newbie question: top balancing cells in series

[m_abukhalid]

Hello all,

I have a prepackaged 12V 4cell 80ah lifepo4 battery if it has a BMS. I am trying to top balance the cells and I was wondering if it is possible to do so using the balance chargers without taking the battery apart to reconnect the cells in parallel. I am still unclear how the BMS is balancing them but I see it trying to when I use the supplied 10A 150W charger and it doesnt seem to be effective. Would perhaps using a lower power charger help out? Would buying a balance charger help?

I guess I am unsure whether the balance charger will do different since the BMS is in the middle regardless of which charger I use.

Would appreciate any advice, including any pointers to good reading material on how the BMS operates during charge cycle. I get that it activates resistors to try to match the cell loads but in that case, with a BMS it should be simple to balance the cells when charging and discharging.

 

[Bob B]

You are asking for specific answers but only providing generic information. Do you know what BMS is in your pack? Do you have a Bluetooth connection to the battery? How far out of balance are the cells?

They all have different capabilities .... but generically, most of the pre-built batteries have a BMS that doesn't have much balance current.
In order to top balance the cells in series, the best way would be to charge with a very small current over a long period of time.

 

[Dzl]

Would appreciate any advice, including any pointers to good reading material on how the BMS operates during charge cycle. I get that it activates resistors to try to match the cell loads but in that case, with a BMS it should be simple to balance the cells when charging and discharging.

I think it is simple and you understand the concept, the BMS uses resistors to bleed off charge from the highest voltage cells (when certain conditions or thresholds are met, such as above a certain voltage during charging). But while it is relatively simple, its not effective/capable of correcting very large differences between large cells. An unbalanced pack or a significantly mismatched pack might exceed the ability of a passive balancing (resistive) BMS, because the balance current is so small.

We are seeing this a lot lately with the grey market 280Ah roughly matched cells people are buying. Because of the large cell size, and because they don't seem particularly well matched, many people are looking to higher current active balancers to keep their packs balanced.


You mention you bought a prebuilt battery, but are trying to top balance. Why? Did it come poorly balanced from the manufacturer/assembler? or go out of balance over time?

 

[m_abukhalid]

Thank you very much for the reply! All I know about the BMS is that its 80A with Bluetooth capability. I have been using the app on my phone to monitor the battery over bluetooth. When charging, the BMS overvoltage protection kicks in when cell 1 reaches 3.65V, with cells 2 through 4 at 3.4V. When I disconnect the charger Cell 1 drops to 3.4V over the span of a half hour and cells 2 through 4 will float at 3.33V. During discharge, under-voltage protection kicks in when Cell 4 drops to 2.6V while cells 1 through 3 are around still around 3.1V to 3.2V. Ive done 4 cycles so far, with pretty much the same results. In terms of overall battery voltage, overvoltage protection kicks in at 13.8V, the battery then quickly settles to around 13.3V and discharge is disabled at ~12.3V. Im debating working through these problems as normal initial setup pains or raising a dispute. Took a while to get this in.

Your recommendation makes sense. I ordered an 80W (5A) balance charger to try charging with a lower current to see if that will help me balance out the cells. For an 80ah battery Im only getting about 55ah right now from max possible charge to "fully discharged".

 

[m_abukhalid]

I think it is simple and you understand the concept, the BMS uses resistors to bleed off charge from the highest voltage cells (when certain conditions or thresholds are met, such as above a certain voltage during charging). But while it is relatively simple, its not effective/capable of correcting very large differences between large cells. An unbalanced pack or a significantly mismatched pack might exceed the ability of a passive balancing (resistive) BMS, because the balance current is so small.

We are seeing this a lot lately with the grey market 280Ah roughly matched cells people are buying. Because of the large cell size, and because they don't seem particularly well matched, many people are looking to higher current active balancers to keep their packs balanced.


You mention you bought a prebuilt battery, but are trying to top balance. Why? Did it come poorly balanced from the manufacturer/assembler? or go out of balance over time?

I only recently received the battery and have had this issue from the get go. I am trying to top balance the cells hoping that afterwards, they would charge more evenly. Ultimately, my goal is just to be able to get the full 80ah out of the battery. Also, I worry that if I constantly fully charge 1 cell only, I will end up wearing out 1 cell more than the rest and have to replace my battery sooner.

I spec'ed it at 80ah knowing that for a full night of imaging, my equipment will use around 50 to 60ah. I do not expect the battery to perform at full capacity when running in the dead of winter (-20 to -10 degC), so the extra 20ah is there as a buffer for any loss of performance due to the cold. Right now, it would be marginal at best.

 

[Bob B]

Does your BMS use the xiaoxiangBMS application?

I would use a charge current even less than 5A. I have an inexpensive adjustable 0-30V power supply. It can be used for parallel or series balancing the cells and can be adjusted to 1A or less so that the cells will remain in the upper knee portion of the charge curve for a long period of time.
How far out of balance are your cells? It is likely your BMS will only provide around 100 ma balance current.

 

[m_abukhalid]

Does your BMS use the xiaoxiangBMS application?

I would use a charge current even less than 5A. I have an inexpensive adjustable 0-30V power supply. It can be used for parallel or series balancing the cells and can be adjusted to 1A or less so that the cells will remain in the upper knee portion of the charge curve for a long period of time.
How far out of balance are your cells? It is likely your BMS will only provide around 100 ma balance current.

Yep! Thats the one! Ive attached a couple of images from the most recent charge cycle (cells actually got to 3.4V)



Looking at the specs of the charger I ordered (Tenergy TB6-B), the current can be customized between 0.1A to 5A. Should hopefully do the job! Next cycle Ill charge it with the 10A and then top off with the new balance charger at very low current and hopefully the BMS will be able to do its job.

Really appreciate the feedback

 

[Dzl]

Dumb question, but how does a 'balance charger' work, I'm unfamiliar with the concept.

 

[Bob B]

Looks like you are about 230 mv out of balance at the top and over 500 at the bottom. The good news is that the #1 cell is the highest on at the top and at the bottom .... so, you may have some capacity to be gained if you can get them to top balance.

 

[m_abukhalid]

Dumb question, but how does a 'balance charger' work, I'm unfamiliar with the concept.

The way I understand it, the balance charger uses the bms to balance the voltage of each cell as opposed to charging through the main leads. Technically it should be possible since the BMS has a parallel connection to each cell while the main leads provide a serial connection for faster charging. Ill understand better once I get it and try.

 

[m_abukhalid]

Looks like you are about 230 mv out of balance at the top and over 500 at the bottom. The good news is that the #1 cell is the highest on at the top and at the bottom .... so, you may have some capacity to be gained if you can get them to top balance.

I hope so. I think the cells are healthy, just not very well matched. If I can top balance them I should be able to get the capacity I need.

 

[m_abukhalid]

As an update, the 50W balance charger I got didnt work. I tried charging with low current (0.5A in normal serial configuration) and the cells didnt balance successfully. I tried connecting the BMS pins to the charger directly to balance the individual cells and the charger error'ed out with low voltage warning (even though the cells were all over 3.3V). Either the balance board was defective or the charger isnt powerful enough. As a last resort I returned it and ordered a 150W balance charger to see if that works.