anything on it directly. Since my volt meter still said I was OK and that I actually still had more than enough to get home I proceeded. Once it the drive I popped the trunk as I had always done to check things I noticed a very hot area in my pack as I waved my hand slowly over the pack. Damn it was hot. I still at the time had the pack strapped tight with a web strap. I released the pressure and two cells just popped out like two fat piggies. I unbolted them and removed them and they were scorching hot. One totally dead and the other even to day still holds a charge but its fat as a pig. Those were the only two cells that died in the mishap and none since at least none that were good from the beginning. I then properly bottom balanced my pack and from that point forward never had an issue with with loosing a cell nor with driving beyond a safe limit. The pack had always stayed cool after that too. The other drives before you'd not notice because the cells had not yet gone into reversal. You just really can't see it. 

Since then I had a few cells get loose connections that needed tightened. I don't have nordlock washers but would highly recommend them or the other company that sells them that are identical to the nordlocks but a tad bit cheaper. Keeping your connections solid is a problem even with the lithium and not just the old lead acid batteries. I do have normal lock washers but they don't always hold solid. Vibration, and heat/cold cycling do have an effect and it really is not hard to loosen a normal lock washer. I will have nordlocks someday. 

As for a well picked pack I'd say no I did not really get too picky. I did cull some out of the pack early on but those were very low capacity cells most likely damaged from the original BMS mishap. 

You should however have a very tight pack with as little of variance as possible after you bottom balance your cells. The closer you are the better the system of bottom balancing works. 

If however you have a pretty wide variance I can see the NEED for a GOOD BMS. At this point I know of only two being actively used. Some more I am sure are being used but not commonly. 

My system is actually a BMS of sorts anyway but not on a single cell basis. I will be checking out a BMS system that has been developed by Synkromotive but to date I have never used one. 

I am not as against them as I sound but I also don't like the idea of hundreds of tiny wires connected either to some circuit boards. Heck, even some of those boards are not fool proof and if you by accident swap polarity you will smoke your board. Ouch. It has happened plenty. 

I am not totally convinced of some of the EV fires being a direct result of the balancing boards actually shorting and burning up but I am convinced that some were a result of the charging being controlled by the BMS system and the BMS fails to communicate that it needs to terminate the charge and some time in the middle of the night which is about right the cells go thermal from over charge and burn. And burn baby they will. 

I know how hot my reversed cells got. 

I trust my charger more than a BMS and I trust my Controller more as well. Since my controller has an excellent cut back and cut off function I just use that now to protect my cells from over discharge but feel that balancing on the bottom is best. 

If I do try out the BMS it will be top balanced. Not something I like. We shall see. 

I have tested cells in the cold as well. So cold in fact that I took a cell to below 58 degrees below zero. The cell at that point actually just goes into hibernation. You can't charge or discharge the cell, it wont let you. If you put a charge on it it will jump to a very high voltge and in an unsafe zone. Once the cell thawed to 20 below zero it allowed you to start discharging slowly. At that point you still could not charge as the voltage level just jumped too quickly. 

Frozen cells just don't work. The cells love heat. I have yet to do a hot charge/discharge and AH check but that is coming. I have the perfect insulated box for that. 

I only have Hi-Power to play with and my A123 pouch cells. They have actually done quite well even at the abuse I have given them running my motor at up to 700 amps. Well above the manufacturers recommended 3C peak. The cells I have are early 08 cells. So right now they are 5 years old and still working.

Pete


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## ricklearned (Mar 3, 2012)

onegreenev said:


> I think folks here either did not read or have forgotten what I have written about my cells.
> 
> ..............


For what it is worth, I have not forgotten what you have written about your cells. Your opinions have been very helpful to me during my build. However this is not about you, this thread is about the getting a variety of opinions on battery management systems. My circumstances and priorities are different than yours and I have reached a different conclusion about how I want to manage my pack.


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## tomofreno (Mar 3, 2009)

onegreenev said:


> I think folks here either did not read or have forgotten what I have written about my cells. When I first got my cells 3.65 volts per cell as the end voltage of the pack was used. Many were still using 3.8 volts. It has since been reduced to 3.55 volts. Some still use 3.65 volts...


I think you have forgotten that JRP3 and I were running bottom balanced packs and telling people here to end charging around 3.45V or so before you even had LiFePO4 cells Pete. I think JRP3 still runs that way, but I have now been using a bms for over 2 years.



> ...I did not willy nilly do this bottom balance thing. I needed to know it worked. It does quite well...


 I started off with a cheap bms that drained some of my cells and others cells due to up to 0.4A leakage current through a T54 device. Then bottom balanced following Jack R's lead and ran without a bms for around 3 months until the minibms became available. I installed the minibms and top balanced due to an issue with using a bottom balanced pack with a Manzanita charger with non-zero timer setting which I have reported here a number of times. Briefly, I came very close (I intervened manually) to overcharging cells a number of times due to differences in pack voltage with charging current magnitude and cell temperature, and the slow cutback of charge current by the Manzanita if the change in pack voltage is small after the pack reaches the limit voltage set on the charger. It works very well with a top balanced pack due to the much larger change in pack voltage with most cell voltages starting up the exponential part of the V versus Ah curve near end of charge rather than just the one or two lowest capacity ones. This is not an issue with Elcon or other chargers that hold voltage constant in CV mode, only with the Manzanitas since they permit pack voltage to rise around 2V or so after the limit voltage is reached and the timer turned on. Not a big deal with a top balanced pack, but it can be with a bottom balanced pack and one or two cells 1 - 2% lower in capacity than others.



> ...Since then I had a few cells get loose connections that needed tightened. I don't have nordlock washers but would highly recommend them or the other company that sells them that are identical to the nordlocks but a tad bit cheaper...


I use the standard flat and lock washers that came with the cell connectors. I check torque about every 3 - 4 months and have not found a loose connection in over 3 years, except for one that I forgot to tighten.



> ...I am not as against them as I sound but I also don't like the idea of hundreds of tiny wires connected either to some circuit boards. Heck, even some of those boards are not fool proof and if you by accident swap polarity you will smoke your board. Ouch. It has happened plenty...


 The minibms has only one wire connection between cell level boards and the two wires of this N.C. loop connected to the main board. Not "hundreds". You can also hook up the boards reverse polarity with no damage. I've done it.



> ...I am not totally convinced of some of the EV fires being a direct result of the balancing boards actually shorting and burning up but I am convinced that some were a result of the charging being controlled by the BMS system and the BMS fails to communicate that it needs to terminate the charge and some time in the middle of the night which is about right the cells go thermal from over charge and burn. And burn baby they will...


 The bms is only back up to prevent overcharging on my pack. The charger limit voltage is set so that HVC (3.6V) is not triggered. It hasn't been triggered in over 2 years. The shunts typically balance the last 10 - 15 minutes of charging on cells that have reached 3.5V, and the shunt resisters aren't even warm to the touch with the about 0.7A through them during this time. So if my pack overcharges it will be because BOTH the charger failed to stop, and the bms failed to stop it. On yours it will be only because the charger failed to stop.



> I trust my charger more than a BMS and I trust my Controller more as well.


I have the low voltage function on my controller set as well, but neither you or I have a good understanding of the control circuitry in our chargers so there is no logical reason for us to have blind trust that they will always operate flawlessly.




> ...I have tested cells in the cold as well. So cold in fact that I took a cell to below 58 degrees below zero. The cell at that point actually just goes into hibernation. You can't charge or discharge the cell, it wont let you. If you put a charge on it it will jump to a very high voltge and in an unsafe zone. Once the cell thawed to 20 below zero it allowed you to start discharging slowly. At that point you still could not charge as the voltage level just jumped too quickly...


 I have had cell heaters since I first converted the car and have reported before that I notice no difference in voltage sag or performance, other than stiff drive train, between summer and winter operation.


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## EVfun (Mar 14, 2010)

The MiniBMS shunts come on at 3.5 volts, right? I wonder if the BMS loop could be pretty easily disabled to cut the 7 milliamp standby power consumption (holding the photoFET on is likely a big chunk of that.) I was just planning to use my EV BMS modules as shunt regs, but it forces me to move the end of charge voltage back up to just over 3.6 vpc. The nice thing about the EVpower modules is that the standby power is about 2 milliamps.

Somewhere here is the pre-miniBMS open source efforts that lead up to it. If I find those perhaps I can see the similarity and how to do that.


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## Ziggythewiz (May 16, 2010)

Why would you need to raise the end of charge voltage to make them shunt? Can't you just continue as normal and when the fresher cells creep up the shunts will keep them in line?


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## EVfun (Mar 14, 2010)

I don't recommend using cell modules unless you hit the shunts regularly. The individual modules won't draw exactly the same standby current and with 8760 hours in a year even a fraction of a milliamp adds up. 

With my pack, 0.15 amp hour is roughly the difference between 3.5 volts at the end of charge and 3.75 volts at the end of charge right now, running without cell boards. At one point I documented the variation in standby current for my BMS modules, I think was about 200 microamps.


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## Elithion (Oct 6, 2009)

EVfun said:


> At one point I documented the variation in standby current for my BMS modules, I think was about 200 microamps.


The cell boards in the BMS in my Sparrow are rated for 10 uA max (standby) and draw 2 uA average. That's so significantly below the self discharge current of the cells, that differences in current cannot possibly have an effect.


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## tomofreno (Mar 3, 2009)

EVfun said:


> I don't recommend using cell modules unless you hit the shunts regularly. The individual modules won't draw exactly the same standby current and with 8760 hours in a year even a fraction of a milliamp adds up...


 Yes, they do drift apart a bit without balancing periodically. I've not seen much over a couple Ah difference though, and most are within more like 1/2 Ah of each other. It would be nice if they drew microamps like the one in Davide's car, but at least to me it's not a big deal. Yes, my minibms boards shunt at 3.50 +/-0.02V.


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## tomofreno (Mar 3, 2009)

Some historical perspective. This debate has recurred at several times in the last 3 or 4 years. The first "discussions" were in late 2009. A later one was started by ElectriCar I think. Only the earliest "pioneers" like Jukka Jarvin and Cedric Lynch were using lithium cells in vehicles much before 2009, but they and others were using them in 2004. I recall them dividing them into pre-2005 and post 2005, with quality much worse in the pre group. There was also much discussion on the old Yahoo Groups Thundersky group (now removed) which Cedric participated in a bit. It also had Jack's early efforts at a crude BMS using a Darlington pair to shunt 2-3A, which got very hot resulting in him railing against the danger of fire with a BMS. Here are some links for late 2009/early 2010 in this site:

Lots of discussion of top v bottom balance by JRP3, Jack, and Dimitri in this thread of 11/22/09 which also references another one of same date, and has Jack’s “ragged top” graph of cell V:
http://www.diyelectriccar.com/forum...lancing-thread-saferi-more-usefuli-38662.html


Here is another thread on BMSs from 10/20/09:
http://www.diyelectriccar.com/forums/showthread.php/minimal-full-function-bms-li-37512.html


Here is Dimitri’s 2/16/10 thread with a poll asking what people want in a low cost BMS:
http://www.diyelectriccar.com/forums/showthread.php/new-cheap-bms-poll-38549.html

Edit: Dimitri's thread on an open source bms:
http://www.diyelectriccar.com/forums/showthread.php?t=38863&highlight=Dimitri

Most of those involved in the earlier debates have long since grown tired of it so no longer participate. Lot of learning has occurred since then.


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## Ziggythewiz (May 16, 2010)

tomofreno said:


> Some historical perspective.


Good info for us noobs, thanks. Added to initial post. I'd seen one of those threads but not the others.


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## kchiangusa (Mar 28, 2012)

Not sure why I can vote anymore. Perhaps the poll has closed?

I decided to save some money and forgo the BMS. Instead, I use cell log 8's with a quick disconnect that let's me measure the cell voltages of all 45 cells in under 2 minutes. In 2 years and 20k miles, I've not had to balance the Calb Sky 180Ah cells too much.

Here's the write up:

http://blog.mr2ev.com/?p=871


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## GerhardRP (Nov 17, 2009)

kchiangusa said:


> In 2 years and 20k miles, I've not had to balance the Calb Sky 180Ah cells too much.


Can you give details of the balancing you have done?


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## nucleus (May 18, 2012)

Elithion said:


> The cell boards in the BMS in my Sparrow are rated for 10 uA max (standby) and draw 2 uA average. That's so significantly below the self discharge current of the cells, that differences in current cannot possibly have an effect.


What was that? Self discharge? What lithium cells self discharge?


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## Siwastaja (Aug 1, 2012)

nucleus said:


> What lithium cells self discharge?


All and every.


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## PStechPaul (May 1, 2012)

About 2% to 3% per month:
http://www.mpoweruk.com/performance.htm

Lead-acid is about twice that, 4% to 6% per month.

NiMH is worst at 30% per month.


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## Zak650 (Sep 20, 2008)

Siwastaja said:


> All and every.


Lifepo4 cells only discharge if they are connected to something, like a bms, dc-dc converter, car radio, etc.

No pain(parasitic load), no drain


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## Siwastaja (Aug 1, 2012)

Zak650 said:


> Lifepo4 cells only discharge if they are connected to something, like a bms, dc-dc converter, car radio, etc.


... which is physically impossible, but feel free to invent your own laws of physics. 

Everything has resistance, and every battery, capacitor etc. self-discharges due to it's own resistance across the terminals. It is just slow. Now, batteries have very thin layers of separators and very large surface area so they need excellent properties to minimize self-discharge, which they indeed have. 

Many report that the typical 3%/month figure is exaggerated, and they instead report significant/measurable self-discharge in the course of a few years or so. So, it is practically insignificant, but it's there. 

I guess the point was that this particular BMS had current consumption that is _even lower_ than the already insignificant self-discharge, and this is perfectly possible with a well designed modern BMS. (designed using the 90's or 2000's parts; the 80's won't cut it.)

It is also well known that lower-quality cells may develop higher self-discharge over time but otherwise work pretty well. In fact, _higher than normal_ self-discharge is a typical failure mode of a li-ion cell. A BMS allows some of these cells to continue on their duty.


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## Zak650 (Sep 20, 2008)

Siwastaja said:


> ... which is physically impossible, but feel free to invent your own laws of physics.
> 
> Everything has resistance, and every battery, capacitor etc. self-discharges due to it's own resistance across the terminals. It is just slow. Now, batteries have very thin layers of separators and very large surface area so they need excellent properties to minimize self-discharge, which they indeed have.
> 
> ...


What is physically impossible about an off switch?

If you have the choice to purchase 4 virgin cells in their original box having sat there for 2 years or identical cells connected to a bms or some other electrical circuit for the same two years with no connection to any power source, which would you buy?


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## nucleus (May 18, 2012)

PStechPaul said:


> About 2% to 3% per month:
> http://www.mpoweruk.com/performance.htm
> 
> Lead-acid is about twice that, 4% to 6% per month.
> ...


That link appears to lump all lithium chemistries together. In other words, worthless.


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## nucleus (May 18, 2012)

Zak650 said:


> Lifepo4 cells only discharge if they are connected to something, like a bms, dc-dc converter, car radio, etc.
> 
> No pain(parasitic load), no drain


This is the right answer.


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## Tesseract (Sep 27, 2008)

Siwastaja said:


> ...
> I guess the point was that this particular BMS had current consumption that is _even lower_ than the already insignificant self-discharge, and this is perfectly possible with a well designed modern BMS. (designed using the 90's or 2000's parts; the 80's won't cut it.)...


Low current drain implies high impedances and high Z circuits tend to be very sensitive to noise (especially E-field). That is to say, the more you try to minimize the quiescent current consumption of a BMS the more prone it is to acting erratically when the motor controller is in operation.


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## PStechPaul (May 1, 2012)

The following link specifies 1.2%/month.
http://www.creup.com/storagepack.htm

Remember that it is not a reduction of voltage by that amount, but as reduction of capacity, or energy, which is not as easily measured. There will also be some change in effective capacity caused by temperature, and the very process of discharge and charge cycles can change the capacity as well. 2% per month would seem to be 24% per year and 100% in 4 years, but it is more likely a percentage of remaining capacity. So it may be like this

Month 1 100%-2% = 98%
Month 2 98% - 0.02(0.98) = 96.04%
Month 3 96.04% - 0.02(0.9604) = 94.12%

Month 12 0.98^12 = 78.5%

Month 24 0.98^24 = 61.6%

Month 36 0.98^36 = 48.3%

Month 48 0.98^48 = 37.9%

10 years (120 months): 8.85%

(or something like that)


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## EVfun (Mar 14, 2010)

I let 2 new spare cells sit with no use or load for 30 months (Feb. 2010 to Oct. 2012.) At that point I charged the cells and it took very near 30 amp hours, which is what I found for the rest of the cells from the same batch that I installed in my EV in May 2010. It isn't realistic that the cells lost more than 10% of their charge in 30 months. Less than 10% would be small enough that could have escaped my notebook measurement of the power required to charge them, though I doubt they had lost even that little. I can't really "buy" a self discharge rate for good condition LiFePO4 cells greater than 0.25% per month. I doubt a self discharge rate of even 0.1% per month because I would expect "drift" in a pack. My cells are in a street legal beach buggy, so they see winter storage. I don't see the cells out of balance on the first charge after storage.


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## nucleus (May 18, 2012)

PStechPaul said:


> The following link specifies 1.2%/month.
> http://www.creup.com/storagepack.htm
> (snip)


Just because someone puts something on the internet doesn't make it true. 

By your numbers cells that sit for 30 months unused will lose over 40% of their capacity, but EVFun's real world numbers say not even close to that.

LiFePO4 cells don't self-discharge, at least not in a measurable, real-world way.


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## onegreenev (May 18, 2012)

We all know they don't self discharge. Some just want to continue the BS to the Nth degree. If it is found that they do self discharge by any measurable amount it has not been found and people have been looking for many years now. The LiFePO4 cells have no measurable amount of self discharge and none has been found. Whats been found is loss when connected to equipment that drains current. Shorts, Components that take power to run and the like. But no SELF discharge like you find in Lead Acid batteries.


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## Zak650 (Sep 20, 2008)

In one of Jack's videos a couple years ago he talked about the feds wanting the lifepo4 battery manufactures to put some value on the import form for self discharge rate so they put something down that seemed reasonable in comparison to lead acid cells just to satisfy the import official, not that that value had anything to do with reality. If new cells really got damaged from sitting around this would have shown up mixing older cells with newer ones of the same size and manufacturer.


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## kchiangusa (Mar 28, 2012)

kchiangusa said:


> In 2 years and 20k miles, I've not had to balance the Calb Sky 180Ah cells too much.
> 
> Here's the write up:
> http://blog.mr2ev.com/?p=871





GerhardRP said:


> Can you give details of the balancing you have done?



Looking back in my battery logs, monthly, I'd measure the cell voltages of each cell after driving the car to about 30% charge remaining. The average cell voltages at this state of charge is typically 3.22 +/- 0.02 volts. If they are significantly lower than this, I will use my GT Power battery charger and individually charge the cells until their voltages are in par with the rest of the cells. I do this in increments so I don't have to watch the charger or risk forgetting about the charger. I'd preset the amount of energy to add to the battery, when it is done, I'll measure the voltage again until it reaches voltage. In my experience, it hasn't been more than 2-3 AH that I would add to the low cells to get them balanced.

I've had to add charge to low cells 2 times in the almost 2 years that I've been driving, the first time was on 3 batteries and the 2nd time on 2 batteries. It has not been bad at all. 

Anybody else that is doing the same? What's been your experience?


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## Siwastaja (Aug 1, 2012)

Zak650 said:


> What is physically impossible about an off switch?


Even the off switch conducts some current!  Well okay, that is _very_ negligible because the switch has a large opening (possibly millimeters) and only a little surface area next to it.

Li-ion batteries, on the other hand, have separators in range of a few dozen micrometers, and a huge surface area, and liquid between the electrodes. It is impossible to _totally_ eliminate resistance. The purity of the electrolyte is very important. This should explain the differences in self-discharge; process contaminants are quite random.

It is clear to everyone that the 3%/month figures do not hold true for current prismatic LiFePO4 cells at all. Indeed, many have suggested something like 0.1%/month. OTOH, there have been reports that some cells have drained quicker than that, without parasitic load. Go figure. I guess 3%/month is a guarantee type of thing. As said before, there have always been "substandard" cells that work perfectly otherwise but have higher self-discharge. If you have 50 cells, you may not have them, but some other may have a few of those, which may explain differences in experiences.

Some no-BMS users have reported noticeable (but not catastrophic) balance shift in about 2 years of use. I think it's more than great. For user, it means the possibility to go without a BMS if they know what they are doing. For a BMS designer, it means that you can use very low shunting current to overcome the very minute unbalance; this allows tighter fusing and removes the burden of thermal design. For example, a proper sealed lead-acid BMS may need to shunt 5A, whereas a properly designed li-ion BMS might be able to do it at 5 mA.

You still need to make differences between saying:
- there is very little self discharge
- there is no self discharge

You are missing the original point that this particular BMS supposedly had quiescent current so low that it was _even lower_ than the miniature self-discharge of the cells, which already is so low that it is practically nonexistent. IIRC, Elithion has reported before that this particular BMS was connected to a pack for a few years without noticeable discharging of the cells. To put it in your terms, if you want to say that the cells do not self discharge, then you can simply say that this BMS does not consume current from the cells. It's exactly the same thing; the current consumption is below your threshold.

OTOH, I fully agree that a current-greedy BMS may be top 1 reason for cell unbalancing. But these BMS's are supposed to balance the cells on every charge. It becomes a problem when the car is not used. Therefore, if I were to design a BMS, I would calculate that the minimum cell size intended for the BMS would not overdischarge from starting SoC of 30-40% during one year or so. For example, miniBMS takes 10 mA which is HUGE and will discharge batteries with some time. I found 500 µA a good compromise with easy design and good noise immunity. It's still a lot more than Elithion has reported in this particular case.


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## dtbaker (Jan 5, 2008)

Ziggythewiz said:


> My pack is top balanced with no BMS.
> 
> I see a BMS as a large expense that is unnecessary with proper care and feeding.
> 70 or 80% DOD is recomended to prolong cell life, while charging is nearly always to 90-100% SOC. It's easier to stay away from the bottom than it is to stay away from the top, so better protection on the top end is more useful.
> ...


sums my position up quite well.
I've got almost 20k e miles on Swift, and 2k miles on Miata now. I checked balance on swift after 1k, 3k, 6k.... re-top-balanced even though nothing was very far off. checked at 15k, still balanced at end of charge.


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## arber333 (Dec 13, 2010)

Hi Can you help me with a problem?

I am using DIY BMS modules to assist with bottom balancing. I discharged the pack to 2,8V and charged untill one cell hit 3,6V couple of times and they went together untill full discharge quite nicely. I drive 65km to work and 50 of those are highway. Speeds are 110km/h and discharge is cca 130A. Sinopoly 200Ah cells.

However day before yesterday i was returning home and i noticed one cell no. 22 voltage was falling below 2,6V at 130A load. I managed to get home and checked this cell. It bounced back to 3V. Nothing to cry about.

I charged them trough the day. First thing i noticed was the soc reading was 92% and cell no. 22 has shut the charger off. I went to work and by the time i arrived there i only had cca 50% SOC left!!! After work i drove back. 15km before home I noticed cell 22 and 23 voltages were falling. I went off the highway and drove at cca 50A - 90A. Nothing changed and just before home my BMS lost comm with those batteries because the modules need at least 2,2V of power to work. I checked cells in my garage and i found them quite warm. They cooled much slower then the rest. But when they cooled down they had 3,1V again. WTF!!! 
My front box lid is lined with neopren rubber to press the cells in their place. Could this cause the cells to heat up? But why only two of them?

Do you have a solution? 

Arber


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## dougingraham (Jul 26, 2011)

arber333

I see that you posted this query in at least three places. You should have started a new thread.

I've thought about this more and I am going to go with a loose or bad interconnect between two cells. This will cause heating, lower indicated voltage during discharge and higher indicated voltage during charge. It explains all three of your symptoms.

You can test for this by setting your DVM to the most sensitive scale you have and placing one probe on the negative terminal (not touching the interconnect strap) and the other probe on the positive terminal of the cell that strap connects to. With no current flowing you will read zero volts. If you turn on your charger you will see a small voltage between the terminals across the strap. The reason to not touch the strap is so you can see the voltage drop across the whole connection. I see a resistance of 0.0000409 ohms between a pair of cells I tested. With a 30 amp charge current I was seeing 0.001227 volts between the terminal pads when I had the bolts torqued down properly with a thin application of NOALOX grease. At 130 amps this connection would display 0.0053 volts and would act as a 0.7 watt heater. It doesn't take much of a bad connection to get real hot real fast. At 1000 amps the voltage between the terminal pads on my test connection would be 0.0409 volts but the strap would be seeing 40 watts. You don't want to do this for too long as every strap in the pack would be dissipating 40 watts.

I hope it turns out to be something simple like a loose terminal bolt.


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## arber333 (Dec 13, 2010)

dougingraham said:


> arber333
> 
> I see that you posted this query in at least three places. You should have started a new thread.
> 
> ...


My ride: http://www.evalbum.com/preview.php?vid=4534
I will service my battery and measure Ri (4R resistor method) and then clean every copper strap. SO you would also recommend NOALOX grease? What is that? I use contact spray to degrease surface and then join the contacts.
I also hope you are right. I see now how i thought connection trivial. My mistake

tnx

A


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## dougingraham (Jul 26, 2011)

arber333 said:


> SO you would also recommend NOALOX grease? What is that? I use contact spray to degrease surface and then join the contacts.


I believe NOALOX is short for NO Aluminum OXide and was originally intended for connecting aluminum wires together to keep the oxidation film from becoming a problem at the interconnect. I think the active ingredient is Zinc. My approach to cleaning the terminals was a dot of noalox and a quick scrub with a Scotchbrite pad to remove the oxide layer and then a rub with a paper towel that has some noalox on it leaving a very light coating. I did this immediately before attaching the terminal. I used a torque wrench to tighten the terminal bolts for consistency and NordLock washers to keep everything tight. At my 6 month test I didn't have anything loose but I did have to retorque all the bolts. I think it was from settling or compression of the copper straps as none of the bolts backed out.

Some sort of dielectric grease is recommended. Aluminum oxidizes so quickly and copper oxide is a poor conductor. Also there is the galvanic action from the contact of the dissimilar metals to be concerned with.

So you used some sort of contact cleaner spray? Those either have some lubricant in them or they are just cleaners in my experience. If it has some lubricant then it might also help prevent the oxidation layer from forming on the contact surfaces.


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## nucleus (May 18, 2012)

*Noalox, preventing corrosion*

Jack Rickard misses the boat on this. 

Clean, put on the Noalox, join. 

The advantage of Noalox over regular dielectric grease is that it has the zinc particles to dig into both sides of the connection and conduct electricity.

I put dielectric grease underneath my heat shrink terminations to keep the oxygen out too.

For the absolute best corrosion protection you may even want to paint your over your completed battery connections with a liquid vinyl like liquid electrical tape. This will keep the Noalox from drying out.

You can just peel it off when you want to do maintenance.

Here is a link to a short article on "Anti Corrosive Zinc Pastes" that is insightful.

http://sw-em.com/anti_corrosive_paste.htm


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## onegreenev (May 18, 2012)

*Re: Noalox, preventing corrosion*

Got pictures of your setup to show?


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## kchiangusa (Mar 28, 2012)

I just swapped out a 22AH 12V auxiliary battery in my electric MR2. While doing this, I measured the amp draw of the 12 V system. I think the only thing pulling amps is the EV display 2 SOC gauge, since I am not using a BMS.

http://blog.mr2ev.com/how-to-size-the-12v-battery-for-an-electric-car/

I am curious for those of you that use BMS's, what's your amp draw from your 12V system?

Thanks!


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## octagondd (Jan 27, 2010)

I chose No BMS, Bottom Balance because that was the option that fit the most. I am planning on bottom balancing and using a BMS. Here is my system:

LVC - N/A - Bottom Balance
LVC backup - Open Revolt Controller
HVC - Elcon 2000+ charger
HVC backup - Manual or digital timer plus some basic math based on my AH monitor
AH monitor - E-Xpert Pro
Voltmeter
**Special Bonus** - 4 more 160AH cells which gives me a larger range cushion by a couple miles and gives me a higher voltage for higher speed.

I am being a bit sarcastic here, but this is the path I have chosen and feel comfortable with. I used to be in the top balance, no BMS camp because I was scared of the unmonitored charging, but there have been very few, and possibly zero, Fire events since the specs have been revised down to 3.8 and 3.6 for HVC on charge. I also believe the BMS hardware and knowledge/installation has gotten better, so the charge voltage is partly correlation and not causation, but since the theory of keeping away from the ends of the curve came into play, the issues with catstrophic events have largely diminished.


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## PStechPaul (May 1, 2012)

I just came up with an idea, although it seems rather simple and probably has been discussed before. It should be possible to run a test on all of the cells in a pack, and label them according to measured capacity. Then you should be able to install a single cell BMS monitor on the lowest cell. When charging, stop the charge when the lowest capacity cell reaches peak charge. And similarly, cut back on discharge when the weakest cell reaches its maximum DOD. There should also be a monitor for the entire pack as well, but unless individual cells age differently, this method should be safe for all of the "better" cells. 

The only disadvantage may be that the BMS on the weakest cell might discharge it a little faster than the others, but this would only be a problem if the pack were left unused for a long time, and at worst it might drain the weakest cell too much. But it should trigger an alarm and/or disconnect itself before this happens.


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## octagondd (Jan 27, 2010)

PStechPaul said:


> I just came up with an idea, although it seems rather simple and probably has been discussed before. It should be possible to run a test on all of the cells in a pack, and label them according to measured capacity. Then you should be able to install a single cell BMS monitor on the lowest cell. When charging, stop the charge when the lowest capacity cell reaches peak charge. And similarly, cut back on discharge when the weakest cell reaches its maximum DOD. There should also be a monitor for the entire pack as well, but unless individual cells age differently, this method should be safe for all of the "better" cells.
> 
> The only disadvantage may be that the BMS on the weakest cell might discharge it a little faster than the others, but this would only be a problem if the pack were left unused for a long time, and at worst it might drain the weakest cell too much. But it should trigger an alarm and/or disconnect itself before this happens.


If top balanced, could you run the BMS off the largest cell and sense the smallest cell to trigger the charger shutdown, or would it still be a drain on the small cell?


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## EVfun (Mar 14, 2010)

*Drain* -- that is the killer when you start with a BMS. Either you need modules that draw within a 10 *micro*amps of one another or you will end up with it moving cells by more than 0.2 amp hour per year. With a top balance that seems to be about what it takes to move the end of charge voltage up about 0.2 volts. If you choose to run a BMS I could only recommend a module on all cells with at least minimal shunting (just a bright red LED should be enough once balanced) and GO that route. I think trying to live halfway between no BMS and BMS is likely to only get you the worst of both. Assured imbalance and unlikely to be warned of a problem.


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## GizmoEV (Nov 28, 2009)

PStechPaul said:


> I just came up with an idea, although it seems rather simple and probably has been discussed before. It should be possible to run a test on all of the cells in a pack, and label them according to measured capacity. Then you should be able to install a single cell BMS monitor on the lowest cell. When charging, stop the charge when the lowest capacity cell reaches peak charge. And similarly, cut back on discharge when the weakest cell reaches its maximum DOD. There should also be a monitor for the entire pack as well, but unless individual cells age differently, this method should be safe for all of the "better" cells.
> 
> The only disadvantage may be that the BMS on the weakest cell might discharge it a little faster than the others, but this would only be a problem if the pack were left unused for a long time, and at worst it might drain the weakest cell too much. But it should trigger an alarm and/or disconnect itself before this happens.


Bad idea. Take for example the BMS boards I used to have on my pack. They are from Black Sheep Technology and only draw 1mA when not shunting or triggering an alarm. 1mA = 0.001A. There are 8760hours/year so 8670h/yr * 0.001A = 8.76Ah/yr. That is significant. It doesn't matter whether the pack is left idle or is cycled each day, the same amount of charge will be drained each year.

In about 6 weeks it will have been 2 years since I top balanced my pack. I removed the BMS boards right after I balance the pack. If I had left one board on the smallest capacity cell group (I have a 2p20s pack) that would be over 17.5Ah used up by the one BMS board. That would knock that cell group nearly 9% out of whack with the rest of the pack (200Ah pack). As it is, the pack appears to be in the same balance as it was when I balanced it. The only instrumentation I have on the pack besides a CycleAnalyst is a split pack voltage monitor. While it does drain the pack a little any uneven drain is always to equalize the two haves. My meter very rarely reads more than a 0.01V when the pack is sitting idle.


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## PStechPaul (May 1, 2012)

Well, the idea is to have the single cell BMS connected only when charging or discharging, and not while idle. Or, if the pack must be idle for several months, it could be set up to sleep for ten minutes and only wake up for a second or so to take a quick reading and trigger an alarm if the cell has gone too low. Something like a PIC12LF1840 can sleep with a current draw of 20nA and its watchdog timer can run at 500nA, so for even a small 20 Ah cell it would take more than 2000 years to drain it to 50%. Even if you charged a capacitor for 2 minutes at 20 uA and flashed an LED for 0.1 seconds at 20 mA (plenty bright enough to see, and long enough to sound an alarm like a smoke detector low battery chirp), it would drain to 50% in 500,000 hours. 

Moreover, if that weak cell was made weaker by this deliberate extra discharge, it would just take a little bit more charging before the BMS started shunting the charge for that cell. This might mean that the other cells could be overcharged if that single cell had been severely unbalanced, but the total pack voltage cutoff should prevent that. 

During discharge, the weakest cell, with the BMS, would be the first to complain, and if it had been discharged more because of the BMS current, it would just reduce or shut off the discharge a little bit sooner, but that still errs on the safe side. You could always install a manual bypass for the cell with the BMS if you find that it shuts you down prematurely, and in that case you could (and should) do a quick manual check of all the cells with a DVM you should always have with you anyway. 

It should not be too great an effort to characterize all the cells and then install them in order from highest to lowest capacity, and if they are really close, it may be an advantage to add a small but significant additional drain on one of them, which will assure that it will be the first to report a low battery condition on discharge. And when charging, it will simply shunt the additional charge current until the entire pack is fully charged as measured by the total voltage and Ah count.


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## Siwastaja (Aug 1, 2012)

EVfun said:


> *Drain* -- that is the killer when you start with a BMS. Either you need modules that draw within a 10 *micro*amps of one another or you will end up with it moving cells by more than 0.2 amp hour per year.


You seem to forget that the BMS is supposed to do a fresh top balance on every full charge cycle. This fixes the imbalance created between the full charges. You charge your pack more often than once in a year, no?

So, for example, if you have a BMS with 1 mA difference between the modules (which is HUGE, and no proper BMS should have this much drain, let alone drain difference), it works out about 0.7 Ah per month, so you would maintain proper balance if you do full charge about once per month. During this first full charge after a month, the BMS module would dissipate max. 2.5 Wh of heat. (Which, IMO, is a bit too much, but then again, no real BMS system has drain difference of 1 mA used in the example.)

Remember, a BMS system needs cells to be in balance only for maintaining full energy, not to protect the cells. Of course the proper top balance adds an extra level of security in case of BMS failing and leaving the charger on, given that the charger has a properly configured CV voltage.

Of course, the drain should be minimized so that leaving the car unused for a while won't hurt the cells; and the difference between the modules should be minimized so that giving only partial charges instead of full charging for a while won't hurt the balance and hence reduce capacity. I have stated many times that 500 uA drain with 100 uA max difference between the modules should work in practically all situations and should be the design basis. Many BMS systems apparently do a lot better than that. The numbers I give are very easy to reach using modern digital components.

AFAIK, there is just one popular BMS (miniBMS) which has current drain so high that it requires special attention; that is, regular full charging, and disconnection of modules if the car is left unused for longer time. This is the price to pay for the super-simple and "failsafe" analog construction it has. Still, it seems that people have had no problems of cell imbalance or BMS discharging the cells even with this worst-case system. So it appears to me that this "problem" is again one of those not based on reality at all, or based on _faulty_ systems working outside of their design basis.


Also, when speaking about "drain difference", there are two kinds of drain difference between modules. If it is random, not depending on voltage or inversely proportional to voltage, then this drain difference indeed has an unbalancing effect on the cells. OTOH, most of the time components (including any voltage measurement resistor dividers) have current consumption _proportional to voltage_, which means these constructs are actually balancing; they drain higher-voltage cells more until the voltages match. Parallel-resistor balancing network is actually used to balance series capacitors.


As a general note to everyone. Stop and think for a while. There's a lot of info on electronics on the Internet. You don't need to rely on the myths and cult created by Jack Rickard; you can think for yourself. Do your own calculations.


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## EVfun (Mar 14, 2010)

Siwastaja said:


> You seem to forget that the BMS is supposed to do a fresh top balance on every full charge cycle. This fixes the imbalance created between the full charges. You charge your pack more often than once in a year, no?


Oh no, I didn't forget. The poster I was replying to, PStechPaul, was talking about only installing a cell module on the smallest cell. 


> Then you should be able to install a single cell BMS monitor on the lowest cell.


If you continue reading my post you would have found that is exactly what I'm talking about. 


> If you choose to run a BMS I could only recommend a module on all cells with at least minimal shunting...


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## Siwastaja (Aug 1, 2012)

EVfun said:


> Oh no, I didn't forget. The poster I was replying to, PStechPaul, was talking about only installing a cell module on the smallest cell.


Oh, sorry, I skipped his long post because they most often contain ideas with little practical meaning 

A more usable way to decrease the number of cell modules would be grouping 2-3 cells together, because with that low number of cells, the sum voltage follows the lowest/highest cell well enough. You just need a bit more leeway in the limit voltages. 

This is how 12V battery modules work; both LiFePO4 modules and traditional sealed lead acid (which needs a BMS, at least a powerful shunt balancer, but not for every cell, one for every six cells is enough).

However, grouping 2 Li cells together would raise the voltage over the range of modern IC's. This approach could be usable in the miniBMS-style analog BMS design.

I would still just do it the good old way, one module per every cell. Again, electronics is cheap to "mass produce". The difficulty in designing a BMS is in designing and careful testing of one cell module and the control logic; then it's somewhat irrelevant whether you need to manufacture 25 or 50 or 100 cell modules.


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## EVfun (Mar 14, 2010)

The biggest problem I see with monitoring cells in groups of cells is shunting. I think that once you have something attached to any point on the pack other than the ends you will have some (even if slight) difference in discharge cell to cell and so need some shunting to return the cells to balance. With a already balanced pack and good cell boards even 20ma from a red LED could likely provide the needed balance current. It just has to counter a tiny bit each cycle. 

I think that using a series of comparator circuits on each pair of cells could protect the pack without having to worry about actual minimum and maximum voltages. At some point one of those cells will either jump in voltage because it is full or fall off the voltage cliff toward dead on discharge. It would allow extra sag in a cold pack or a pack at low SOC while _hopefully_ catching a cell going dead.

For example, it would be able to avoid false reporting at 20C discharge rate sags (for cells that can take it.)
1.8 vs 2.0 would not flag an issue (top and bottom cells within 0.2 volts) 
but when the smallest cell starts falling toward dead...
1.8 vs 1.4 could flag an issue (top and bottom cells vary by 0.4 volts)


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## GizmoEV (Nov 28, 2009)

Siwastaja said:


> As a general note to everyone. Stop and think for a while. There's a lot of info on electronics on the Internet. You don't need to rely on the myths and cult created by Jack Rickard; you can think for yourself. Do your own calculations.


And you should do some direct measuring for your self and not just "thinking." While my data is limited and only one data point, it is supporting what Jack Rickard is suggesting. No myth to it. It will be 2 years this July since my pack was balanced and it shows absolutely no signs of being out of balance any more than when I started. This is no myth or cult for that matter.

Theory is fine but it can not be relied upon 100% without some real measurements to back it up.


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## PStechPaul (May 1, 2012)

While most LiFePO4 cells may last for many years without significant degradation, it seems likely that, eventually, some cells may grow weaker or develop internal leakage or other failure mechanisms, that will result in imbalance or even catastrophic failure. For that reason I would want to monitor every cell, at least on an occasional basis, to detect this. A very sensitive total pack monitor may be able to sense when this occurs, which would probably be evidenced by a faster-than-normal discharge or charge, but for a high voltage pack, this may be hard to detect. Perhaps cells could be grouped as modules of 24 or 48 volts with their own BMS and then have isolated communication from each module to the charger and/or controller.


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## octagondd (Jan 27, 2010)

PStechPaul said:


> While most LiFePO4 cells may last for many years without significant degradation, it seems likely that, eventually, some cells may grow weaker or develop internal leakage or other failure mechanisms, that will result in imbalance or even catastrophic failure. For that reason I would want to monitor every cell, at least on an occasional basis, to detect this. A very sensitive total pack monitor may be able to sense when this occurs, which would probably be evidenced by a faster-than-normal discharge or charge, but for a high voltage pack, this may be hard to detect. Perhaps cells could be grouped as modules of 24 or 48 volts with their own BMS and then have isolated communication from each module to the charger and/or controller.


I believe some people use the lee hart batt bridge to split the pack and see if either side is varying in anyway. The idea being if they see a larger than normal variance in one half the pack, then a cell may be having a problem.


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## GizmoEV (Nov 28, 2009)

octagondd said:


> I believe some people use the lee hart batt bridge to split the pack and see if either side is varying in anyway. The idea being if they see a larger than normal variance in one half the pack, then a cell may be having a problem.


That is exactly what I built for my pack. Naturally if similar issues happen simultaneously in each half of the pack it would be missed by the meter but I don't figure that to be much of a risk.


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## kchiangusa (Mar 28, 2012)

kchiangusa said:


> Looking back in my battery logs, monthly, I'd measure the cell voltages of each cell after driving the car to about 30% charge remaining. The average cell voltages at this state of charge is typically 3.22 +/- 0.02 volts. If they are significantly lower than this, I will use my GT Power battery charger and individually charge the cells until their voltages are in par with the rest of the cells. I do this in increments so I don't have to watch the charger or risk forgetting about the charger. I'd preset the amount of energy to add to the battery, when it is done, I'll measure the voltage again until it reaches voltage. In my experience, it hasn't been more than 2-3 AH that I would add to the low cells to get them balanced.
> 
> I've had to add charge to low cells 2 times in the almost 2 years that I've been driving, the first time was on 3 batteries and the 2nd time on 2 batteries. It has not been bad at all.
> 
> Anybody else that is doing the same? What's been your experience?



I just measured the parasitic amp draws of the cell log 8 if they were switched off (i.e. with pin 1 disconnected) and with the cell logs switched ON. here are the results. For anyone using cell log 8s as a BMS and leave them on while you charge, you could be significantly draining a couple of your cells and causing imbalance.


PIN	cell log 1, OFF, amp draw (mA) cell log 2, OFF, amp draw (mA)
9	0.57	0.76
8	0.02	0.02
7	0.33	0.35
6	0.04	0.02
5	0.12	0.15
4	0.20	0.18
3	0.27	0.35
2	0.30	0.32
1	—	—

PIN	cell log 1, ON, amp draw (mA)	cell log 2, ON, amp draw (mA)
9	0.81	0.78
8	0.762	0.74
7	14.82	15.60
6	0.02	0.02
5	0.02	0.02
4	0.02	0.08
3	0.04	0.04
2	0.02	0.03
1	15.84	16.64



http://blog.mr2ev.com/should-you-in...to-protect-those-expensive-lithium-batteries/


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## PStechPaul (May 1, 2012)

I don't know the details of the cell log 8s but I found this:
http://ev-power.com.au/webstore/index.php/instrumentation/junsi-cell-log-8s.html

It seems to use an LM358 and an LM324 as amplifiers and other circuitry that is not really low-power. They typically draw 1-2 mA plus whatever loads are being driven. The power seems to be taken from two places on the 8 cell string and that may be why the current draw is higher on some pins. This may not necessarily mean there is more current draw on those cells, however. The current flows equally through all the cells used to power the module, and the current measured on intermediate points is just the input to the amplifiers which do the actual measurement.

There are ways to greatly reduce the current drain of a BMS or cell monitor such as this, where the average power supply current is less than 1 mA and the measurement current is only a few microamps.


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