basic BMS question parallel vs series

[BlueDee]

Hello guys, have been reading upon the topics around here a lot and most of the answers I found last few years made me conclude I needed even more reading. However I have learned a lot reading here (and elsewhere). So thank you guys for sharing all your info.

I have a very basic question about a BMS (probably not needed for now but I got a question on which I can't clearly find the answer and the mind gets angry when it does not get it). I'm not going to put anything in reality right away but I am thinking out stuff and my info got stuck somewhere and I simply need to understand. I know I need tons more of knowledge but this one bothers me.

let's say we take a simplified battery made up of 4 series packs each containing 5 cells in parallel.

If I understand correctly, the usage of a bms is to make sure the series packs don't get out of balance to much in relation to the other packs, let's assume pack 4 has a lower voltage after charing, I have the following questions

1. a bms works by lowering the highest packs to the voltage of the lowest pack, when evened out it starts charing the battery up again,... If I assume correctly my follow up question is, If by default the 4th pack is lower, how does the bms help? doesn't this process repeat itself each recharge cyle?

2. what if 1 cell in pack 4 gets bad (and thus drags down the pack) if I understand the working correclty the bms does not help here itself at all, basically trying to compensate the pack voltage but if a cell is very bad this is a process with only disadvantages (giving unnecessary discharge and cycles to the other packs)

3. lowering the voltage in the other "good" packs to the voltage of the bad pack and raising the voltage again seems to me like a very energy inefficient method als also influencing life of the better packs.

I hope it is possible for you guys to answer this in a way I can get my mind around it. Nothing I have been reading and watching really answers these questions (or i missed it).

ps I tried the search function but it did not yield sufficient results, hope I didn't overlook a thread containing the clue. I know I still need a lot of eduction before going to real world mode, but learning starts somewhere

Thanks for your time
Bert

[max2k]

Hello guys, have been reading upon the topics around here a lot and most of the answers I found last few years made me conclude I needed even more reading. However I have learned a lot reading here (and elsewhere). So thank you guys for sharing all your info.

I have a very basic question about a BMS (probably not needed for now but I got a question on which I can't clearly find the answer and the mind gets angry when it does not get it). I'm not going to put anything in reality right away but I am thinking out stuff and my info got stuck somewhere and I simply need to understand. I know I need tons more of knowledge but this one bothers me.

let's say we take a simplified battery made up of 4 series packs each containing 5 cells in parallel.

If I understand correctly, the usage of a bms is to make sure the series packs don't get out of balance to much in relation to the other packs, let's assume pack 4 has a lower voltage after charing, I have the following questions

1. a bms works by lowering the highest packs to the voltage of the lowest pack, when evened out it starts charing the battery up again,... If I assume correctly my follow up question is, If by default the 4th pack is lower, how does the bms help? doesn't this process repeat itself each recharge cyle?

2. what if 1 cell in pack 4 gets bad (and thus drags down the pack) if I understand the working correclty the bms does not help here itself at all, basically trying to compensate the pack voltage but if a cell is very bad this is a process with only disadvantages (giving unnecessary discharge and cycles to the other packs)

3. lowering the voltage in the other "good" packs to the voltage of the bad pack and raising the voltage again seems to me like a very energy inefficient method als also influencing life of the better packs.

I hope it is possible for you guys to answer this in a way I can get my mind around it. Nothing I have been reading and watching really answers these questions (or i missed it).

ps I tried the search function but it did not yield sufficient results, hope I didn't overlook a thread containing the clue. I know I still need a lot of eduction before going to real world mode, but learning starts somewhere

Thanks for your time
Bert

I have no practical experience but from my own reading I can say:
1. it depends-
1.a. 'passive' BMS would try to 'undercharge' higher voltage cells (because they have lower capacity) bypassing part of the charge current during charging process either starting from some threshold voltage or through entire charging process. The total Ah of the battery will be limited by capacity of the lowest capacity cell in the battery.

1.b. 'Active' BMS would take part of the charge from higher voltage/lower capacity cells and put it into lower voltage / higher capacity cells during charge process and in reverse- during discharge. This way total Ah of the battery will be averaged between Ah of individual cells. They're much more complex than passive ones.

2. This is usually handled by monitoring voltage of each cell and triggering some alarm. BMS should be sophisticated enough to reliably detect this situation and not mix it up with normal charging / discharging behavior.

3. I never came across such system. Please note cell with lower voltage during charging phase often has it so due to its higher capacity so in a sense it's actually 'good' cell. All BMS are trying to address 2 problems: not to overcharge less capacity/higher voltage cells during charging process and prevent the same cells from over- discharging during discharge as they would discharge and lose their voltage sooner than other cells.

all of the above assumes battery that consists of multiple cells of the same type connected in series. When connecting cells in parallel there's no way to easily distinguish one cell from another within multiple parallel connected cells 'pack'.

[BlueDee]

Thanks for your Time Max, I got another knowledge boost, definately helping me on the way again.

3. I never came across such system. Please note cell with lower voltage during charging phase often has it so due to its higher capacity so in a sense it's actually 'good' cell. All BMS are trying to address 2 problems: not to overcharge less capacity/higher voltage cells during charging process and prevent the same cells from over- discharging during discharge as they would discharge and lose their voltage sooner than other cells.

I should call myself stupid for overlooking it since the logic is very easy but completely mislooked it here.

all of the above assumes battery that consists of multiple cells of the same type connected in series. When connecting cells in parallel there's no way to easily distinguish one cell from another within multiple parallel connected cells 'pack'.

Yes, that is where my mind goes in error mode. from my understanding parallel packs are measured in their series connection, but this being the "pack" as a whole, my mind could not figure out what a bms could do to help against a faulty cell in this individual pack. So basically I think I was right to assume there was nothing.

If anyone cares to answer again the follow up question. (off course assuming you start with brand new batteries from the same batch)

So if one cell goes faulty the parallel pack's individual voltage will raise slightly (compared to the average) and the bms will compensate the packs voltage relation. Doing so you will save your battery as a whole from ruining, but in the long run it will however ruin the pack with the faulty cell right?
So basically If you notice a systematic recurring lower voltage in one of the packs you better check the individual cells.

thanks guys!
Bert

[karrak]

If you have say a 12 volt LiFePO4 (LFP) battery made up from 8 cells, 2 in parallel and then each block of two cells wired in series where all the cells in the battery have the same capacity and SOC (i.e. the battery is balanced) and we suddenly get one cell going open circuit we have the following scenarios.

1. If the cell goes open circuit when the battery is fully charged the voltage of the 4 blocks of 2 cells will be the same after the fault occurred. As we draw energy from the battery the voltage of the 2 cell block with the defective cell will decrease faster than the other 3 good blocks. When the battery SOC reaches 50% the voltage of the 'bad block' will crash as the SOC of the good cell in that block will have reduced to 0%. If you have a BMS installed it would detect this and protect the good cell. It is unlikely that any cell balancing circuitry would be able to counteract such a fault. If you didn't draw more than say 40% of the capacity of the battery you might not be aware of any fault unless you studied the individual block voltages closely.
2. If the cell goes open circuit when the battery is ~0%SOC when charging the battery the voltage of the defective block will go through the roof when the battery SOC reaches 50%. Again the BMS should protect the good cell in this case.
3. If the cell goes open circuit when partially full you would get scenario 2 when trying to charge when the battery SOC reaches between 50% and 100% and scenario 1 when discharging when the battery SOC gets between 50% and 0%.

Of course if one cell went open circuit the other would get double the charge and discharge current which would decrease its lifespan

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[max2k]

...
If anyone cares to answer again the follow up question. (off course assuming you start with brand new batteries from the same batch)

So if one cell goes faulty the parallel pack's individual voltage will raise slightly (compared to the average) and the bms will compensate the packs voltage relation. Doing so you will save your battery as a whole from ruining, but in the long run it will however ruin the pack with the faulty cell right?
So basically If you notice a systematic recurring lower voltage in one of the packs you better check the individual cells.

thanks guys!
Bert

From my experience with SLI (car batteries) the type of faults they develop are:
- electrical leak
- loss of capacity due to age

BMS cannot help with failed cell but it should help with detecting it and protecting other cells. I'd expect good BMS to make battery pretty much unusable in case of faulted cell as it would be forced to disconnect battery from charging/discharging circuits so often that it will definitely get your attention.

In case of electrical leak it will discharge cell to itself and if there are other cells in parallel- all of them. BMS should 'notice' this development long before complete discharge happens and set alarm / disconnect battery from output circuit. Disconnecting won't save parallel cells from discharge but at least it will help to save others. Leak will also cause loss of part of the charging current slowing down cell charge and creating impression of 'higher capacity' cell except during discharge it will fall in voltage faster than others.

Loss of capacity can be compensated by BMS as it happens to all cells in the battery so while losing capacity individual cells might no go too far from each other. Eventually this loss will make you replace all of cells. This replacement better happen at once as mixing new cells with old in the same battery is very similar to mixing cells of different capacities in the same battery: they'll be severely unbalanced all the time. Again I'm referring to the battery here with cells in series. This applies to battery which consists of parallel cell packs connected in series too if you consider each pack as equivalent 'cell'.

I've never seen cell become 'open' as it would require one of its active electrodes to disconnect from the post. This would be indicative of poor design or using battery in the environment it was not intended for, like high vibrations.

[BlueDee]

I thank you guys and make a humble bow for your quick and clear answers.

Have a nice off-grid future!

Bert

[Adeleras]

Can anyone tell me what bms i need to finish my 18650 battery bank i have 4x80 cells and 7 packs do i need a bms for each pack or 1 for the whole lot its also a 24v set up

[Adeleras]

Is their anyone who can help with what bms is need for a 4x80 cell x7 pack lithium 18650 battery set up please and thank you for your time

[Ampster]

Before any one can answer your question, more information is needed. What chemistry are the 18650 cells, How are they configured? 4S80P ? That is correct how do you get to 24 volts?
Do you have pictures or a diagram about how they are connected?

[Adeleras]

Oh ok so they are 4 lines of 20 3.7v 2600 mah

[Adeleras]

We are totally off grid and are using 22 lead acid batterys at 24v

[Ampster]

Okay do you have the Lithium batteries in groups of 20 in parallel and each parallel group is in series for a voltage of 14.2v. There are plenty of hobbyist BMSs available for that. Where are you located? That may determine where your best source is.
However for a pack that small there are a lot of opinions that suggest you don't need a BMS. How are you trying to use that pack off grid? What does the Lead Acid pack have to do with this?

[Adeleras]

Ok lead has nothing to do with this sorry im in australia so they are all plus 1 side and minus the other side in 4 lots of 20 in each row

[Ampster]

My opinion is you dont need a BMS as long as you have low voltage cutoff on your load and high voltage cutoff on your charger. There is a good BMS sold in Australia. It is configurable, and I think has those cutoffs. What application are you using this pack in?