# Did anyone just run battery pack without a BMS ???



## batterytang (Nov 17, 2011)

Just heard from other people, saying BMS is a waste of money, only need to use a charger charging battery packs and do balancing, no need for a BMS? Anybody have similar experience?


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## WarpedOne (Jun 26, 2009)

Real BMS is also an automatic top balancer.
If you love to manually balance your cells after every few charges, you are welcome. Others like to have it done automatically.


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## batterytang (Nov 17, 2011)

Thank you... 
I just wonder if here is any safety problem.


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## Siwastaja (Aug 1, 2012)

a BMS is not optional. It is mandatory.

For LiFePO4, it is not _strictly_ 100% mandatory for safety as they don't catch fire so easily.

A pack without a BMS, however, will last something around 1...100 cycles. With a proper BMS, it can last for 5000 cycles. Do the math for the price.

However, there are (or have been) people out there who decide to be a BMS themselves. Some of them have changed their mind after a long way, some are still happy. If you make a lot of effort to manually balance and monitor your battery pack all the time, it really IS possible! You may get more than 100 cycles if you are good on that and really understand your battery. You can make it a lot easier by deciding to give up a part of your range; don't charge full and especially don't discharge fully, and you may get out of the problems. But from this viewpoint, you can as well buy smaller/fewer cells and get a BMS for the money.

A real passive BMS will give you about 20-50% more range than "human BMS". An active (charge transfer) BMS may give you a further 10% on the top of that of a passive BMS. This is for a NEW battery pack. After 1000 cycles, a "human BMS" pack is probably dead while proper BMS pack still has almost full capacity left.

However, the BMS critique may be justified, and the BMS opposition is understandable. Apparently, there have been some faulty and totally unusable BMS products available. Some have drawn too much current when idle, destroying the cells. Some have overheated from wrong shunt design. And most are a bit expensive so I understand why people want to avoid them if possible.

So, to choose a BMS, it _*MUST*_ include the following two features as an absolute minimum:

- A cell-level monitoring for every cell that COMMUNICATES WITH THE MOTOR CONTROLLER. If ANY of the cells drop above minimum voltage, it *instantly* limits or cuts power. If the motor controller does not support this, an alarm sound is acceptable.

- A balancer that shunts off or redistributes extra voltage on any cell when charging.

Without those features and without a proper connection to the motor controller (or alarm), it is not a BMS.

*Without a BMS*, you typically just leave enough leeway on the both sides. This means you limit the charger voltage much BELOW numberOfCells * 3.65 V, and when the pack is nearing this point, go around with a volt meter to check random cells and stop if any of them is nearing 3.65V. This way you may have a SoC of 90% when you stop charging and you are probably fine.

Similarly, when you are driving, you must stop BEFORE you have any signs of heavy voltage sag. Without simultaneous monitoring of all cells during accelerations, it is impossible to know when your pack is near empty; when you start seeing the symptoms, you already are on the very dangerous region for the cell life. If you know what you are doing, you may be able to stop at around 30-40% SoC. With a BMS, you can stop at 10-20% SoC. This is why a BMS gives you more range; you can run your battery more flat without risking destructing it.

Simply put the usable SoC range is:
No BMS: 90 ... 30 % of the weakest cell
Passive BMS: 100 ... 10 % of the weakest cell
Active BMS: 100 ... 10 % of the average cell (becomes important after the pack starts getting older)

So just use a BMS. For me, it's a natural choice because I can do my own BMS without relying on products I know nothing about. So I don't need to take a risk, I can just do one that works -- internally, a BMS is very simple electronics. I'm going the active way to get every single drop of juice out of the expensive pack, but OTOH, I have access to some free components such as small ferrite cores to make the charge-balancing transformers.

Also, the lower the cell uniformity (even good quality cells can have variation if the production involves a lot of manual steps or they are from different batches), the more important the BMS becomes.

Hope this helps.


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## Duncan (Dec 8, 2008)

Siwastaja said:


> a BMS is not optional. It is mandatory.


That's NOT what a lot of other people say

There seems to be no reason for cells to go out of balance and people without BMS's run for years without problems

The main reason for cells to go out of balance is ...... the BMS!

Personally I am going for a split pack check circuit and no BMS


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## Siwastaja (Aug 1, 2012)

Duncan said:


> That's NOT what a lot of other people say


Hah, I was quite sure that someone will quote that out of context by stopping reading my post after the very first sentence. 

Later you can see that I propose a solution of a "human BMS", what is what you are going to do. I just count it as a BMS, just an inferior one but indeed with zero initial cost. There is a reason why I count it as a BMS. Simply, it does not come completely without a cost -- at least a cost of your time. It will work, as I say, but you cannot just ditch a BMS concept completely and forget about it; you have to be the BMS, at least to some level. You MUST understand this, or you can ruin your $10 000 pack on your few first test drives by driving it flat or charging it full without knowing what you are doing. But it's of course up to you. I am poor and can overcome my monetary situation by using my brain. There are people in opposite situation, too. Most are somewhere in-between. Nothing wrong in any of this.



> The main reason for cells to go out of balance is ...... the BMS!


I _know_ there are broken and dangerous BMS products out there. At least the rumors say so. Instead of spreading the misinformation about the BMS concept, we should create a list of those exact products. Say the names. We need to know what to avoid! We need to do some BMS testing. Which is a bit sad because the BMS _should_ protect our battery, not destroy it, but the world is cruel.

One more professional view on the matter: http://www.metricmind.com/education/ (see 2. Do I need a BMS)


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## DIYguy (Sep 18, 2008)

This topic has been beaten to death. Do some searches and read about it. 

There are many examples of BMS-less lifepo4 (chemistry is important here) running for years, including my own.


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## DIYguy (Sep 18, 2008)

WarpedOne said:


> Real BMS is also an automatic top balancer.
> If you love to manually balance your cells after every few charges, you are welcome. Others like to have it done automaticaly.


There is no need to balance manually if you have no parasitic, unbalance loads. Have you tried? There is no mechanism for cell drift other than perhaps extreme temperature differences within the same pack. I have never re-balanced my pack after 15000 km and going on two years. Others have longer/more miles than me.


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## Jan (Oct 5, 2009)

DIYguy said:


> ...never re-balanced my pack after 15000 km and going on two years.


It seems a little that people with no problems with their choice of components only share their positive experience when provoked.


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## jeremyjs (Sep 22, 2010)

Jan said:


> It seems a little that people with no problems with their choice of components only share their positive experience when provoked.


That works both ways. Many a BMS has failed and killed cells/or burned buildings, boats, cars, etc. We're only talking lifepo4 here obviously. There is little evidence of any cell drift in this chemistry without unbalanced loads on the pack and several years worth of evidence with certain people bottom balancing, using a reliable/accurate charger, slightly under charging the pack and never having a problem, even with instances of running the pack nearly flat. There are conditions for going BMS-less, but in my eyes it's been well proven to work very well and reliably. I might say even more reliably than running with a cell level BMS. I'm not saying you shouldn't check up on the pack once a year or so, but balancing should only be necessary once unless you did something wrong from the start. There's more than a little evidence showing this to be the case.


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## Jan (Oct 5, 2009)

jeremyjs said:


> That works both ways.


I totally agree, I'm already long convinced BMS-less is a perfect option for a DIY car. And way better than a unreliable BMS.

Rereading my post I think that can be taken sarcasticly, but that's not what I meant. People with no problems do not share this on a regular basis. Because that's just the way it is. And that's a pity. A provokative 'You must use a BMS" post like placed her is needed so once in a while to get this info on the forum.


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## MalcolmB (Jun 10, 2008)

Siwastaja said:


> You can make it a lot easier by deciding to give up a part of your range; don't charge full and especially don't discharge fully, and you may get out of the problems. But from this viewpoint, you can as well buy smaller/fewer cells and get a BMS for the money.


I think this is a very good point. EV manufacturers install a BMS for a very good reason, it makes financial sense to squeeze the maximum range from the smallest battery pack.

I'm in the middle of bench testing a relatively small LiFe pack for my mini, and watching the cell voltages diverge rapidly once they get over 3.4V. I can't afford a good BMS at the moment so I'm just using cell-logs to monitor cell voltages and cut the charger at cell Vmax. When I charge to an average voltage of just over 3.4V the cell voltages don't diverge too fast, so this is how I plan to run until I get a BMS. The downside is that, according to the cell manufacturer's specifications and my own amp hour measurements, I'm undercharging by about 15% by doing so. Since my pack is small to start with this is a considerable loss. The way it looks to me, you pay your money and take your choice.


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## jeremyjs (Sep 22, 2010)

Jan said:


> I totally agree, I'm already long convinced BMS-less is a perfect option for a DIY car. And way better than a unreliable BMS.
> 
> Rereading my post I think that can be taken sarcasticly, but that's not what I meant. People with no problems do not share this on a regular basis. Because that's just the way it is. And that's a pity. A provokative 'You must use a BMS" post like placed her is needed so once in a while to get this info on the forum.



Sorry misread your original post.


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## jeremyjs (Sep 22, 2010)

MalcolmB said:


> I think this is a very good point. EV manufacturers install a BMS for a very good reason, it makes financial sense to squeeze the maximum range from the smallest battery pack.
> 
> I'm in the middle of bench testing a relatively small LiFe pack for my mini, and watching the cell voltages diverge rapidly once they get over 3.4V. I can't afford a good BMS at the moment so I'm just using cell-logs to monitor cell voltages and cut the charger at cell Vmax. When I charge to an average voltage of just over 3.4V the cell voltages don't diverge too fast, so this is how I plan to run until I get a BMS. The downside is that, according to the cell manufacturer's specifications and my own amp hour measurements, I'm undercharging by about 15% by doing so. Since my pack is small to start with this is a considerable loss. The way it looks to me, you pay your money and take your choice.


Looking at it a different way, you're not really giving up anything. Your pack is no more capable than your smallest cell regardless of whether or not you use a cell level BMS. So so long as the pack is balanced at the bottom you should get just as much range and greatly reduce the risk of damaging batteries if you ever over discharge them accidentally. There's no better way to quickly kill cells than cell reversal and there's no other way to tell if you're near the bottom of your pack other than an amp hour or Kw/h counter gauge. Under load, voltage is almost meaningless.


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## Siwastaja (Aug 1, 2012)

I think we need more data. As honest data as possible. We need to share both successes and failures. When I went through old discussions, Jack Rickard showed a strong attitude and some wrong conclusions, but he also showed honest data. When he had cells die in his non-BMS system, he shared it. IIRC, his conclusion was something like that it was not a problem of a "BMS-less apporach" but a user failure, discharging the battery too empty. This is exactly correct and I mean exactly this when I say "human-BMS".

Of course cells do not drift suddenly and quickly, as said above by DIYguy. There is no such mechanism. (Or if they do, they are bad anyway.) However, there are slow mechanisms. For example, a small difference in resistance causes variable hotspots during discharge, and those with higher resistance end up increasing the resistance even more and lose capacity. But if the starting condition is good (small differences), this process takes a long time. However, it should speed up when time goes by as this is a "positive feedback". It's an unstable system but very slowly so.

Actually, a simple passive BMS doesn't quite solve this problem, it just saves you from increased need of manual balancing _when this starts to happen_, and prevents _you_ from accidentally _doing extra damage when this starts to happen_. To really solve the drifting problem, you need an active (charge transferring) BMS which not only sees that some cells are sagging more during a drive, but supply current from other cells to them to ease their load. This also leads to a system where the full capacity of the pack can be used; when the first cell gets empty, it gets charged from the rest of the pack, instead of just shutting down / limiting the drive. This is what we are going to build and have some prototyping already going on. This process should cut the spiral of worst cells becoming even worse all the time. 

I guess a somewhat carefully maintained human BMS works well for maybe 3-5 years. A passive BMS or a _very_ carefully maintained human BMS can then give a few years more. An active BMS will make any real difference only after that point. These numbers are just a hunch. We just can't know; this is a relatively new chemistry and new field of study.

But one thing is sure: a _properly designed_ BMS will never cause damage and will _always _be better than no BMS or human BMS. (Furthermore, I claim that a BMS system is quite easy to design properly.) If you have any experience of the opposite, *please provide the details* so that others can avoid the mistake. I've tried to find details but find only blanket statements, usually referring to other people without giving references. It seems most are happy with the road they have chosen, expect for small number of people who either cooked their cells by no-BMS or a similarly small number of people who had faulty BMS to cook their cells.

I believe that _if_ there are any differences, we will be seeing then after five years or so! I just can't believe in no-bms even if it seems to work _for now_, because it does not work in _theory_ too well, and because the packs are expensive, so I wouldn't want to take the risk. Still, I feel very happy when I hear that even a no-bms system has worked perfectly for years.

"Mandatory" is a funny thing. When something is considered mandatory, it opens an endless business opportunity for all kinds of people. When you must buy something (required by law, or as in this case, required by the "community" and experts), you can sell everyone anything with that title. This is a bit sad, because I believe that the experts are right. Of course a bad BMS can be even worse than no BMS. Much worse. There is no limit. It may not perform the minimum tasks to be a BMS. It may just have the title. In addition, it can overheat and catch fire. As can any electrical device if designed by a complete moron or by a person with bad intentions.

This is why I stated the absolute minimum requirements earlier. But in addition to implementing those in theory, the modules shouldn't take too much idle current, nor should they overheat when shunting. And they should come with clear instructions / user manual how to connect them with the charger and motor controller.

These "no bms" experiences are actually very valuable even for us who are absolutely pro-BMS. The more "mishandling" LiFePO4 handles, the more expectations I can have when handled properly. It seems to be a VERY robust chemistry as a lithium chemistry.

We should all agree that we want our very expensive packs to last _as long as possible_ and get _every bit_ of juice and lifetime out of them. Even if it is 5% more range and 9 years of use instead of 8, it will be worth it. Today, electronics is very integrated, simple and cheap.

We should also all agree that electronics and computers are good in doing things such as measuring something all the time, switching things and controlling where electricity flows, without losing their attention even for a second.

If there are problematic BMS systems, let's warn others about them. If there are good experiences with good BMS systems, let's recommend those to others.


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## Siwastaja (Aug 1, 2012)

jeremyjs said:


> Looking at it a different way, you're not really giving up anything. Your pack is no more capable than your smallest cell regardless of whether or not you use a cell level BMS.


Actually, this is not completely true even though in most cases it is. However, an active BMS is able to transfer energy from higher capacity cells to the lower capacity cells on the fly. Active BMS systems are still quite rare among hobbyist conversions, but due to their only slightly increased complexity (contrary to what is usually claimed) compared to passive, I believe they will become more common. I'm working on one design and probably will be open-sourcing it.

A properly designed active BMS should make the battery look almost like one big cell. Of course there is some energy loss when transferring the energy (currently we have around 80% efficiency), but it's so much better than losing that energy completely. And when the battery is balanced ALL THE TIME, there's no need to talk about top, middle or bottom balancing anymore.

Edit: ah, you said "cell level BMS". Active clearly isn't, it is both battery and cell level, so you're right.


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## DIYguy (Sep 18, 2008)

REad this thread it went on for a while as I recall.
http://www.diyelectriccar.com/forums/showthread.php/bms-not-bms-53768.html


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## Ziggythewiz (May 16, 2010)

MalcolmB said:


> The downside is that, according to the cell manufacturer's specifications and my own amp hour measurements, I'm undercharging by about 15% by doing so. Since my pack is small to start with this is a considerable loss. The way it looks to me, you pay your money and take your choice.


Except that by undercharging and underdischarging your cells will live much longer as well. Does 50% improved cycle life make up for 15% reduced usable capacity?



Siwastaja said:


> Even if it is 5% more range and 9 years of use instead of 8, it will be worth it.


That depends on how big your cells are. I could buy a pack to fit my needs for $4k. If a BMS could guarantee 5% more lifetime range, which it can't, it would be worth about $200. Can I get a good BMS for $200? I don't think so.


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## jeremyjs (Sep 22, 2010)

Ziggythewiz said:


> Except that by undercharging and underdischarging your cells will live much longer as well. Does 50% improved cycle life make up for 15% reduced usable capacity?
> 
> 
> 
> That depends on how big your cells are. I could buy a pack to fit my needs for $4k. If a BMS could guarantee 5% more lifetime range, which it can't, it would be worth about $200. Can I get a good BMS for $200? I don't think so.


It also depends on how long the lifepo4 batteries actually last. 10 years should be easy and much longer seems likely barring defects or user error/BMS killing cells. Last I read some of the first lifepo4 cells commercially produced are still going strong in delivery trucks. I wish I could find the article. It's a few years old.


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## octagondd (Jan 27, 2010)

Siwastaja,

What do you think of Jack's bottom balancing, undercharging and checking the cells once every 6 months or year? I realize that checking cells is time consuming, but 2 hours every year for 10 years is not the same as buying a $1000 or more BMS. His solution is simple. If you have planned for a bms in your budget but also might have space for a few more cells, buy the cells and not the bms. You get a higher voltage, less amp draw, and better cushion at the ends of the charge/discharge curve for your anticipated mileage.

I have a 22 mile commute and was originally planning a 128 Volt 160AH system, which would get me about 35 miles at 70%DOD. I decided to buy 5 more cells instead of a $1000 BMS. Now I am getting ready to install a 144V system which has the benefits described above and can go 40 miles. I have no experience with cycling the cells yet, but everything I have read from the people who have used the cells in exactly this manner have not had problems yet. I know people say they may not have problems until they get further along in the life of the cells. We will see how it turns out. I believe cycle life will be very good, even without a BMS.


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## EVfun (Mar 14, 2010)

Siwastaja said:


> But one thing is sure: a properly designed BMS will never cause damage and will always be better than no BMS or human BMS. (Furthermore, I claim that a BMS system is quite easy to design properly.)


It doesn't seem so easy to get right, considering there have been a number of failed modules and failed central boards. The top ones seem to be:

Cell reversal failures of the BMS. Will it report the problem after it happens? That pack could get ugly if it is charged again without removing that cell. Could anything result in the shunt getting stuck on, insuring a cell reversal down the road? If a module was touched to the cell backwards on installation will this lead to module failure at that time or in the future? Would a unit failure be obvious to the owner before it destroys a cell?

Inability to handle the kind of electrical noise that can happen on the BMS monitoring loop. What happens if pack voltage touches a terminal on the loop? What happens if there is a pack short to frame eliminating the isolation between the monitoring loop and the traction pack? What about the transients spikes that can AC system when charging. Plenty of EVs charge with non-isolated chargers (Manzanita Micro PFC series chargers, Russco chargers...)

That said, I bought 42 cells from one batch. I ran 32 of them in my Buggy for a year and then added 8 more of those cells to the pack. Based on my most recent discharge test the 32 cells have stayed right where they where before. The 8 cells I added seem to slowly climb in SOC until I have to bleed some power off of them. I'm not done yet so I can't say how much in 5 months. Likely less than 1 amp hour but enough that when charging without a BMS a couple of those cells where getting very close to 4.0 volts while the rest where around 3.5 volts.


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## EVfun (Mar 14, 2010)

Nice sales pitch. Isn't there a folder for that?


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## Siwastaja (Aug 1, 2012)

Guys,

the point here is that you are playing with an idea previously unheard of, and discouraged by every single battery expert for simple, technical reasons and theory.

The only argument for non-BMS is that it seems to work quite well in practice. I agree this is a very good argument, but...

The problem in this is that we don't have practical data of running no-BMS for more than just a few years. This is still a small fraction of the expected lifetime of LiFePO4 cells.

Has anyone run more than 1000 cycles with no BMS?

When we have several people running their no-BMS systems for 8 years and 3000-4000 cycles of 70% DOD average, and still have 70-80% of the original range, it is proved. Until then it's a wild theory and completely unproven.

This battery chemistry is specified to more than 2000-3000 cycles, or even 5000 if we accept some drop in capacity. If the capacity would halve due to wrong use, you would see this around 1000-2000 cycle point. And *if* a BMS could double your pack life, it would be very worth the money.

Don't pretend you know because you don't. No one knows. There just is no adequate testing done. So it's all theory. 

Theory of BMS giving longer range and longer cell life is at least logical and very believable for any who understands how the cells work. The only question is, will the cells be so high quality in the long run that this does not matter? We HAVE cell measurements and they can usually show around 5% variation in internal resistance and capacity, so the starting conditions for slow drift DO exist. It's only about how quick this drift is.

Some technical details from implementation point of view were discussed;



> It doesn't seem so easy to get right, considering there have been a number of failed modules and failed central boards


First, it starts to get funny to hear about "number of failed modules" when seemingly no one can give any reference even to just one such case! (I hope this provocation helps getting one.)

The fact(?) that there are faulty units, is not a sign it wouldn't be easy to get right. It only shows that there are people who are ready to take your money, using the "obligatory BMS" attitude for their own good. If this is really true, I think I understand _very_ well why so many are going no-BMS way. But this is the third time I ask for this: PLEASE provide the references (at least names) to the faulty units!

A BMS should not be a toy. It should be a SUPER reliable safety unit. It should be as simply constructed as possible, as cheap as possible, but at the same time, as reliable as possible. But good thing in electronics is - it can be copied with little recurring costs. The problem here is that there are dozens of quickly designed BMS units which probably only sell a few thousand units each, so all of them have quite a bit of non-recurring costs on their price tags.



> Cell reversal failures of the BMS. Will it report the problem after it happens?


A _minimum_ expectation for a BMS is an instant cell level LVC. Therefore, it reports the problem much _before_ any cell reversal failure can happen, preventing it. If a cell reverses, the BMS has failed miserably (or the cell was very bad to begin with). Of course, a good BMS would report this failure, too.



> Could anything result in the shunt getting stuck on, insuring a cell reversal down the road?


Do your motor controller get stuck on so that you drive full speed and cannot stop? Do electrical devices tend to get stuck on anyway? If it happens, it's a _terrible_ design mistake and easy to avoid by having more than just air in the designer's head.



> If a module was touched to the cell backwards on installation will this lead to module failure at that time or in the future?


The module can be protected from reversal voltage. If it isn't, it is practically 100% sure it will be damaged instantly. Of course, it should include some kind of status display, either via a central unit or at least a LED.



> Would a unit failure be obvious to the owner before it destroys a cell?


There are two approaches to this.

_Passive security_ tries to make the modules as safe as possible so this won't happen in any situation. For a product as simple as BMS, it should be fairly easy. But I admit that people make mistakes. They can just forget some important protection. This is exactly why we need data on the experience of the users. Now it seems everyone does their own little BMS module to sell, with too little testing and too short time-to-market.

_Active security_ adds self-diagnostics to detect failure.



> Inability to handle the kind of electrical noise that can happen on the BMS monitoring loop.


IMO; Use only distributed BMS. It's not only easier to install, but also not so prone to noise problems. A small electronic module at the cell is actually not too noisy environment. It should be fairly easy to design a reliable unit.



> What happens if pack voltage touches a terminal on the loop?


You should get smoke out of the BMS module. No, it's not going to break down in a way that would show years later.

And I would add one point of concern more:
How much current does the BMS draw? How long it takes to drain the cell? Is the current constant so that the drain is balanced?

IMO, this should be designed to have current so low that it doesn't matter. And it's so easy to do it.

We are probably going to use an Atmel ATTiny25 MCU powering on the cell voltage. Other components (voltage dividers, comparator for active rectification, etc.) added, the total idle current draw will be around 0.5 mA. For a worst case of 40 Ah cell sitting at 50% SoC, it would take 16000 hours = almost two years to drop to 30% SoC, which would still be ok. So this design has a HUGE leeway and NO special effort done to minimize the current draw. If needed, we can drop the idle current to around 0.1 mA easily.

If there is a BMS unit that eats more than 1-2 mA when the car is sitting down not charging nor driving, please provide a reference to such a system.

As you can see, I don't have experience on real commercial BMS modules. OTOH, what I can read here, people who have bought and used those, seem to be happy. People who do not buy them seem to be unhappy. Go figure!

Still, if the rumors of cell-killing BMS units have anything to them, it's horrible and should be elaborated on, as it is a much more important issue than debating around "to BMS or not to BMS".

As an end remark, I can imagine how funny this forum could be for battery experts or battery manufacturers to read -- their basic knowledge and science turned completely upside down by hobbyists playing with their products!


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## GizmoEV (Nov 28, 2009)

MalcolmB said:


> When I charge to an average voltage of just over 3.4V the cell voltages don't diverge too fast, so this is how I plan to run until I get a BMS. The downside is that, according to the cell manufacturer's specifications and my own amp hour measurements, I'm undercharging by about 15% by doing so.


My data and yours don't agree then. The most I got into a TS-LFP100AHA cell between 3.45-3.65V was 1.0056Ah. A lot depends on your ending current. Let the current taper to well below 0.05C and you will get your cells easily 99% full any way.

I top balanced my pack and charge to 69.1V or 3.455vpc. I used to charge to 69.3V or 3.465vpc but I found that this summer the resting voltage was just a tad over 3.400V after several hours so I lowered my ending voltage. My Zivan NG1 doesn't know how to terminate when the current drops below a certain point so it just tapers back to nearly zero before timing out.

FWIW, I used to run a Black Sheep Technology BMS setup and it only draws 1mA of standby current for the individual boards. I saw that there was little if any balancing going on so I have been running with only a Battery-Bridge circuit and my Ah counter. You definitely need an Ah counter. Don't drive blind. A CycleAnalyst is relatively inexpensive for all of the features it has.


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## GizmoEV (Nov 28, 2009)

Siwastaja said:


> Similarly, when you are driving, you must stop BEFORE you have any signs of heavy voltage sag. Without simultaneous monitoring of all cells during accelerations, it is impossible to know when your pack is near empty;...


You are implying that without a BMS the user doesn't have an Ah counter. Why? I don't have a cell level BMS installed at the moment and I can easily tell where what my pack SOC is all the time. In my case it is a CycleAnalyst.


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## batterytang (Nov 17, 2011)

The first time I heard of Non-BMS usage comes from Jack Rickert, he is influence so I bring this topic to forum looking for a right answer. 
P.S. I am very dislike lowcarbon-idea advertizing his website on this thread...


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## Siwastaja (Aug 1, 2012)

GizmoEV said:


> You are implying that without a BMS the user doesn't have an Ah counter. Why? I don't have a cell level BMS installed at the moment and I can easily tell where what my pack SOC is all the time. In my case it is a CycleAnalyst.


You are correct; I missed the point. However, I think that it should be a responsibility of those who suggest going no-BMS to mention the precise Ah counter, as it is _very_ important in that case.

You see, the problem here is that people tend to provocate and get provocated on both sides. Instead, we need to give all information that is needed. For BMS, it is admitting that there may be faulty BMS units out, and giving out the details. For non-BMS, it is listing what you need instead of a BMS (most importantly an Ah counter) and how you are going to be the human BMS.

To summarize:

No BMS:
- Get a precise Ah meter!
- Never run the battery more flat than say, 80% DOD (80 Ah on 100 Ah cells, for example)
- When you get near to that point, avoid strong accelerations etc.
- Set the motor controller LVC considerably higher than just number of cells * mimimun acceptable cell voltage (e.g. 2.50V)
- Set the charger voltage considerably lower than just number of cells * maximum acceptable cell voltage (e.g. 3.65V)
- Check random cell voltages towards the end of charging every now and then. Take action if some of them are too high.
- Check random cell voltages towards the end of discharging every now and then. Take action if some of them are lower than others, even if they were over the acceptable minimum. (The problem here is that this is hard to do under load.)

Anything else?


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## Duncan (Dec 8, 2008)

Siwastaja said:


> You are correct; I missed the point. However, I think that it should be a responsibility of those who suggest going no-BMS to mention the precise Ah counter, as it is _very_ important in that case.
> 
> You see, the problem here is that people tend to provocate and get provocated on both sides. Instead, we need to give all information that is needed. For BMS, it is admitting that there may be faulty BMS units out, and giving out the details. For non-BMS, it is listing what you need instead of a BMS (most importantly an Ah counter) and how you are going to be the human BMS.
> 
> ...


Take advantage of 20% extra range and power caused by swapping BMS for more cells

Much less chance of failure because of massively reduced connection number

Simple easy to understand system requiring annual check-up


_Get a precise Ah meter!_
You need one of these anyway - otherwise you don't know when you are going to run out of power
If you wait for the BMS to tell you you *will *get stuck somewhere



_Set the motor controller LVC considerably higher than just number of cells * mimimun acceptable cell voltage (e.g. 2.50V)_

Not a bad idea if you want to maximize your battery life - lose 1 or 2 % in capacity gain increased cell life


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## MalcolmB (Jun 10, 2008)

GizmoEV said:


> My data and yours don't agree then. The most I got into a TS-LFP100AHA cell between 3.45-3.65V was 1.0056Ah. A lot depends on your ending current. Let the current taper to well below 0.05C and you will get your cells easily 99% full any way.


I should have mentioned that I'm using PSI cells (charge curve on this page: http://mvp090-1.104web.com.tw/cetacean/front/bin/ptdetail.phtml?Part=e40138f1). 15% may be exaggerating, sorry. It's probably more like 10%. I'm getting 8.1 Ah out of these 9 Ah (nominal 10Ah) cells, after taper charging to 3.44V and discharging to 2.70V at C/2. I was a bit surprised myself at the low SOC, as I wouldn't have expected much difference in charging behaviour between these cells and Thundersky. I'll try to repeat the capacity test after charging to 3.65V at C/20.

Maybe I gave the impression that I'm firmly in favour of a BMS. That's not the case. I like to keep things as simple as possible and strip away anything that's unnecessary. Unfortunately for me I also agree with Siwastaja's description of the way cells should gradually drift with time. It would be great to be proved wrong.


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## WarpedOne (Jun 26, 2009)

On a sidenote: 
I still feel and believe that charger and BMS should be one singe integrated unit in a form of a parallel charger(s). You get 'automatic' top balancing without dedicated shunts etc. Yes, it is a wiremess but nothing worse than a wiremess of a good BMS (i.e. fully functional one).


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## Siwastaja (Aug 1, 2012)

WarpedOne said:


> On a sidenote:
> I still feel and believe that charger and BMS should be one singe integrated unit in a form of a parallel charger(s). You get 'automatic' top balancing without dedicated shunts etc. Yes, it is a wiremess but nothing worse than a wiremess of a good BMS (i.e. fully functional one).


This is a simple solution, but it is very expensive and heavy due to the need of high number of relatively high-power power supplies. It also has low efficiency due to low voltage output (rectification losses).

For example, a 100-cell and 10 kW charger system would require 100 x 100W chargers. A 100W 3.65V charger, however, has considerable weight, price and efficiency below 80%.

It is much cheaper and less heavy to have one 10 kW charger at 90% eff and to provide most of the charge to the battery, and 100 say 5W "chargers" at 80% eff to provide cell-level balancing. This does exactly the same as your proposition given that the cells are similar _enough_.

The real catch is here: these small "chargers" can be _very easily_ made *bidirectional*; then they can work any time, also while driving, and this becomes an active BMS.

The same idea can be stretched to discharging, too. You _could_ have a "small" transformer or boost converter for every cell instead of series connection. For a 100-cell system, 100 pcs of 300W transformers to provide 30 kW for motor. Then, you could use any type cells, even different capacities and there would never be any balance problems. This would be an "ultimate" solution. However, it is practically impossible due to weight and price. But if we can say that all our cells will be within, say, 10% from each other, we can use typical series connection to provide 90% power and have transformers 10x smaller for every cell to provide active redistribution. This is the core idea of an active BMS. With correct design choices, it can also be made relatively simple, small, efficient and reliable, and not too expensive. At least it would be better use for money than some quickly constructed, half-baked BMS that fails to do its task.

Our minimum setup is currently two FETs and a small ferrite flyback transformer weighing 7 grams, having 5 and 20 turns of wire in primary and secondary, able to transfer maximum of 5-10 watts of power in both directions with efficiency of around 80%.

I think I understand the BMS issue now; either use a really good BMS that has some real benefits, or do not use one at all. No idea of wasting money in something in-between.


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## WarpedOne (Jun 26, 2009)

> For example, a 100-cell and 10 kW charger system would require 100 x 100W chargers. A 100W 3.65V charger, however, has considerable weight, price and efficiency below 80%.


Yes. 10kW 100way parallel charger looks like a no-go.
But typical voltage of today conversions is not 360V but closer to 120V, reducing those 100 units to say 36. Also, 10kW onboard charger is of little use because of limited power of "convenience" charging points, here in Europe 3kW is max what i can wish for reducing a bit the 'required' power from 100W down to around 70W per unit. You can always install a bigger charger at home where you have a stronger electrical connection.

40/80W (40way 3.2kW) charger/BMS unit is something I would buy.


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## Ziggythewiz (May 16, 2010)

Siwastaja said:


> Guys,
> The only argument for non-BMS is that it seems to work quite well in practice. I agree this is a very good argument, but...
> 
> The problem in this is that we don't have practical data of running no-BMS for more than just a few years. This is still a small fraction of the expected lifetime of LiFePO4 cells.
> ...


Other arguments for non-BMS is that BMS CAN BURN CARS!
And cheaper and simpler.

How many years of BMS experience do you have with LiFePO4? 
The problem in this is that we don't have practical data of running BMS for more than just a few years, if that.


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## DIYguy (Sep 18, 2008)

Siwastaja said:


> Guys,
> 
> the point here is that you are playing with an idea previously unheard of, and discouraged by every single battery expert for simple, technical reasons and theory.
> 
> ...


First let me say, that I like your earlier list of things for non BMS users.

Secondly, I would suggest you read the thread I pointed you to. . . including watching the video from Jay Whitacre. Regarding experts opinions I will cut and paste a question I posed to one of the worlds leading professors on this subject. I will also post his answer.

Question;
I sent the following email to Jay Whitacre. This is the fellow in the video. Jay Whitacre, a Carnegie Mellon professor with a joint appointment in Materials Science and Engineering and Public Policy, gives a quick overview of Lithium-ion batteries. Starting at the chemical level, he explains the properties and mechanics of the battery which give rise to macroscopic behavior, especially focusing on issues relevant to electric vehicles and other high-power systems.

Here was my email.... 

Hello Jay.
> 
> I watched with interest your video on Lithium batteries. It was very informative. It is also very much needed information in the DIY community. (this would be for a series string of batteries in an motive application) There are intense, sometimes heated, discussions regarding many aspects of the current Prismatics in particular. One significant topic is the need for a BMS (management, not monitor). Some say it is a must. Others say, it is not cost justified and if you know how to set up your battery, including charging and use... you may not need one.
> One of the finer points of this discussion is relative to "drift". The BMS proponents say that a well balanced pack of cells with close IR (matched... as close as reasonably possible - I could give numbers) and Ah capacity does "drift" as a result of charge/discharge cycles. That is to say, the cells get further apart with respect to SOC. . and that active cell level balancing is a requirement to prevent usable capacity from shrinking over time and eventually causing catastrophic failure.
> The non-BMS believers, including some real world users..... just aren't seeing this drift in actual practice. They find that if they bottom balance and stay away from top SOC (stop charging at the knee) and not push the bottom on discharge... the cells are fine. No drift. thousands of miles so far, no issues. The BMS guys say... you'll be sorry. The wall will come crumbling down.
> 
> I'm trying to sort my way through it. Can you offer any wisdom in this regard? It would be soooo helpful to this community, which is really trying to do their small part in this thing.

Answer;
Hey Gary,

This is pretty easy to answer: well balanced (especially capacity wise) Li-ion strings that are strung up when at the exact same SoC should never drift with respect each other as long as they see the same thermal environment. I'd testify to this in court: there is no mechanism to cause such drift.

I worked at JPL in the group that put the Li-ion batteries on the 2 mars rovers that are still working. No cell level BMS. The Chevy volt, Nissan leaf, and Prius PHEV do not have cell level BMS.

you only need BMS when you have poorly matched cells and you plan on using more than some critical amount of SoC swing (say 80% or so - depends on chemistry and degree of mismatch).


If I were a hobbyist, however, I would monitor cell level voltages to identify failures point and to be safe (to avoid over charging cells in a string where one cell has failed to short or has lost much capacity).


Cheers,

-Jay


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## DIYguy (Sep 18, 2008)

Siwastaja said:


> As an end remark, I can imagine how funny this forum could be for battery experts or battery manufacturers to read -- their basic knowledge and science turned completely upside down by hobbyists playing with their products!


lol, you must mean like the same guys like from TS who's specs first said to charge to 4.2 volts later revised to 4.0 volts later revised to 3.8 volts.. . ?? and the same guys who spec their discharge to be "less than 5% month"...


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## Jan (Oct 5, 2009)

DIYguy said:


> lol, you must mean like the same guys like from TS who's specs first said to charge to 4.2 volts later revised to 4.0 volts later revised to 3.8 volts.. . ??


I'm a bit under the impression that this shifting opinion is based upon market research. Well, not really scientific research, but just trying to find a position in this small market between the competitors. 

Because this has some influance on two important parameters: Wh/kg versus cycle life. I think that the exact same prismatic cell is sold by two different suppliers, maybe even different factories. One sells them as 180Ah with an impressive cycle life, and the other with a shorter cycle life but as a 200Ah cell, and therfore with a higher Wh/kg. There's some freedom to play with.


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## jeremyjs (Sep 22, 2010)

DIYguy said:


> lol, you must mean like the same guys like from TS who's specs first said to charge to 4.2 volts later revised to 4.0 volts later revised to 3.8 volts.. . ?? and the same guys who spec their discharge to be "less than 5% month"...


4.2 is a perfectly reasonable charging voltage for a single cell run with the proper cc/cv charging curve. It shouldn't harm the cell other than reducing it's cycle life due to using a larger portion of it's capacity. That said there's no reason to go that high as the gains in capacity are minimal and the benefits of using less of the capacity improve cycle life. On top of that when you put them into a string you never want any single cell to ever go over 4.2 volts as it can damage the cell when charging them with the specified cc/cv charging curve so you under charge them, loose a few percent capacity and keep your pack safe. Although this 4.2 maximum ending voltage is almost meaningless if you're not using the recommended charge curve. 

For example if you're charging at extremely high rates. The voltage can actually be well above this and the battery isn't full and you're not damaging it from over charging. Heat may be an issue at this point though.

Another example is if you're amp rate at the end of charge is much lower than recommended. You could potentially over charge the cells at anything over 3.4v over a long enough period.

The meaning of the voltage of a cell is conditional on what rate you're putting power in or taking out. This is also why a LVC is almost useless under driving conditions with the lithium iron phosphate cell chemistry. You could easily sag below your cutoff voltage under load and still have plenty of battery left.


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## DIYguy (Sep 18, 2008)

jeremyjs said:


> 4.2 is a perfectly reasonable charging voltage for a single cell run with the proper cc/cv charging curve.


Technically yes. Practically, no. Do you charge to this voltage with or without a BMS?
Tell me more about your pack and your car. . . I'm interested to know your experiences.

What are your thoughts on Jay Whitacre's comments and the video?


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## DIYguy (Sep 18, 2008)

Jan said:


> I'm a bit under the impression that this shifting opinion is based upon market research. Well, not really scientific research, but just trying to find a position in this small market between the competitors.
> 
> Because this has some influance on two important parameters: Wh/kg versus cycle life. I think that the exact same prismatic cell is sold by two different suppliers, maybe even different factories. One sells them as 180Ah with an impressive cycle life, and the other with a shorter cycle life but as a 200Ah cell, and therfore with a higher Wh/kg. There's some freedom to play with.


Maybe. Maybe not. We all know there is next to no capacity held within these voltage ranges. It's more a protection measure in my little brain. Either from warranty/complaints. . . or feedback.. Jack figures they are watching his EVTV. lol


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## DIYguy (Sep 18, 2008)

jeremyjs said:


> Another example is if you're amp rate at the end of charge is much lower than recommended. You could potentially over charge the cells at anything over 3.4v over a long enough period.


Baloney. Ur typing yourself smart. U will never damage lifepo4 charging at 3.4 volts in cv .05C rate... or typical end of charge current levels.

Listen to the professor's video. He discussed charge voltage levels and damage as I recall.


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## jeremyjs (Sep 22, 2010)

DIYguy said:


> Baloney. Ur typing yourself smart. U will never damage lifepo4 charging at 3.4 volts in cv .05C rate... or typical end of charge current levels.
> 
> Listen to the professor's video. He discussed charge voltage levels and damage as I recall.


I think you misread everything I typed. For example this quote is incomplete. I say *OVER* 3.4 volts for long periods of time at low currents; which would be more like a float charge and very bad for lithium iron batteries. My point about 4.2V is as a single cell is they can be charged to that voltage just as described without damage, but there's no point. There's almost nothing that high on the knee and in a pack you'll go a good deal lower, say 3.4-3.5ish, to keep any one of the cells going over that voltage, ever, during charging with the recommended charge curves. 

I don't think we really disagree on much of anything. reread what I typed in my previous posts. My main point was almost everything with these batteries is conditional on current and duration.


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## GizmoEV (Nov 28, 2009)

Quote:
Originally Posted by *jeremyjs*  
_Another example is if you're amp rate at the end of charge is much lower than recommended. You could potentially over charge the cells at anything over 3.4v over a long enough period._





DIYguy said:


> Baloney. Ur typing yourself smart. U will never damage lifepo4 charging at 3.4 volts in cv .05C rate... or typical end of charge current levels.
> 
> Listen to the professor's video. He discussed charge voltage levels and damage as I recall.


You might want to re-read what Jeremy wrote. Note that it is impossible to charge a LiFePO4 to 3.4V at the 0.05C rate indefinitely. The current will drop. The laws of Physics dictate this. If the voltage is held above 3.4V for a long enough period then it will over charge the cell. The higher the voltage is above 3.4V the sooner it will get overcharged. I have noticed that with my charger set to 3.465vpc that my cells end up resting above 3.4V now that they have warmed up with the summer temps. I have dialed back my charger to end at 3.455vpc and now they rest right at 3.4 or just below. Note that my Zivan charger does not terminate charge when the current drops to 0.05C or any other particular value. It just tapers back until the average current is a few mA and it finally times out. The longer the charge takes the longer this very low current time is. I have a 200Ah pack so a few mA of current is quite tiny by comparison.


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## GizmoEV (Nov 28, 2009)

jeremyjs said:


> My main point was almost everything with these batteries is conditional on current and duration.


It appears that many people miss this very point! It is both ending voltage AND current that is important.


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## cpct (May 31, 2012)

drgrieve and me were in a similar discussion some days ago.

I would like to point out that there are some publications available on IEEE Xplore that show negative effects when no cell balancing is present:

http://www.diyelectriccar.com/forums/showpost.php?p=316500&postcount=36


But I would agree with the idea that problems will appear mostly when cells were not really equal when the pack was assembled.


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## GizmoEV (Nov 28, 2009)

Siwastaja said:


> You are correct; I missed the point. However, I think that it should be a responsibility of those who suggest going no-BMS to mention the precise Ah counter, as it is _very_ important in that case.


I think it is assumed that an Ah counter is going to be used but when we have many people just getting into LiFePO4 they may not know this so it is important to point out.



> You see, the problem here is that people tend to provocate and get provocated on both sides. Instead, we need to give all information that is needed. For BMS, it is admitting that there may be faulty BMS units out, and giving out the details. For non-BMS, it is listing what you need instead of a BMS (most importantly an Ah counter) and how you are going to be the human BMS.


Which is why I finally started to do my own tests in my Gizmo. It only has one battery box and the temperature differences between cells is minimal. I went with a top balanced pack because I have no control except slight voltage adjustments on my charger for when it terminates. We need to be concerned with the truth not who's right.



> To summarize:
> 
> No BMS:
> - Get a precise Ah meter! This should always be a given.
> ...


Batteries should be at the same temperature. If not, you should probably monitor more closely.


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## DIYguy (Sep 18, 2008)

jeremyjs said:


> I think you misread everything I typed. For example this quote is incomplete. I say *OVER* 3.4 volts for long periods of time at low currents; which would be more like a float charge and very bad for lithium iron batteries. My point about 4.2V is as a single cell is they can be charged to that voltage just as described without damage, but there's no point. There's almost nothing that high on the knee and in a pack you'll go a good deal lower, say 3.4-3.5ish, to keep any one of the cells going over that voltage, ever, during charging with the recommended charge curves.
> 
> I don't think we really disagree on much of anything. reread what I typed in my previous posts. My main point was almost everything with these batteries is conditional on current and duration.


Of course current and duration are important. But I took what I read "I say *OVER* 3.4 volts for long periods of time at low currents". So, I charge to 3.45 volts and finish in CV as normal curve would dictate. No damage. If u r in CV mode, it can't hold a particular current level. . . it will diminish to nothing at some point. So, u charged to 3.45, or 3.5 or 3.6. What did I miss?

Most of the people who have been on here for a long time are really tired of this debate about BMS, top and bottom balancing etc. They won't even join the discussion any more. That's why I suggested searching for past discussions. If you understand the battery behavior (particularly from experience) you can pick the method that suits u and u know what is important to watch for.
New posts with blanket statements are a bit troubling at times, when people don't always look for what has been hashed out.
You want examples; talk to JRP3 who has had a bottom balanced pack for like 3 years now. I don't even know how many cycles. Bottom balanced and even drained pretty much out a couple times with no issue. You will find lots of support for either camp. I'll just leave u with one cautionary note. If you are planning to use a BMS and have the BMS shut off your charger, you had better design a secondary (redundant) end of charge control. If you u choose not to, I hope you have a lot of faith in your design. . . . + don't park in the garage of your house.


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## jeremyjs (Sep 22, 2010)

DIYguy said:


> Of course current and duration are important. But I took what I read "I say *OVER* 3.4 volts for long periods of time at low currents". So, I charge to 3.45 volts and finish in CV as normal curve would dictate. No damage. If u r in CV mode, it can't hold a particular current level. . . it will diminish to nothing at some point. So, u charged to 3.45, or 3.5 or 3.6. What did I miss?
> 
> Most of the people who have been on here for a long time are really tired of this debate about BMS, top and bottom balancing etc. They won't even join the discussion any more. That's why I suggested searching for past discussions. If you understand the battery behavior (particularly from experience) you can pick the method that suits u and u know what is important to watch for.
> New posts with blanket statements are a bit troubling at times, when people don't always look for what has been hashed out.
> You want examples; talk to JRP3 who has had a bottom balanced pack for like 3 years now. I don't even know how many cycles. Bottom balanced and even drained pretty much out a couple times with no issue. You will find lots of support for either camp. I'll just leave u with one cautionary note. If you are planning to use a BMS and have the BMS shut off your charger, you had better design a secondary (redundant) end of charge control. If you u choose not to, I hope you have a lot of faith in your design. . . . + don't park in the garage of your house.


It will diminish and shut off your charger if it's set to do that as it should, but over 3.4v resting voltage, after the battery is fully charged using the recomended cc/cv charge curve, it can still accept some current. That current will be plating lithium once the battery is fully charged. It may only be a milliamp at a time at those voltages, but it can happen; since the fully charged resting voltage of lifepo4 is in theory 3.4 volts on a 100% fully charged cell. It would take a long time to damage the battery like that, but damage the battery it would. That's why I said over 3.4 V for a long period of time.


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## Dennis (Feb 25, 2008)

Siwastaja said:


> This is a simple solution, but it is very expensive and heavy due to the need of high number of relatively high-power power supplies. It also has low efficiency due to low voltage output (rectification losses).


False, any engineer with a brain would realize that the relatively low voltage output requirement requires synchronous rectification for good efficiency. This is the same technique being used on your computer motherboard to power the microprocessor because of the low voltage requirement of the microprocessor. The voltage drop of such a setup is merely a few millivolts.





Siwastaja said:


> For example, a 100-cell and 10 kW charger system would require 100 x 100W chargers. A 100W 3.65V charger, however, has considerable weight, price and efficiency below 80%.


You must be assuming that the charger is going to use 60/50 HZ transformers. Again, any engineer with a brain would use SMPS topology as is used in your computer power supply to reduce weight. The high frequency transformers can have multiple isolated secondary taps that can service more than one channel output.

In your computer power supply a single transformer will have multiple isolated output taps that are rectified by Schottky diodes and filtered with capacitors and inductors that service the different voltage levels. 

In the modern SMPS computer supplies they even do synchronous rectification of the 5 volt and 3.3 volt rails that feed off a single, high current, 12 volt rail, to meet the new efficiency requirements. You can now buy a platinum plus rated computer power supply that is 90% efficient.


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## dtbaker (Jan 5, 2008)

batterytang said:


> Just heard from other people, saying BMS is a waste of money, only need to use a charger charging battery packs and do balancing, no need for a BMS? Anybody have similar experience?



looks like this sparked the usual religious war... my $.02 is that IF you decide to skip the BMS, this means you subscribe to top-balancing to let the charger do its job and shut down as designed at some specific voltage, and trust that the cells do not drift (which they don't seem to), which means you *should* do a very careful job of top-balancing initially.

Taking all that as part of the BMS-less design, a good initial top balance is crucial. on a power supply at something like 3.9 in parallel until amps are close to 0, THEN checking and tweaking after putting in series to make sure all cells are 'finishing' at very nearly the same target vpc based on your ending pack voltage average.


all that being said..... BMS-less is certainly possible and practical as long as you stay away from the bottom!

a good ah counter is helpful, but not the end all.... eventually (hopefully a long time) the capacity will drop, and you won't ge the same ah out of the pack.... only indication will be that pack voltage starts to droop... so the long term safety indicator when capacity starts droppig is NOTICING whenpack voltage starts to sag earlier under load, and not recover back to nominal at rest (at a stop light for instance). This will be your only indication tha tthe pack capacity is not what it was when new.....


I am a believer in BMS-less by the way, and have had no problems.


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## DIYguy (Sep 18, 2008)

dtbaker said:


> looks like this sparked the usual religious war... my $.02 is that IF you decide to skip the BMS, this means you subscribe to top-balancing to let the charger do its job and shut down as designed at some specific voltage, and trust that the cells do not drift (which they don't seem to), which means you *should* do a very careful job of top-balancing initially.
> 
> Taking all that as part of the BMS-less design, a good initial top balance is crucial. on a power supply at something like 3.9 in parallel until amps are close to 0, THEN checking and tweaking after putting in series to make sure all cells are 'finishing' at very nearly the same target vpc based on your ending pack voltage average.
> 
> ...


Hey Dan, how r u?. . Just wanted to clarify that you can bottom balance and go BMS-less also. As long as you have a decently matched pack wrt capacity, particularly. This is how I manage. I have done top balancing in the past and have no issue with it . . as long as you pay attention to your consumption. I'm lucky enough to have a pack that is so well matched that it works exceptionally well bottom balanced. With Charger set to 3.5, I have no cells reach 3.8 when topped off. Of course CALBs SE's can live with a little lower finish than the TS/Winston.

The nice perk with this method as you mention, is that you can run the pack to empty (not that u want to or plan to) and not damage it. Where a top balanced pack will drive that low capacity cell into reversal and kill it.

Cheers.


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## dtbaker (Jan 5, 2008)

DIYguy said:


> Hey Dan, how r u?. . Just wanted to clarify that you can bottom balance and go BMS-less also.



I would not recommend this.... by definition with a bottom balanced pack, the end-of-charge is unbalanced and *some* cells will be considerable higher than others without a BMS to flag the charge to stop before it sees the programmed pack voltage. you may get lucky and have a pack where the high ones aren't 'too' high, but as a general rule you are running a risk with every charge.


my decision to run top balanced is basically because the DAILY case of end of charge is *safe* without additional BMS, although the unusual case of running too deep is risky. I much prefer this to risking unbalanced daily end of charge and being safe(er) running deep DOD, because I hope to *never* run down to deep DOD.


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## Ziggythewiz (May 16, 2010)

Good points. Everyone knows staying away from the edges will promote long life, and CALB recommends only operating at 10-90% SOC. 

We also know lower DOD means longer cycle life so for me it would make sense to try to keep it between 10-70% DOD. That leaves a much larger cushion on the bottom so where you want the cells tight is on the top.


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## EVfun (Mar 14, 2010)

dtbaker said:


> my decision to run top balanced is basically because the DAILY case of end of charge is *safe* without additional BMS, although the unusual case of running too deep is risky. I much prefer this to risking unbalanced daily end of charge and being safe(er) running deep DOD, because I hope to *never* run down to deep DOD.


I like that simple and clear explanation, it captures why I run top balanced without a BMS.


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## dougingraham (Jul 26, 2011)

jeremyjs said:


> That current will be plating lithium once the battery is fully charged. It may only be a milliamp at a time at those voltages, but it can happen; since the fully charged resting voltage of lifepo4 is in theory 3.4 volts on a 100% fully charged cell. It would take a long time to damage the battery like that, but damage the battery it would. That's why I said over 3.4 V for a long period of time.


And yet A123 (at least for the 26650 cells) specs a float voltage of 3.45 volts. I tested this with four cells initially top balanced and floated for 3 months at 3.45 volts per cell. No measurable change in capacity and no drift. The current tapered off to a few microamps after several days and stayed at that level.


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## Jan (Oct 5, 2009)

DIYguy said:


> Maybe. Maybe not. We all know there is next to no capacity held within these voltage ranges. It's more a protection measure in my little brain. Either from warranty/complaints. . . or feedback.. Jack figures they are watching his EVTV. lol


Yes, you're right, there's little energy in that part. It's only cycle life. 

But seeing evtv there are suppliers who sell 180Ah batteries with the capacity of close to 200Ah. That's the way to increase the cycle life. 80% of 180Ah is a lot less than 80% of 195Ah. Which means more cycles to get to 80%.


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## Jan (Oct 5, 2009)

dtbaker said:


> my decision to run top balanced is basically because the DAILY case of end of charge is *safe* without additional BMS, although the unusual case of running too deep is risky. I much prefer this to risking unbalanced daily end of charge and being safe(er) running deep DOD, because I hope to *never* run down to deep DOD.


Mmmm. That also makes sense. Pitty, because I was convinced by the idea of bottom balancing. Now you've killed the clear and simple choice. I hate that. Does anyone know how to middle balance?


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## ElectriCar (Jun 15, 2008)

Siwastaja said:


> You are correct; I missed the point. However, I think that it should be a responsibility of those who suggest going no-BMS to mention the precise Ah counter, as it is _very_ important in that case.
> 
> You see, the problem here is that people tend to provocate and get provocated on both sides. Instead, we need to give all information that is needed. For BMS, it is admitting that there may be faulty BMS units out, and giving out the details. For non-BMS, it is listing what you need instead of a BMS (most importantly an Ah counter) and how you are going to be the human BMS.
> 
> ...


So at first you come on here telling us that we're not very smart for not using a BMS. Now you're asking us if there is anything else you may not know about how to safely run without one?

There is a lot you don't know evidently about running safely without one. As was pointed out earlier by DIYguy, there is an exhaustive thread on here that many of us have debated the subject ad nauseum with a wealth of information and empirical data that we've come up with. Take the time to read it then come back and talk to us about it. I suggest you take a day or two away from your daily schedule to tackle it as it's quite long and extremely informative.


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## onegreenev (May 18, 2012)

Jan said:


> Mmmm. That also makes sense. Pitty, because I was convinced by the idea of bottom balancing. Now you've killed the clear and simple choice. I hate that. Does anyone know how to middle balance?



Ragged top or Ragged bottom. Both work. Ragged bottom does require you to be a bit more diligent so you don't go too far and ruin a cell. Let someone else drive the car unassisted and that pretty much goes out the window even if you educate them before use. Ragged top really only requires a good charger that shuts off when you tell it to shut off. Elcon comes to mind here. I personally chose to use the bottom balance method for a few reasons. 

1). I don't trust BMS systems. A factory BMS ruined a minimum of 20 cells in my possession. ZERO TRUST. 

2). I had an unbalanced pack of cells in my MG when I put them in and when I drove it to find out how far I could go I ruined two more cells but my VOLT meter said I had PLENTY OF POWER LEFT. Well in all but two cells I did. 

3). If I let anyone else drive my EV I want to know that if THEY drive it a bit too far I won't loose any cells. It's sooooooo easy to do. 

4). I figured that since 99.9% of my driving is below FULL I decided to bottom balance than top. Even with bottom balancing with in seconds all the cells are nearly at the same voltage anyway. That top bit is so small it just goes away nearly instantly. Why bother then with top balancing. 

It's safer to bottom balance. It's not that its not the only way to go but it is a very logical and safe way. 

I choose the bottom. 

No biggie if you don't.

Pete


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## GizmoEV (Nov 28, 2009)

Assuming a bottom balanced pack stays balanced at the bottom regardless of the capacity loss over time of the cells a bottom balanced pack is safer unless the top gets more raged as the capacity drops.

Too many seem to think that they need to charge to 3.65+V to get a full charge. That is obviously not true to any one who has actually played around with LiFePO4 cells. Even Jack R. is now talking about charging to only 3.5V. With my Zivan I can't terminate based on current so I just lowered my ending voltage. Right now I'm charging to 3.455V.

If you bottom balance and have a consistent charger the raged top isn't an issue. Just charge to a lower voltage. In my case I'd only lose 500 feet of driving range.

What ever you do, it is important to know the risks. I'm still running a top balanced pack because it makes it easier to document any cell drift at the end of charge. I fully realize that one day I may have a problem as the capacity decreases but that is why I have a Batt-Bridge circuit installed. I also have a Gizmo which I don't loan to people except for short test drives. Trust me, they will not want to drive it 70+ miles to run it flat.


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## onegreenev (May 18, 2012)

Might even be tough on the ol behind for 10 miles. You have a good point in your case. 

Pete


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## ElectriCar (Jun 15, 2008)

Onegreenev, is that a Volkswagen locomotive engine? 

I've been top balancing since charging the first time after bottom balancing. Probably will bottom balance again one day. Just takes forever to drop the voltage that low! 

I have 50 Calb SE200 with about 3000 miles on them. Today after reading this thread I recorded individual battery voltages after charging and thought I'd post a little info, FWIW. 

Resting pack voltage was 170.2, an average of 3.404V/cell.

There are 12 at 3.420 or more with the max at 3.428V. 

The lowest was 3.55V and there are 3 at 3.56-3.66V. These I'd like to bring up a bit but don't have a charger to do so that I feel good about. I have however used a 6V automotive charger on one that was quite low and it won't approach 6V as my cells will suck up the amps and keep the voltage from approaching 6V!


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## Ziggythewiz (May 16, 2010)

Why would you bottom balance, then top balance, then bottom...? makes no sense!


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## ElectriCar (Jun 15, 2008)

Ziggy what are you talking about??? I bottom balanced initially then when I reached the top I had some cells going high so I top balances. I really don't get anywhere near the bottom but it's not a bad idea in an emergency. And I really doubt it's very practical to do on a regular basis.


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## onegreenev (May 18, 2012)

> Onegreenev, is that a Volkswagen locomotive engine?


Big Fat 11" Kostov. Nice torque and speed at 156 volts. Getting it ready to put into the Red Roadster.


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## jeremyjs (Sep 22, 2010)

dougingraham said:


> And yet A123 (at least for the 26650 cells) specs a float voltage of 3.45 volts. I tested this with four cells initially top balanced and floated for 3 months at 3.45 volts per cell. No measurable change in capacity and no drift. The current tapered off to a few microamps after several days and stayed at that level.


I find that really interesting. Have you tried this on any other lifepo4 cell chemistry or even an a123 pouch cell? I ask; because I'm just wondering if a123 just has a slightly higher voltage; because of some special doping they use? Did the cells rest at 3.4 or lower after the current was taken off?


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## DIYguy (Sep 18, 2008)

jeremyjs said:


> I find that really interesting. Have you tried this on any other lifepo4 cell chemistry or even an a123 pouch cell? I ask; because I'm just wondering if a123 just has a slightly higher voltage; because of some special doping they use? Did the cells rest at 3.4 or lower after the current was taken off?


They use nano phosphate. . . and the nominal is spec'd higher at 3.3 volts I believe. I doubt it's much different for other LiFePo4.


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## bruceme (Dec 10, 2008)

batterytang said:


> Just heard from other people, saying BMS is a waste of money, only need to use a charger charging battery packs and do balancing, no need for a BMS? Anybody have similar experience?


The "M" in BMS is an overloaded acronym that the EV community uses two ways... 

Monitoring is "for fun" and not required

Management means "balancing", and that is required long term.

LiFEPo4's do not rapidly go out of balance. In fact it is so slow, that there are some that balance by hand with a small 3.4v charger. Long term (100's of cycles) you _MUST_ have some system for balancing. 

I do a manual balance on my Prius' 4kwh Enginer pack as it doesn't have a balancer, only a BMS. This was a mistake as my EV conversion BMW, doesn't have a BMS, it only has a top-regulator (miniBMS). 

My BMW is a fraction of the hastle and worry. It is well balanced all the time and after 2+ years and 25000 miles of commuting, my 100Ah TS are still running very strong. I'm sure they aren't "like new", but they are plenty strong still (80-90% after at least 1000 partial cycles). 

The simplest/safest battery insurance is a regulator system like miniBMS

FYI... many EV fires happen during charging because of BMS/regulator failures. So chose wisely, an EV is really just a rolling battery.

-Bruce


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## JRP3 (Mar 7, 2008)

ElectriCar said:


> I bottom balanced initially then when I reached the top I had some cells going high so I top balances.


So you aren't bottom or top balanced at this point, but sort of middle balanced. Should be fine as long as you stay away from the ends, but if you do a deep discharge you are no longer bottom balanced and could potentially harm some cells. I drove my car till it stopped moving once, but being bottom balanced no cells were harmed.

As to the general BMS argument, it all comes down to cost and comfort level. You pay for a BMS because cells are expensive, but an expensive BMS means you get a much smaller pack than you could have had. I can replace a lot of cells if I damage them, (which has not happened), for the price of a good BMS. A BMS also means many more potential failure points in your vehicle. It's purely anecdotal but we've seen more reports of cells lost from bad BMS systems than we have from running no BMS at all.


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## JRP3 (Mar 7, 2008)

dtbaker said:


> I would not recommend this.... by definition with a bottom balanced pack, the end-of-charge is unbalanced and *some* cells will be considerable higher than others without a BMS to flag the charge to stop before it sees the programmed pack voltage. you may get lucky and have a pack where the high ones aren't 'too' high, but as a general rule you are running a risk with every charge.


I've been running that risk for three years now 



> my decision to run top balanced is basically because the DAILY case of end of charge is *safe* without additional BMS, although the unusual case of running too deep is risky. I much prefer this to risking unbalanced daily end of charge and being safe(er) running deep DOD, because I hope to *never* run down to deep DOD.


I hope that as well, but I've done it a few times. Because I was bottom balanced no cells were damaged. However, I do agree that your cells need to be closely matched to use bottom balancing, if you can't reach the end of charge with all your cells close enough then you do need to top balance, or do as EC is doing and split the difference with sort of a middle balance. The best thing you can do is get a closely matched pack, even if that means buying a few extra cells to replace any outliers. For my next pack purchase I'd probably buy 4 more cells than I need, that way I 
have a good chance of building a pack that is very close in capacity, and I can use the 4 cells that might be more out of line to make up a nice 12V Li battery that I can use for something else, or as the 12V battery in the car. I'd also check with the battery supplier and see if they would more closely match the cells for me, even for an extra fee.


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## ElectriCar (Jun 15, 2008)

JRP3 said:


> So you aren't bottom or top balanced at this point, but sort of middle balanced. Should be fine as long as you stay away from the ends, but if you do a deep discharge you are no longer bottom balanced and could potentially harm some cells. I drove my car till it stopped moving once, but being bottom balanced no cells were harmed.
> 
> As to the general BMS argument, it all comes down to cost and comfort level. You pay for a BMS because cells are expensive, but an expensive BMS means you get a much smaller pack than you could have had. I can replace a lot of cells if I damage them, (which has not happened), for the price of a good BMS. A BMS also means many more potential failure points in your vehicle. It's purely anecdotal but we've seen more reports of cells lost from bad BMS systems than we have from running no BMS at all.


My cells weren't as well matched from Calb as I thought they would be. They were all labeled and appeared to match well but when charging them, they didn't stay together. I suppose I could top my charge off at a lower level but then I'm giving up capacity, how much I don't know. Right now I'm charging to about 3.45V or so. 

As the pack gets some age, if I see them diverging, getting out of balance more, I may cut back to a lower level. I really don't think I need to go above 3.4V if I were to get a good bottom balance, but that is very time consuming!


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## jeremyjs (Sep 22, 2010)

JRP3 said:


> I've been running that risk for three years now


You're really only running a risk with your first charge with a bottom balanced pack. Once you've got the CV phase set on your charger to where no cell goes over the recommended voltage it shouldn't be a problem. As far as I'm concerned it works and has been proven to work for at least 3-4 years, by several people, save for having a defective battery or a pack made of cells that have a large deviation in capacity.


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## JRP3 (Mar 7, 2008)

ElectriCar said:


> My cells weren't as well matched from Calb as I thought they would be. They were all labeled and appeared to match well but when charging them, they didn't stay together. I suppose I could top my charge off at a lower level but then I'm giving up capacity, how much I don't know. Right now I'm charging to about 3.45V or so.
> 
> As the pack gets some age, if I see them diverging, getting out of balance more, I may cut back to a lower level. I really don't think I need to go above 3.4V if I were to get a good bottom balance, but that is very time consuming!


I do stop my charging around 3.42V average, which keeps the cells closer together and keeps the pack at a lower SOC.


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## DIYguy (Sep 18, 2008)

ElectriCar said:


> My cells weren't as well matched from Calb as I thought they would be. They were all labeled and appeared to match well but when charging them, they didn't stay together.


I agree with JRP on all counts. How far away were the smaller capacity cells? How far up did the voltage go when finishing charge? . . . before you knocked them down? It is pretty normal to have some higher when u bottom balance . . . I don't like to see them over 4 volts but technically, that shouldn't hurt them.


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## ElectriCar (Jun 15, 2008)

Not sure really it's been over a year ago. If it's not in my upgrade thread I'm clueless. 

I saw some get up to 3.8-4V and had to take a lamp and drain them off. Thing is when I top balance them, over time they get off again. I'd like some info on this but I think it's due to a higher internal resistance consuming some of the charge voltage in those with lower SOC now, either that or they're a higher capacity cell, probably a combo of both. The ones which quickly topped out probably were low in capacity, much lower resistance or both. 

According to "the prefessor" in his video, 4V is a no no and I believe is a never exceed point. I did have one get there but I quickly killed power and bled it off.

But yea, why buy a BMS when you can buy some spares and if properly monitored ala split pack monitoring etc just replace a failed cell when detected. 

And OT but as I understand it, due to internal resistance when a cell reads 0V under any amount of current flow it's not actually at 0V, thus it won't destroy the cell or make it reverse.


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## WarpedOne (Jun 26, 2009)

> But yea, why buy a BMS when you can buy some spares and if properly monitored ala split pack monitoring etc just replace a failed cell when detected.


For exactly the same reason you bought a car in the first place - to reduce the amount of manual work.


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## DIYguy (Sep 18, 2008)

ElectriCar said:


> According to "the prefessor" in his video, 4V is a no no and I believe is a never exceed point. I did have one get there but I quickly killed power and bled it off.


Actually, I think the no-no...not to exceed number is 4.2 for LiFePo4 IIRC.


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## WarpedOne (Jun 26, 2009)

What if I do something like this:
1. carefuly go through initial bottom balancing
2. I install DVM (diferential voltage monitor) that:
a) triggers a blinking red light when both halfpack voltages are further apart than say 0.02V
b) triggers atomatic shutdown when both halfpack voltages are further apart than say 0.05V
c) stops the charging when both halfpack voltages are further apart than say 0.05V or the cell pack reaches 3.5Vpc.

As I see it, this 
- won't demand any special work on my part except of intial bottom balancing
- is almost dead simple to implement and sufficiently simple not to pose any special complexity risks
- is always there monitoring the state of the pack and should cacth any drifting cell before it drifts into the abyss and pulls behind itself the whole pack

When that blinking red light finally shows itself I repeat the bottom balancing ritual.
When automatic system shutdown takes place I find and repleace the weakest cell.
When charging stops because of imbalance I would also probably need to repeat the bottom balancing?

Would such a scheme/system suffice?

What would be appropriate voltage limits in above scheme for 36 series LiFePO4 pack?

Would there be any real advantage to monitor/compare 4quarter-packs except for a bit quicker search of offending cell?


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## ElectriCar (Jun 15, 2008)

DIYguy said:


> Actually, I think the no-no...not to exceed number is 4.2 for LiFePo4 IIRC.


I knew some wiseass would correct me if I was wrong!  lol


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## ElectriCar (Jun 15, 2008)

WarpedOne said:


> What if I do something like this:
> 1. carefuly go through initial bottom balancing
> 2. I install DVM (diferential voltage monitor) that:
> a) triggers a blinking red light when both halfpack voltages are further apart than say 0.02V
> ...


Just regarding item 2, all of them, the pack voltage will vary as you vary the current draw off of it. Not sure how much but those numbers you propose aren't practical. I'm looking for 2V or so out of whack. If a cell turns into a busbar, you've lost 3+ volts. If OTOH you have a connection point failing and increasing in resistance, as your current goes up, the voltage drop across it will also, causing an imbalance to appear on a voltmeter which will disappear as current stops.


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## JRP3 (Mar 7, 2008)

Split pack monitoring is a reasonable solution, I might aim for a 1V or so difference between the pack halves as a warning level. If for any reason you have a 1V or more difference between the halves I'd think you have an issue that you need to trace down.


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## octagondd (Jan 27, 2010)

I was thinking about a 3 position toggle switch and a small digital Volt meter in the fuel door to monitor the pack halves.

Park and turn off the car, note the total pack voltage and AH used on my E-Xpert Pro before shutting the door. Get the charge cord, flip the volt meter switch in the fuel door to see the two halves of the pack. If all looks good, plug in and charge. I realize this is not automated and easy, but this is my vehicle and my solution. Also may use a simple timer on the outlet to set a secondary safety for turning the charger off. If Elcon fails to turn off, then timer expires within 10-15 minutes and shuts off the power.


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## JRP3 (Mar 7, 2008)

Voltage readings in the middle of the SOC range while parked might not show a problem that would appear under load.


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## octagondd (Jan 27, 2010)

JRP3 said:


> Voltage readings in the middle of the SOC range while parked might not show a problem that would appear under load.


Yah, I understand that issue. It seems it is tough to tell anything under load. Some people use Batt Bridge, and I have thought about that as well, just don't like the ambiguity of a sort of lit up led. It would certainly be easier to read under load though then trying to look at two volt meters. I remember someone figured out how to use a center positioned analog volt meter to show like a 2-3 volt swing either way. That might work.


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## Ziggythewiz (May 16, 2010)

I'll be running two half-pack voltmeters on the dash, along with a jld or two that can trigger limp mode or cutoff.


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## GizmoEV (Nov 28, 2009)

octagondd said:


> It would certainly be easier to read under load though then trying to look at two volt meters. I remember someone figured out how to use a center positioned analog volt meter to show like a 2-3 volt swing either way. That might work.


Just use a digital volt meter like I did. It is scaled to read double the value of the voltage to give the true voltage difference. It is simple and cheap to build. http://2003gizmo.blogspot.com/2011/07/battery-pack-balance-monitor.html


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