# Beginner BMS question



## Siddy (Jun 28, 2013)

I am planning a 72V 100A LiFePO4 Battery pack.

I know if use 22x 3.2V 100A cells in series I will need a BMS for each cell which is 22 BMS units.

I want to know if I will need a BMS unit in between each cell in parallel.

For example, say I had 12V 4A cells, could I run 6 BMS units between series connections to 4 cell parallel batteries. (Which would be approximately the same volts and amps).


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## Ziggythewiz (May 16, 2010)

I suggest reading much more about BMSs and what they do.

I have 39 cells in series and find no need for one. I have an AH couter and 3 DVMs across my pack which is more than enough protection, total cost ~$100


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## TexasCotton (Sep 18, 2008)

Ziggythewiz said:


> I suggest reading much more about BMSs and what they do.
> 
> I have 39 cells in series and find no need for one. I have an AH couter and 3 DVMs across my pack which is more than enough protection, total cost ~$100


 Images please and what is a DVM aka D Voltage Monitor??BMS can overcharge cells too FYI


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## dougingraham (Jul 26, 2011)

Siddy said:


> I am planning a 72V 100A LiFePO4 Battery pack.
> 
> I know if use 22x 3.2V 100A cells in series I will need a BMS for each cell which is 22 BMS units.
> 
> ...


If you build your pack by paralleling the cells first then you can treat the paralleled unit as if it were a cell. If you don't parallel first then any weak cell in a series string becomes the weak point in the battery pack. You will have several weak points instead of just one.

The real question here is why do you think you need a BMS. Especially with a low voltage pack of LiFePO4 cells. Save your money and buy a decent DVM and a couple of spare cells instead. Do an initial manual bottom or top balance and check the balance every so many months (or years). A BMS will not save a cell from going bad, all they can do is tell you which one is going bad or has failed.


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## EVfun (Mar 14, 2010)

A BMS is often denigrated on this forum, but they remains quite popular. Here is the last thread with a poll on the subject.

Plenty of people are successfully running with a BMS, I'm not one of them. If you don't have a BMS then the operator is the BMS. That means you must do periodic checks of the pack after a cycle and near the end of a charge to make sure no cells are leaving a balanced state or loosing capacity. LiFePO4 cells are quite stable and have few issues with getting out of balance.

A poorly implemented BMS is far worse than no BMS. If the cell modules are not reliable then you will reduce the pack reliability. If your BMS wiring is poorly done you will reduce pack reliability. Reversing a Lithium cell is something that you should never do, the cell is destroyed and it can create a fire hazard. This applies with or without a BMS, thus the flame wars.


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## mora (Nov 11, 2009)

Siddy said:


> I want to know if I will need a BMS unit in between each cell in parallel.
> 
> For example, say I had 12V 4A cells, could I run 6 BMS units between series connections to 4 cell parallel batteries. (Which would be approximately the same volts and amps).


Let's try to answer these questions and not argue if he needs a bms or not.

I'm not sure if you mean having bms board for every single cell in your pack or only one for each parallel group. If you put cells in parallel those should be very close to each other at any time and should require only one bms per parallel group. Nothing prevents you from putting a bms board to every single cell though. Total cost will go up significantly.

Bad off topic feeding: cell reversal didn't cause flames, explosion or cell death in my 144V pack. It was a Winston cell from 2010 or 2011. I drove my pack beyond its low limit while bms screamed at me to stop the car. Measurements were done immediately when I stopped the car. Reversed cell recovered itself back to positive reading in few minutes. I haven't done any cell capacity or internal resistance measurements on that cell afterwards but I know it isn't nowhere near my weakest cell in the pack. Still keeps giving amps as it should. At the time those Winston cells were not nicely top balanced but older TS cells were. Those TS cells were discharged down to 1.5-2.0V and Winstons were between 1.0-2.0V.


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## Siddy (Jun 28, 2013)

Thanks for the replies!




dougingraham said:


> Save your money and buy a decent DVM and a couple of spare cells instead.


What are the spare cells for? Just for when the old ones die?


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## dougingraham (Jul 26, 2011)

Siddy said:


> What are the spare cells for? Just for when the old ones die?


Spare was not the correct word. Extra cells would have been better. In my case I tested every cell and kept out the four weakest ones. Once I got my battery boxes sorted out I found that I could only do 51 cells instead of the 56 I was originally planning on. So I have some extras that hope to never have to use but I fully expect some day might need to get inserted if something happens to a cell. I may also look for a place to tuck in a few more cells. In my case each cell is equal to about 1.5 miles of range or if never used additional pack life.


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## Ziggythewiz (May 16, 2010)

TexasCotton said:


> Images please and what is a DVM aka D Voltage Monitor??BMS can overcharge cells too FYI


Digital Volt Meter. Everyone's most used part of the multimeter, but I have a pile of specific DVMs for different stuff.

These 3 aren't that accurate ($3 from China) but they're good up to 50V without a separate power supply, so no isolated DC-DC needed, and if they ever get a volt or 2 of difference I would know to investigate.

The meters are fed by just a few taps in the pack which have inline fuses near the terminals.


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## frodus (Apr 12, 2008)

BMS is up to you Siddy. I prefer at least having something that monitors cell voltages as I'm riding. Watching pack voltage doesn't give you the whole story. Cell-Log 8's from HobbyKing do that wonderfully and they have an alarm output you can use for a Dash light, buzzer or something that causes the throttle to be reduced immediately. 

Make sure your pack is either top or bottom balanced before you start though. Individually balance each cell before building your pack.

As far as parallel groups of cells, you only need one BMS channel per group. If you have 24s4p (24 in series, 4 in parallel) where you parallel the 4 first and then hook 24 of those in series, you only need a 24-channel BMS.

Also, instead of 22 cells, go for 24 cells. It'll be easier to find a proper charger, DC-DC and the controller will be happy. Just think of 4-cells per 12V.


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## TexasCotton (Sep 18, 2008)

Ziggythewiz said:


> Digital Volt Meter. Everyone's most used part of the multimeter, but I have a pile of specific DVMs for different stuff.
> 
> These 3 aren't that accurate ($3 from China) but they're good up to 50V without a separate power supply, so no isolated DC-DC needed, and if they ever get a volt or 2 of difference I would know to investigate.
> 
> The meters are fed by just a few taps in the pack which have inline fuses near the terminals.


 LOL
thank 
I may go cheap with the HF MM CM LCD aka Harbor Freight Multi metersssss...............


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## TexasCotton (Sep 18, 2008)

Ziggy
I like the CALB's you have.........


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## Ziggythewiz (May 16, 2010)

I'm a big HF fan, but their multimeters are only worth the $ if they're free.


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## ga2500ev (Apr 20, 2008)

Ziggythewiz said:


> I'm a big HF fan, but their multimeters are only worth the $ if they're free.


Enjoy!

The best HF coupons can be found here:

http://slickdeals.net/f/1276399-Harbor-Freight-Coupon-Thread

ga2500ev


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## Siddy (Jun 28, 2013)

frodus said:


> ...Watching pack voltage doesn't give you the whole story. Cell-Log 8's from HobbyKing...


Thanks Frodus, I will take your advice. I was looking for something like that cell-log 8, I am going to get one or two.

I don`t think I`ll go down the BMS road anymore. Just a monitor, logger, alarms and emergency cut off with balanced cells and a good charging system.


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## frodus (Apr 12, 2008)

Cell Logs are a type of monitoring BMS, but also has the ability to hook charger/throttle to the alarm output to stop charging/discharging when a cell goes low/high.

You DO NOT need something that balances the pack (balancing BMS) if you are going to balance yourself. You may need to do this often, or you may not. If you aren't bottom or top balancing then you should consider buying a BMS that balances. Buy a good brand, not something that can fail to protect your cells.

You DO need a way to stop (or reduce) discharge when one cell goes low. If you bottom balance perfectly, this should happen at the same time.... but there's no guarantee that will happen. 

You DO need a way to stop charging when one cell goes high. If you top balance perfectly, this should happen at the same time, but there's no guarantee that will happen.

IMHO, bare minimum is to monitor all the cells and at the very least set the system up to disable the charger when a cell goes high, and reduce/stop throttle when a cell goes low. This protects the pack, regardless of balance of the cells.



Also, consider my advice on switching to 24s, makes life much easier... you don't have to be right at 72V ("12V" lead acid aren't right at 72V either). It'll work fully with 72V controllers and chargers.

If you get 3 cell-logs, you'd have enough for all 24 cells using this breakout board from RWAudio:
http://www.diyelectriccar.com/forums/showthread.php/cell-log-8-breakout-module-70186.html


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## Siddy (Jun 28, 2013)

Lots to consider I see. 
Regarding charging, is it ok to leave the cells fully charged for a long time or is it better to leave it at 80% and then top it off just before using them?

Oh and yeah I see the reasoning for using a number of cells in multiples of 12v or whatever the charger voltage is so I will do that.


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## Ziggythewiz (May 16, 2010)

The manufacturers usually leave them at 50 or 60%. I try to "top mine off" to 90% before use. The closer they are to full the more they wear.


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## jumpjack (Sep 9, 2012)

This is the reason because one single Digital Volt Meter is not enough at all to guarantee health of a battery pack:

http://liionbms.com/balance/index.html

*Always *use a BMS on your battery pack or you will (literally) burn your money.
It's just physically impossible to have cells "so well matched that they will never got unbalanced".

BEWARE:
some BMS just *balance *the cells; and there are PCBs and PCMs (Protection Circuit Board, Power Circuit Management,...), which just protect from overcharge and overdischarge.

A *good* BMS must both balance and protect!


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## EVfun (Mar 14, 2010)

jumpjack said:


> *Always *use a BMS on your battery pack or you will (literally) burn your money.
> It's just physically impossible to have cells "so well matched that they will never got unbalanced".


Wow... have you tried? I'm actually fairly favorable to BMS (though scared of the wiring I see on some EV conversions) but I can see that there are users successfully running LiFePO4 battery packs without a BMS of any type. The last poll on the subject indicated that the members with Lithium powered EVs where about equally divided between those with and those without a BMS.


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## jumpjack (Sep 9, 2012)

EVfun said:


> Wow... have you tried? I'm actually fairly favorable to BMS (though scared of the wiring I see on some EV conversions) but I can see that there are users successfully running LiFePO4 battery packs without a BMS of any type. The last poll on the subject indicated that the members with Lithium powered EVs where about equally divided between those with and those without a BMS.


Yes, you'll find a lot of people using battery without BMS and happily saying "see? it works!".
Unfortunately they don't know, and they can't verify by themselves, that they are damaging the batteries! There's no way, AFAIK, to distinguish a brand new battery from a battery which just had a DoD100% discharge at 10C and then has been recharged.
But in the long time there WILL be a way: multiple 100% and/or high-rate discharges irreversibly damage electrodes, and after few cycles your battery will retain just 80% of capacity , rather than after 1000 cycles.
And if you do not use BMS, you THINK you are discharging for example at 50%.... but it's true only for whole battery, while maybe you are discharging some cells down to 100% and overcharging others over safety voltage! (fire risk).
And once 1 cell fail, whole battery fails!


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## Ziggythewiz (May 16, 2010)

jumpjack said:


> And if you do not use BMS, you THINK you are discharging for example at 50%.... but it's true only for whole battery, while maybe you are discharging some cells down to 100% and overcharging others over safety voltage! (fire risk).
> And once 1 cell fail, whole battery fails!


Where do you get this nonsense? If you think your battery is at 50% when it's really at 100% (either way) you're an idiot.

And if 1 cell fails...1 cell fails.


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## mora (Nov 11, 2009)

I believe Jumpjack is trying to say you don't do well with only one voltmeter monitoring whole pack. You can't really know if there is one single cell in the pack which is way lower than others. And same goes the other way: you can't tell if one single cell jumps over 4.0V when others are at 3.4V during charge.


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## jumpjack (Sep 9, 2012)

mora said:


> I believe Jumpjack is trying to say you don't do well with only one voltmeter monitoring whole pack. You can't really know if there is one single cell in the pack which is way lower than others. And same goes the other way: you can't tell if one single cell jumps over 4.0V when others are at 3.4V during charge.


Exactly.
And by sure I cant waste my time measuring ALL cells of my battery EVERY time I use the scooter! (Not to talk about bringing 20 wires out and fixing them in some place easily accessible but not dangerous...)
There are "machines" supposed to do such a boring job; they're called BMSs. 
Even if you are not mounting a BMS but you're doing above operations... you ARE using a BMS: it's named HBMS (Human BMS).


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## EVfun (Mar 14, 2010)

A balanced pack doesn't change very fast. If you balance them you don't need to check more than once a month, unless there is already something very wrong with your pack.


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## Ziggythewiz (May 16, 2010)

Yup. I have three DVMs across my pack. I switch em on about once a month, while charging or driving farther than typical. Still close? Yup. Switches off.


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## ga2500ev (Apr 20, 2008)

mora said:


> I believe Jumpjack is trying to say you don't do well with only one voltmeter monitoring whole pack. You can't really know if there is one single cell in the pack which is way lower than others. And same goes the other way: you can't tell if one single cell jumps over 4.0V when others are at 3.4V during charge.


Actually you can. A couple of years ago there was a thread here on simple pack monitoring based on tracking the difference in voltage between the two halves of the battery pack. As long as the two halves had the same voltage, then the pack was fine. However, if they were way out of balance, the voltage difference would show up on the meter. Here is a BLOG entry describing the circuit which also contains a link to the original thread. The original "idiot light" version simply lit an LED when the pack was out of wack. The improved versions used a meter to measure the voltage different between the two half packs.

Now to be sure jumpjack, this will not work well if you throw a random collection of batteries with vastly different states of charge together. All BMS-less options are predicated on some level of battery balancing. The simple idea is that if all the cells have the same amount of energy, then they will all change state at the same time. Top balancing ensures that all cells charge to the same voltage so that they all have nearly the same high state of charge. Bottom balacing drains all the cells equally, so they are presumed to have the same amount of energy. In both cases, the presumption is that you are not going to smoke the cells by overcharging them, or that you will not completely drain the cells to completely empty.

The simple idiot light can help in both instances. On charge if one cell gets to 100%, its voltage starts to rise while the others stay put. Whichever half of the pack that overcharging cell is on will get out of wack with the other half of the pack. Light goes on, stop charging.

On the other hand if one cell prematurely drains, then it's voltage will drop off relative to the others (i.e. 2.7V as opposed to the 3.2V steady state). Again the light goes off. Stop driving.

There will not be an exact indication of which cell is the culprit as only half packs are monitored. However, it is more than sufficient to make sure that nothing is destroyed in the process.

So decent balancing + half pack monitoring can do most of the work of the human BMS without having to resort to checking each cell on charge/discharge. And if the pack gets out of wack, stop and rebalance the pack to get it back into good health.

Hope this helps,

ga2500ev


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## PStechPaul (May 1, 2012)

That pack balance monitor is really elegant in its simplicity. A meter with 199.9 mV FS will display 50.0 if a cell is 0.1V low or high, and thus it can detect imbalance of 200 uV per count. This circuit is very sensitive to the values of the divider resistors, so much so that 1% resistors could show a reading as high as 450 mV. So the adjustment potentiometer is needed, and it is important to use low temperature coefficient resistors and make sure they are the same temperature. This is usually done by mounting them on the same heat sink, or by using a resistor network with a common substrate, but in this case the resistors should be mounted close to the ends of the battery pack, which are likely to be far apart. 

I would suggest these 1/8W 0.1% 100k resistors with 5 PPM/C tempco:
http://www.mouser.com/ProductDetail...=sGAEpiMZZMu61qfTUdNhG3EzrPz99APhoC6zXVULOLs=

They would be OK for a pack up to 200V at which point they would each dissipate 100 mW. With 0.1% tolerance you would only need to trim about 50 mV so the zero trimpot can be 200 ohms. These resistors cost $1.50 each but are worth it. If the temperature difference is 10 degrees C, you could have 50 PPM error which is 0.005% and will show up as 1.2 mV on the meter. Compare that to ordinary 1% metal film resistors which are commonly 200 PP/C, or 0.2% for 10C and a meter reading of 47.7 mV! And carbon film or composite resistors are 250 to 800 PPM/C. So this is very important.
http://www.resistorguide.com/carbon-film-resistor/
http://www.resistorguide.com/materials/
http://www.resistorguide.com/temperature-coefficient-of-resistance/

Another bonus for this circuit is that you can use another DMM across the 200 ohm trimpot for a 1000:1 voltage divider, so a 199.9 mV FS meter will read 0-199.9 VDC pack voltage, with no additional loading on the pack. The actual loading for a 150V pack will be 750 uA, so for even a 10 Ah pack it will take 133 hours for a 1% discharge.


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## EVfun (Mar 14, 2010)

I think the disconnect is experience. Those who have time with LiFePO4 packs (and this DOES NOT apply to other Lithium chemistries) have learned by experience that they are fairly stable, have near 100% charge efficiency at proper voltages, and negligible self discharge. If you leave the safe window all bets are off.


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## Siwastaja (Aug 1, 2012)

ga2500ev said:


> Actually you can. A couple of years ago there was a thread here on simple pack monitoring based on tracking the difference in voltage between the two halves of the battery pack. As long as the two halves had the same voltage, then the pack was fine. However, if they were way out of balance, the voltage difference would show up on the meter.


Yeah it adds some protection, and you can add more taps for more protection, and in the end you have a tap for every cell and a full BMS for full protection.

Your suggestion is based on a hope that two cells on the different sides of the battery are not similarly unbalanced. Or, for example, two cells at the first side are slightly unbalanced and one cell on the other side is more unbalanced. Etc. There are various ways in which it can fail, but indeed, half-pack monitoring is better than nothing at all. Just don't trust it alone, and realize you are taking a risk.


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## ga2500ev (Apr 20, 2008)

Siwastaja said:


> Yeah it adds some protection, and you can add more taps for more protection, and in the end you have a tap for every cell and a full BMS for full protection.
> 
> Your suggestion is based on a hope that two cells on the different sides of the battery are not similarly unbalanced. Or, for example, two cells at the first side are slightly unbalanced and one cell on the other side is more unbalanced. Etc. There are various ways in which it can fail, but indeed, half-pack monitoring is better than nothing at all. Just don't trust it alone, and realize you are taking a risk.


As I said in my original post, and Lee Hart said in the original document, it's an "idiot light". While there are a number of failure modes such as the ones you onlined above, the most common failure mode is a single cell in a string either overcharging or draining before the others. The "idiot light" can detect that so that there isn't a catastrophic failure.

It seems that many think that Lithium is complex. It really isn't. I personally think that thought process comes from the facts that lithium is high in cost, and is very unforgiving of overcharging and overdisharge. The rules for managing it are simple.

1. Balance all the cells so that they have about the same energy. Whether this is top or bottom balancing, the idea is to have each cell carry an equal load. Unbalanced cells are what leads to problems outlined below.

2. DO NOT OVERCHARGE! This is one area where lead and lithium are vastly different. Overcharging is death to lithium cells. You need step 1 above because if the cells do not have about the same energy, then one will be full while the others are still charging. On the other hand, if they have about the same amount of energy, they will all reach that charging knee where the voltage shoots up fairly quickly. So when the first cell reaches that state, stop charging. So you need some High Voltage cutoff on your charger to ensure this happens.

3. DO NOT OVERDISCHARGE! Lead and lithium are actually the same here. Overdischarge of a cell leads to cell reversal and destruction. Again if the cells have about the same amount of energy, then once the voltage of even a single cell starts to drop off, then there's no more energy in the pack as a whole. A Low voltage cutoff is needed.

That's it. A well balanced pack works fine as long as the pack is not driven to extremes. The half pack monitor can assist with #2 and #3 because it can detect when the first cell changes relative to the others. And no matter if it's a charge or a discharge, once that change happens, then the pack is at an extreme and the activity needs to stop immediately.

So a BMS that monitors each and every cell isn't as crucial as structurally setting up the pack so that it stays away from the extremes and there is an indicator that one or the other of the extremes has been reached.

ga2500ev


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## Siwastaja (Aug 1, 2012)

ga2500ev said:


> It seems that many think that Lithium is complex. It really isn't. I personally think that thought process comes from the facts that lithium is high in cost


Well said -- I think this idea also comes from the "new tech" or high-tech status.

Whereas in reality, li-ion chemistries are the easiest batteries to understand because they very closely resemble the "ideal battery" model which is a voltage source and a series resistance.

There is practically no peukert effect, the charge efficiency is always 100%. This concept alone is so difficult to grasp for most people that it's great to get rid of it.

There is practically no self-discharge in most cases.



> 1. Balance all the cells so that they have about the same energy.


This means bottom balancing. In bottom balancing, the cells have the same energy.

In top balancing, every cell has the maximum energy, which differs between the cells due to different capacities.




> the idea is to have each cell carry an equal load.


Nope, balancing has nothing to do with this. In fact, in a series string, every cell inevitably will carry an equal load; the same current will flow through all of them.

Balancing only affects at which point first cells get full or empty.

Top balancing maximizes the available energy by utilizing maximum SoC point from all cells which means they release their charge at highest possible voltage.

In practice, today's production cells being of consistent quality, the energy maximizing feature of top balancing is very slight. So, people running without a BMS use balancing as an obligatory protection feature, and people who run with a BMS use balancing as an _extra _protection feature.

I want to remind that:
Bottom balancing only protects at bottom
Top balancing only protects at top

You need to make sure that you protect the other end in some other way, if you don't use a BMS. Top balancing people mostly use Ah counters. Bottom balancing people mostly just use leeway on the charge side and may overcharge a bit.



> 2. DO NOT OVERCHARGE! This is one area where lead and lithium are vastly different. Overcharging is death to lithium cells.


Wouldn't say they are _so_ different. Depends on lead and lithium! In fact, the LiFePO4 cells seem to be _surprisingly_ tolerant to some overcharging, as shown by multitude of people having bottom-balanced system with CV charging point set too high, inevitably causing some overcharging to some of the cells. Also seen from the initial instructions to charge to as high as 4.2V, then reduced to 4.0V, then to 3.8V during the years... Yes, the consensus now seems to be that you only should charge to 3.65V, but at least it didn't cause any _massive_ damage to charge to 4.2V once of twice.

OTOH, sealed lead acid cannot take much overcharging either (flooded can). This leads (pun intended) us to a situation where a long series string of SLA batteries needs a BMS (a strong shunt balancer is enough) for good cell life, and OTOH people are charging LiFePO4 without a BMS, even an unbalanced string (bottom-balanced, that is), and they seem to be just fine, at least after a few years...

So, what can we say? Don't overcharge any chemistry, but most can take some.



> The half pack monitor can assist with #2 and #3 because it can detect when the first cell changes relative to the others.


... which is still based on an assumption that there is clearly one single cell which differs from the others, but not two similarly weak cells on the different sides. In _reality_ however, you can see people having several very similarly performing cells in their packs. It's all about luck.


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## PStechPaul (May 1, 2012)

Perhaps you can at first test each cell for actual capacity, and mark it with the value. Then when you build the pack you can string them in order of capacity. That will pretty much guarantee that one half of the pack will be weaker and thus trigger the imbalance alarm.


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## EVfun (Mar 14, 2010)

PStechPaul said:


> Perhaps you can at first test each cell for actual capacity, and mark it with the value. Then when you build the pack you can string them in order of capacity. That will pretty much guarantee that one half of the pack will be weaker and thus trigger the imbalance alarm.


That is what I have done with my pack, though I haven't put a Lee Hart type battery balance indicator. I have a 39 cell pack, but the same trick can be done 1 cell off-center by compensating with slightly different resistor values. 

I may go back to a simple shunt reg or BMS. I have EVworks cell modules. I would like simple shunt regs (about 0.2 to 0.6 amp shunting) that tripped at about 3.5 volts and without the standby current caused by a green LED or keeping an opto on. My pack is made from a single batch of cells, but there is some small amount of "drift" like behavior. The cells that have been used less cycles (the EV Buggy started with 32 of my 42 cells installed) creep up in end of charge voltage over time.


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