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Bottom Balancing

85K views 318 replies 51 participants last post by  IamIan 
#1 ·
Hi All,

The Intro
I regularly get asked by customers, potential customers and other forum members about what I recommend for charging of LiFePO4 cells. Rather than repeating myself every time I thought I'd post what I think is right and get some debate going!

I have a ton of research, tests and practical experience in this area. I think now have a methodology for keeping these cells safe down to a tittle.

To summarise the rest of this post. No single cell should touch the top end of the charge curve, ever. No single cell should touch the bottom of the charge curve, but if it does, all other cells should do so at the same time.

The below graph is of a single Sinopoly 60AH (B) cell being discharged at a little over 30A (0.5c) measured capacity was exactly 60AH. This cell had been over-discharged and abused in the past so probably not the best example! They normally come in closer to 70AH.



Note how the voltage spikes at the start and begins to fall out rapidly at the end.

I'm a firm believer that *most* BMS usually do more harm than good, and cost a bloody fortune! The only time I can see any use for them is when you need to squeeze every AH out of the pack. generally, I'd say spend the money you'd of spent on a BMS on more cells.

I think the method Jack Rickard (EVTV) often describes is more or less spot on. I just want to elaborate on this and add some extra thoughts.

Identifying your overall voltage and cell drift.
I would recommend bottom balancing each individual cell (once) to 2.70v and then charging in series until any single cell reaches 3.5v (identifying it with a normal multimeter by running around the pack). The overall voltage when that first cell reaches 3.5v then becomes your charge cutoff voltage for all subsequent charges. You just need to check that that cell (the weakest) every few hundred cycles to see if it is still the first to reach 3.5v (in case another cell becomes your weakest as they age differently). And also re-program your charge cutoff voltage.

All loads on the cells (charge and discharge) should be equal. (The following will be controversial!) Cell drift doesn't exist, except for unequal losses in capacity over time. So adding unequal loads such as a BMS, or voltage sensing (split pack method etc.) means that cells become out of balance, very quickly.

In short, the only wires on the cells should be to connect to the cell next to it and at either end of the pack to connect to the HV components. Nothing else. The whole pack needs to be treated as one.

Why bottom balance?
By bottom balancing you are protected for over-discharge as all cells will reach zero capacity at more or less the same time meaning no single cell will go into reversal (at least not significantly with hundreds of amps passing through it, none of the other cells should have enough charge to allow that to happen). I'm pretty sure everyone will over-discharge their cells at some point, and without a bottom balance you could be looking at dead cells and possibly worse!

I have successfully brought back all cells in a 4 cell bottom balanced pack that went down to less than 0.1v. No noticeable losses in capacity.

By cutting of the charge when the first cell reaches 3.5v (or higher depending on your risk appetite), you ensure that your weakest cells (the lowest in capacity) never get overcharged.

Charge Cutoff
I would recommend having your charger set to cutoff when at the voltage determined above. And then using something like a JLD404 as backup in case the charger fails (voltage sensing etc.).

The JLD404 has some really good features (AH counting, programmable relays etc.) You can use one of the relays to trigger the BMS or EOC (end of charge) control on your charger (if available) at just above (as in 0.5v above) the charge cutoff voltage determined above. This ensures safe shutdown of the charger if something goes wrong.

You could then use a second relay output on the JLD at say 1.0v above the pre-determined voltage to trigger an appropriately sized AC relay/contactor on the chargers input. Just in case the other two systems (above) fail.

Using a latching relay tied in to both of the above relays you can set up some kind of alerting system (beep, LED etc.) the next time you turn the key. This will let you know that the JLD has had to intervene, allowing you to investigate.

Monitoring and cell loading
Finally, you just need to keep monitoring the pack as a whole. If you get in your car and realise the voltage is higher than normal or an extra 5AH has gone in (or not gone in!), something isn't right and needs investigating. The JLD404 can monitor all of this.

These prismatic LiFePO4 cells are really robust and age very slowly when treated right. The characteristics of a single cell will change very slowly as it is cycled and time goes by (2-3,000 cycles to 80% of its original capacity is about right when treated as above).

As previously mentioned. If the cells are all equally loaded (exactly) and treated as one they will not drift apart. They will however age differently depending on things like environmental conditions (one cell next to a heat source will age slightly differently to one that isn't, slight differences in production etc.)

This is why it is worth running around with the multimeter every few hundred cycles just to ensure that your weakest cell (the first to reach 3.5v) is still your weakest and then re-setting your cutoff voltage. If that cell is no longer he weakest you need to find out why and go through the above process again. If anything ever goes wrong (it shouldn't), re-bottom balance and start again.

Hope this helps and let the debate begin!

Cheers,

Mike
 
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#2 ·
There should be no debate on this any longer, for its the only way to do a lithium pack correctly.

I may want to add that after discharging the cells to 2.7v per cell, to wait at least 24 hours, I like to wait 48, because some cells will come back up a bit, and to bring those down to 2.7v till all of them are equal, and wait another 24 to make sure.

It takes time to do this right, spend the time, so later on you wont have to buy more cells, cause you were in too much of a hurry to get your ev up and running.

Roy
 
#3 ·
Your argument follows very nicely from this one premise:

"No single cell should touch the top end of the charge curve, ever. No single cell should touch the bottom of the charge curve, but if it does, all other cells should do so at the same time."

As you presented that premise without substantiation, would you care to tell us very precisely how you derived it?

Thanks
 
#18 ·
Hi Elithion,

I deliberately didn't get *too* techy so not to scare of any newbies!...

As a result of actively testing lifepo4 cells and discussing with others who have done the same (not with huge, expensive prismatics but smaller cylindrical cells). I can make 3 destructive conclusions bout the chemistry:


  1. Overcharging any single cell will significantly reduce capacity or destroy it.
  2. Overdischarging any single cell on its own will reduce its capacity, by how much depends on a few environmental factors, how deeply discharged and point 3
  3. Overdischarging any single cell and then passing current through it (how much depends on the capacity remaining in the cell) will cause it to go into a state of reversal. This is identifiable when a negative voltage can be read across the cell terminals where a positive one would normally be read. the cell will have a significantly reduced capacity and likely be destroyed.
So by having a pack that is not bottom balanced will make point number 3 much more likely to happen.



By bottom balancing, all of the cells are fully discharged at the same time so there is no potential for (much) current to flow and cause reversal.



If a pack is not bottom balanced, there is very likely to be potential in some of the cells that can be drawn through a depleted cell and cause reversal.



For anyone who wants to try this. Buy four small, cheap LiFePO4 cells on ebay, only need to be a couple of AH. Fully charge (3.5v+) three of them and discharge (2.5v) another. Put them into a four series string and load the pack with something 12v (bulb or something). Monitor the voltage of the depleted cell and watch it go negative as the pack depletes.


You can then measure the capacity of each of the cells. The easiest way to do it is to fully charge all for cells individually. Put them back in series and discharge again.


If you can get the reversed cell to hold a charge, you have done well. If it does, measure the voltage of it compared to the other three while it is being discharged lower by any chance?


Obviously if you have something like a powerlab you can test the capacity using that.



Hope this helps with the understanding.


Cheers,


Mike
 
#5 ·
(The following will be controversial!) Cell drift doesn't exist, except for unequal losses in capacity over time.
Please tell me what I have experienced then...

I bought 42 ThunderSky cells in February 2010. Initially, 40 where run in one car for about 6 months. Over the winter I removed them and installed 32 of them in my EV Buggy. I drove it like that for 1 year. Over the next winter I added 6 of the 8 cells back to the Buggy pack to increase the voltage. The pack runs without a BMS or any connections to the pack except at the ends. It is a top balanced pack so I can expect all the cells to reach 3.5 volts at the same time. The 6 cells I added back had quite a habit of creeping up in voltage.

I would start with all the cells charging to 3.50 volts average (range 3.47-4.55, always the same cells low and high.) The 6 cells I added back where in the bottom part of that range (3.47-3.50) initially. After a couple months those 6 cells where all up to 3.7 to 3.9 volts, the rest of the cells slightly lower for the same end of charge voltage. Using a resistor I removed 0.2 amp hours from the 6 cells as a block. In following charges the cells where once again near the bottom of the pack voltage range at the end of a charge. Over a couple months they cells again showed the same increase in end of charge voltage, and where once again knocked down about 0.2 amp hour as a block to lower them back into range. I watched this happen 3 times, over about 3 months each time.
 
#6 ·
In my humble opinion,
I think that would be classed as uneqlossy repayment over time due to adding the 6 cells. The cells in the pack have not been treated the same over time. Also the pack is not bottom balanced and also not (re- bottom balanced) when the 6 cells were added.

If I'm talking Rubbish Please Explain:eek:

I have no experience what so ever in looking after lithium cells but I've ordered 72, 100ah sinopoly cells for my TT and am trying to learn as much as I can before they arrive, hopefully early June.
At this point in time everything I have read on these forums and else where suggest for me, bottom balance as per mikes post is the way to go, but I'm open to any advice as I don't want to end up with a very expensive mistake:D
 
#8 ·
<snip>
Note how the voltage spikes at the start and begins to fall out rapidly at the end.<snip>
Mike
This is a discharge curve.
Does the voltage similarly "spike" on charge. Or alternatively, how sharp is the knee on charging. Or another alternative form of the question is: can you detect one single cell being fully charged by monitoring the full pack voltage during charge [assuming no failed cell]?
 
#9 · (Edited)
Does the voltage similarly "spike" on charge.
The Open Circuit Voltage? Yes, identically: on a first order, the OCV vs SOC curve is fixed, and independent of current.

The terminal voltage? Mostly. But it depends on the IR drop due to the charging and discharging current.


can you detect one single cell being fully charged by monitoring the full pack voltage during charge?
In general no, because the pack voltage does not divide equally among cells, due to unbalance, variations is cell capacity, variations in cell DC resistance. The exceptions are:

  • Top balanced pack, at 0 current, 100 % SOC (professional EVs)
  • Mid balanced pack, at 0 current, 50 % SOC (professional HEVs)
  • Bottom balanced pack, at 0 current, 0 % SOC (No-BMS school of thought)
In those 3 cases, and those 3 cases only, all the cell voltages are identical, so you can deduce the cell voltage from measuring just the pack voltage and dividing by the number of cells.

But you asked specifically about during charging. Then, your answer is: only if the pack is top balanced.
 
#14 ·
Bottom balancing should only need to be done once unless you need to marry in a cell or if you put in a parasitic load then you'd need to do it a lot. Don't put in any parasitic loads. Loads that come from the entire pack are fine as long as you don't leave it sit with that load continuing. But being bottom balanced you may be ok if you let the pack drain. Best to just disconnect all loading when sitting for a week or more. If you forget you could have trouble.
 
#13 ·
So you have a cell that can hold 40ah and another that can hold 45ah. Now you bottom balance both to 2.6 volts each. Then charge them up and stop when you have 38ah put into them both since you are charging them in a string. What you have is two cells that both have exactly 38ah within both cells. No more no less. What you will find if you monitor the pack is that the one cell that can only hold 40ah will have a little higher end voltage than the other but both still have exactly 38ah. That cell will show different behaviors while driving also but it still started with exactly 38ah and when your done driving you will have taken out exactly 38ah.

Each cell in a pack is slightly different and will show a wandering behavior under loads but by no means does it change the end result. It does not cause drift, self discharge nor imbalances unless of course you have a load on a few cells which many actually have.

Bottom balance your cells to 2.7 volts. This has been confirmed and validated and repeated. There is no doubt.

Charge your pack to 3.5 or if you want 3.45 which is well in the safe zone. Without a BMS you can actually safely charge your pack and not worry about sending cells off the charts. If by chance you do have a cell that is OFF the charts you need to replace it. Simple.

I fully concur with Mike and know what happens when you do drive an improperly bottom balanced pack to the end of charge where one or more cells reaches empty and you drive those into reversal. You WILL kill the cell and bloat it beyond belief fast and there is no recovery.

Charging a few cells into the 3.7 volt range with little amperage is not going to damage your cells.
 
#17 ·
Excellent Thread!

...what I recommend for charging of LiFePO4 cells. Rather than repeating myself every time I thought I'd post what I think is right... Cheers,

Mike
This is a great go-to post with all the basis procedure for how-to-do-it.

We did it this way for Paul's Celica (Weighs ~ 3000 lbs) and he is up and running down the road. It appears that his 44 cell 100Ahr pack comes off the 160 Volt (3.65) cut-off charger in the mornings at 150 Volts (3.41) but quickly drops to 144 (3.27) in the first mile or so. Then follows a very linear sag burning about Weight/10 or 300 Wh/mile down to pack of 136 V (3.1). By then he had better be getting close to home because at 132 (3.0) he is 'out of gas' and will need a tow...
 
#19 ·
Hello Mike,

To summarize, your premises are:

1) "No single cell should touch the top end of the charge curve, ever."

2) "No single cell should touch the bottom of the charge curve, but if it does, all other cells should do so at the same time."

> Overcharging any single cell will significantly reduce capacity or destroy it.

Not quite: it mostly increases its resistance. But your premise doesn't follow from this point.

> Overdischarging any single cell on its own will reduce its capacity

That's just restating your first point. And your premise still doesn't follow it.

> Overdischarging any single cell and then passing current through it ... will cause it to go into a state of reversal.

Very true. But, again, your premise doesn't follow from this point either.

> If a pack is not bottom balanced, there is very likely to be potential in some of the cells that can be drawn through a depleted cell and cause reversal.

True. (Though that can also happen if a pack is bottom balanced.)
Again, your premise doesn't follow from this point.

---

You presented your premises as if they were facts.

In absence of corroborating evidence, please allow me to think of them as your opinions instead of facts. And I do respect your opinions.

Carry on!
 
#25 ·
Maybe I haven't written clearly or yo have misunderstood my points.

I don't see why you don't think my points follow?

Either way, let me just state the following.

I personally prefer bottom balancing, It protects the pack from overdischarge (which I think is most likely) but not so much from overcharge although this is easily managed by undercharging the cells slightly

Nothing wrong with top balancing, it protects the pack from overcharge but not overdischarge, perhaps getting slightly more capacity from the cells but at a slightly higher risk of something going wrong.

BMS have there uses. I think in 95% of EV conversions they are an added expense that is not necessary (sorry!). The only use I can really see is when one needs to squeeze every possible watt of energy from the cells without burning them up, probably where space and weight are issues (motorcycles?). Although there are some that would say that a BMS increases the risk of fire. I don't think this is the case when implemented correctly with degree of common sense.

Better of spending the funds on additional cells to make up for the slight undercharge on a bottom balance in my opinion.

I think BMS do have a place in this market, but only for a select few that have a requirement for them. They can be replaced by good education, a little bit of manual monitoring, a top or bottom balance and even a few extra cells.
 
#26 ·
Hi All,
<snip>
The below graph is of a single Sinopoly 60AH (B) cell being discharged at a little over 30A (0.5c) measured capacity was exactly 60AH. This cell had been over-discharged and abused in the past so probably not the best example! They normally come in closer to 70A

Note how the voltage spikes at the start and begins to fall out rapidly at the end.

<snip>
Cheers,

Mike
My earlier question about recognizing overcharge, answered by Davide was meant to be rhetorical. I think it can be done. During the main phase of charging, there is a linear rise of voltage vs. Coulombs delivered. It should be trivial for a modern microprocessor based system to recognize that the total measured voltage slope is larger than expected and therefore the charge should be terminated.

Gerhard
 
#28 · (Edited)
I have to speak up here because I did buy a BMS, an Elithion Pro. I did it because while I was very much into building an electric car, I knew eventually I would rather just drive it and wouldn't want to have to check my batteries every so often. I read all of the posts about people who had over charged their batteries because of the bms, and decided that most of those occurrences probably came from operator error. When I installed my Elithion, I was careful to follow all of the instructions. I have the proper cutoff relays, the extra 12 volt power source that runs during charging, and just in case, an Elcon charger set to turn itself off if the pack voltage exceeds 3.7 volts * 38. I tested all of the turn-off methods before charging, and then on the first pack charge, (after top balancing) I watch the whole thing through my computer. I have had this system now for 1 and 1/2 years, and haven't had any issues. I simply come home, plug in my car and the next morning the BMS has finished the charge and top balanced the pack. I ran the pack to the end, set to 2.7 volts. The bms set it's fault line, and I had my controller set to go to a limp home mode, (max allowable 80 amps). This all worked as well. This winter, the BMS warned me a few times of low voltage because of the cold. That is also programmable. I also got a SOC meter out of it. I built a circuit to run my gas gauge off of its SOC output. It correlates very well (as in I think it is dead on) to my miles driven after a full charge. If I forget to re-set my trip odometer I have my gas gauge to go by. I now should go out and check my cell voltages, because at the end of the charge it seems it takes longer for the balance to finish. All I need to do is hook up the computer and read the cell voltages reported by the BMS. No individual voltmeters needed. I understand the bottom balance arguments, and I think that it is a perfectly good way to go if you're willing to put more time into your car. This thread hasn't gone so far as to bash bms systems like some others, and I glad to see that in forum posts. I just wanted to post a very positive experience with a bms system.
 
#29 ·
Purchased a group of CALB 100 AH cells from Portland. Spec. sheets show a tested 118 AH and factory instructions state cells are 50% charged.

Everyone has their opinion of charging lithium cells: Mine is to bottom charge to equal and use a cell log 8 for monitoring LV during driving and HV during charging.

Your opinion may vary. That's fine.

Reading over the DIY for the past several years, I have an idea of recommended charging of the new cells. Please correct me if my procedure is wrong.

First Step: Take a single cell and connect to a resistive load that draws around 10 amps to discharge the cell. Connect just the first channel of the Cell Log 8 to shut off the load automatically when the cell reaches 2.7 volts.

Second Step: Repeat the first step two more times after allowing the cell to rest.

Third Step: Repeat for the remainder of cells.

Fourth Step: Connect all cells in series in the vehicle and set the charger for (number of cells) (3.500v) Set the Cell Log 8 to shut off when any cell reaches 3.5 volts on charging and 3.0 volts when driving.

Fifth step: Place a 10 amp load across the series string and monitor cell voltage with a 4-1/2 digit DVM. All cells should hit 2.7 volts at the same time. If there are some early sprinters out front, bleed a little juice off that cell with a small 2 amp load.

How's that sound? Thank you for your help. :)
 
#31 ·
Fifth step: Place a 10 amp load across the series string and monitor cell voltage with a 4-1/2 digit DVM. All cells should hit 2.7 volts at the same time. If there are some early sprinters out front, bleed a little juice off that cell with a small 2 amp load.

How's that sound? Thank you for your help. :)
Sounds a bit off. First, you can't bottom charge. You bottom discharge. And that last bit you're discharging when you should be charging.
 
#30 · (Edited)
If you have a JLD404 or JLD 5740, a contactor that you probably have for the car anyway, two 10' pieces of 3/8 rebar you can setup a 160 amp bottom balancer, here's a video from my thread on this system. Set the cells up in parallel and the rebar in series, at 3.2V the rebar gets to 160 deg F.

Zak's 160 amp bottom balancer in batteries and charging
 
#33 ·
Cells slowly lose capacity with usage, but what I want to know is, does a cell lose more capacity based on its SOC? Does it hurt the cell more to draw 200-300 amps at 80 or 90%DOD as opposed to 50%DOD?

I think I found some general lipo studies a while ago that basically state this in their cycle testing saying that recharging around 50-70%DOD gave optimal cycle life. Too shallow of cycles hurt life and too deep hurt life. I am not sure if that holds true with LiFePo or not.
 
#46 ·
Does anyone have any thoughts or data on my question here? Is capacity loss greater at a larger DOD?

Here is my theory. Top balancing with shunt balancing BMS is absolutely doing its job of protecting the life of the cells. In fact, it may be doing too good of a job, and it may be a detriment to the pack in the future. If, (and this is a big, huge giant IF,) the capacity loss is larger at deeper discharge levels, then a cell that is 10% larger will be experiencing a slightly smaller capacity loss on a daily basis than a small cell.

ie. Let's say you have a 100AH cell and a 110AH cell and one cycle removes .01% of the capacity. 100.00 AH cell after 100 cycles may now have 99.00 AH. One might expect the 110AH cell to have 108.9AH after the same 100 cycles, but what if it loses .008% per cycle instead of .01% because it is slightly larger and therefore not as deep in the DOD as the small cell? It would then be at 109.12AH instead of 108.9AH. This sounds nice on the surface because you have had a lower percentage capacity loss, but then you charge, and now your shunt balancer has to stay on a bit longer because your smallest cell has decreased its capacity at a greater percentage than your largest and reaches its "full" voltage quicker than the larger cell. Every cycle increases the gap and although you are still using your smallest cell at the normal rate of capacity loss, your shunt BMS has to work harder and harder each time to keep from overcharging the small cell. Has anyone with a shunt system noticed anything like this?

Does this make any sense or am I just talking out of my ass? Its ok if you think I am. I am just throwing stuff at the wall and seeing if it sticks.
 
#34 ·
Hi Can you help me with a problem?

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

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

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

Do you have a solution?

Arber
 
#35 · (Edited)
Hi Can you help me with a problem?

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

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

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

Do you have a solution?

Arber
It's beyond me why people think that any electronic circuit or components are more reliable than no circuit at all. If you bottom balance your cells and do not attach any bms device to them then charge them together in series. There is only one thing attached to them to make them exceed the set voltage and that's the charger. Compare that system to each bms component attached to each cell plus the control module and all it's connections in addition to the charger. The sheer numeric possibilities of a failure overwhelms the simplicity of a single component, the charger.
After bottom balancing the cells and connecting them in series, keep an eye on the voltages of all the cells as you charge them. The one with the least capacity will reach the highest voltage first. Your charger should shut off when the that cell reaches 3.45 to 3.5 volts. There are probably several cells that are of fairly close voltage at this point make note of them and watch them periodically.

You have to think of these cells as something that tend to measure at 3.2 volts in a rested state. They are like a basket ball with only a slight amount of air pressure. It's basically a sphere that doesn't change it's size or shape a great deal with varying pressures. Increase the pressure enough and the weakest spot will bulge or decrease the pressure and a dimple will appear.

Keeping away from the extremes is cell nirvana and happiness will reign. Live at either edge and chaos will have it's day.
 
#37 · (Edited)
Any connection, device, electronic component, or circuit that is connected between the terminals of a battery cell has the ability to cause an electron flow between the terminals of that cell. Multiply that possibility times the number of cells with similar circuits and this is what makes a different number of electrons pass through each of the other cells in the pack and thus unbalances the pack.

Z
 
#39 ·
Any connection, device, electronic component, or circuit that is connected between the terminals of a battery cell has the ability to cause an electron flow between the terminals of that cell. Multiply that possibility times the number of cells with similar circuits and this is what makes a different number of electrons pass through each of the other cells in the pack and thus unbalances the pack.
Do you have any evidence that the BMS failed the pack, as opposed to the BMS warning about a failure within the pack?

The bottom-balance-no-BMS group likes to tell me about how over-discharge is virtually inevitable, but so far not one has been able to share end of discharge pack voltages on a deep cycle after a significant period of cyclic use. Personally, my latest work was top balance no BMS and my next setup will most likely be top balance shunt reg.

If I set my controller minimum voltage to cell count times 2.5 I can draw 5C without issue for 75% of the charge, I will feel power loss between about 10% and 25% SOC, and I can't use the bottom 10% in any practical way for driving. I can't get a end of discharge voltage lower than 3 volts with a test load of 0.05C after the minimum voltage limit has rendered the pack useless. On the plus side, I can accurately determine pack balance on any charge cycle. How far do they vary from 3.50 volts when the charger is holding the pack at 3.5v * cell count?

As I recall, the last poll on the BMS issue shows that half of Li battery users here are using a BMS. Could it be they post less because they have fewer issues? I'm starting to wonder.
 
#38 ·
(The following will be controversial!) Cell drift doesn't exist, except for unequal losses in capacity over time.
Goes too far for my tastes.

I'd be happier with something like "Is very small" ... or ... "smaller than ___" ... etc.

Proving a absolute negative like the way you wrote it ...is virtually impossible ... and yet you state it casually.

Just my 2 bits ... and my own personal preferences.

- - - - - - - - -

As for the Bottom vs Top Balancing :

To me it's either a personal preference ... and a person always does one or the other even when it is not the best option for a given scenario.

Or , the person knows the pros and cons of both methods and will use either method interchangeably as they best fit the context of a given scenario without personal bias one way or the other.

- - - - - - - - - -

As for BMS vs no-BMS :

What I find is that most people who advocate 'no-BMS' in general when asked about the details ... are not actually advocating no-BMS ... instead they are advocating Human manual BMS done at some interval ... and an active human BMS at some level ... because they trust Humans to error less often at these tasks than the computer systems of some BMS.

Which computers and such do fail ... but I don't always share others faith in a human having fewer errors ... especially without personal knowledge of the particular human involved.
 
#43 ·
As I recall, the latest poll on Battery Management System shows that half of Li battery users here are using a BMS. Though this case is an exception, I'm not seeing enough posts about BMS problems that suggest they are causing significant issues. I used one at one point and I never had an issue. I had good access to the cell tops so I didn't have the need (human BMS.) I have seen cell voltage creep though. Never seen an explanation.
 
#44 ·
I won't disagree with that. I too don't see but then there are countless others doing conversions that just flat out don't bother with the forums except to glean information totally under the radar. I'd say there may actually be more users of BMS systems than not but I have also not seen any real solid reasons for needing them at least for the LiFePO4 cells. I fully understand the issues with BMS systems and what they are to do or supposed to do.

I just choose NO BMS and speak it. I don't agree with the reasons people tout for top balancing. Not that others have not been successful with top balancing but I just don't buy the argument.

Anyway it all boils down to personal preference. Giving as much information both for and against and top or bottom is very useful.

One nice thing is that there ARE fewer fires being reported now that the bottom balance issue and no BMS has come to light. Been watching very carefully.
 
#49 ·
If you charge your pack to 3.5 and you have a cell that reaches 3.6 by the end of charge what is the problem? All cells are below the top. If however you have a cell that is in the 4 volt range then you need to replace the cell. Shunting with a BMS will not change the fact that you are still limited to the lowest capacity cell. Just replace it and be happy. Be sure your pack is well balanced. When you are talking 100ths or 1000ths of a volt difference your'e nit picking.

Go drive your car and enjoy it.
 
#51 ·
If you charge your pack to 3.5 and you have a cell that reaches 3.6 by the end of charge what is the problem? All cells are below the top. If however you have a cell that is in the 4 volt range then you need to replace the cell. Shunting with a BMS will not change the fact that you are still limited to the lowest capacity cell. Just replace it and be happy. Be sure your pack is well balanced. When you are talking 100ths or 1000ths of a volt difference your'e nit picking.
There is some distance between 3.6 volts and 4 volts. At what point do you pull the smallest cell? (3.7 volts, 3.8 volts...) Most people don't have much of a stash of spare cells.
 
#56 ·
John Hardy
21 minutes ago
An addition to Anne’s thoughtful exhortations on having good electrical connections in your battery pack: I just had an intermittent problem with my battery test rig with a resistance in the charger circuit fooling the charger into terminating early.
BTW I now have over 400 cycles on a set of CALB SE 40s. I am undercharging and under discharging slightly on all cycles apart from every 50th where I am doing a maximum effort charge and discharge to measure capacity. At cycle 400 there was about a 4.5% capacity loss compared with cycle 50. Just like the Headways there is no discernible voltage drift: minimum variance between all eight cells at some point in the charge/discharge cycle has remained consistently around 11 – 14 millivolts with no balancing since a bottom balance on assembly.
More or less they will loose capacity during use. Even by staying off the top and bottom but I will still stay off the top and bottom. My Leaf has shown no discernible loss of capacity even after 2 years and 23,500 miles. The Leaf stays off the top and bottom too. :)
 
#57 ·
That appears to be looking like standard capacity loss. Did he say what his full test specs were? Is it in a thread here? I am curious if he is doing a test based on factory charge and discharge specs, or a more real world DIY electric car type of test. He probably has to do constant discharge, but if you set it at 1.5C or 2C you could probably get a reasonable idea.
 
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