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16 cell 12v battery. What is the best configuration?

vtx1029

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My plans changed for some cells I have and I’d like to put them into my rv. I’d rather not go 24v right now and stay with 12v. That said I have 16 90ah cells and 4 bms’s on hand. I won’t exceed the current draw even for one bms so there’s multiple ways I could go. My first thought is just make 4 12v packs and parallel them
 
My plans changed for some cells I have and I’d like to put them into my rv. I’d rather not go 24v right now and stay with 12v. That said I have 16 90ah cells and 4 bms’s on hand. I won’t exceed the current draw even for one bms so there’s multiple ways I could go. My first thought is just make 4 12v packs and parallel them

4s4p would be the easy way to go given that you have four BMS already.
 
4s4p would be the easy way to go given that you have four BMS already.
Indeed. While it is more costly to do (4s+BMS) by 4p than 4p4s+BMS, there are certain maintenance/monitoring advantages being able to monitor each cell by itself. If wired correctly you could easily remove one of the parallel strings to work on it and leave the other 3 in service, should a need arise.
 
But there are caveats as well, I run 4 packs in a bank and indeed have tripped over a few, some forcibly.
- Monitoring amps to determine charge cutoff is pointless, it's unreliable.
- Each BMS has to be able to handle the full charge/charge potential. Any one could be left as the "last man standing" if the others cutoff.
- Each battery will share load/charge if properly wired in parallel. If there is too much difference between battery capacities, the "share" can become uneven.

Fortunately, with 4S packs you have a few advantages by having less cells.
Ideally, top charging the cells to 3.60 and then top balancing in sets of four is the way to go.
Preferably, if you have a method to capacity test or test the Resistance & Impedance of the cells to match them up as close to each other as possible would be best. With 16 cells to make 4x4S packs you could probably manually match them up.

Another way is to capacity test each cell and match them up that way, many capacity testers also report the mOhm resistance values for the cells.

If pre-balancing is necessary, there are a couple common methods for pre-balancing cells:

Method 1: Charge all cells individually.
Before assembling the cells into a pack, each individual cell may be charged independently by using a battery charger designed for a single lithium ion cell. It is essential that if this method is used, the charger MUST be configured so that it does not overcharge the cell. Never leave a cell charging without an automatic method for shutting of the charger.

Method 2: Put all cells in parallel and fully charge together. Before assembling the pack, cells can be connected in parallel and charged together. This method may not work with some types of cells or if the cells are significantly out of balance, as significant currents may flow from one cell to another. Current flow can be calculated by taking the difference in voltage from the lowest to highest cell and dividing by the internal resistance. If you are unsure if current will flow or if you are using a chemistry other than iron phosphate, use one of the other methods. The charger must be setup with a maximum voltage no higher than the maximum cell voltage specified by the cell manufacturer. In order for this method to work, cells must have a charge applied while in parallel (simply connecting in parallel will not allow enough current to flow to balance the pack.)

Method 3: Measure voltages of each cell and manually adjust using a charger or load. This is the most difficult method, but it may be significantly faster than the others if only a small number of cells are significantly out of balance while the rest of the pack is well balanced already. This is an especially useful technique after a cell in a pack has been replaced. Use an isolated load or isolated charger to manually adjust the state of charge of the particular cell requiring adjustment to bring it into balance with the rest of the pack. Careful attention must be given to ensure that a cell is not over charged or over-discharged with this method. This method can be used to balance a cell after it has been assembled into a series pack, but it is absolutely essential to ensure that the load or charger is isolated so that a short circuit is not created within the pack.

Method 4: Have the manufacturer “bin” the cells. While this is not usually an option for small scale systems, in production environments, cell manufacturers can “bin” the cells and match them based on current state of charge, internal resistance characteristics, and cell capacity. Building a pack using cells which are all approximately the same not only eliminates the need to pre-balance, but also keeps the pack in the best health long term since all cells are likely to age similarly.
SOURCE: Pre-Balancing Cells | Orion Li-Ion Battery Management System
 
But there are caveats as well, I run 4 packs in a bank and indeed have tripped over a few, some forcibly.
- Monitoring amps to determine charge cutoff is pointless, it's unreliable.
- Each BMS has to be able to handle the full charge/charge potential. Any one could be left as the "last man standing" if the others cutoff.
- Each battery will share load/charge if properly wired in parallel. If there is too much difference between battery capacities, the "share" can become uneven.

Fortunately, with 4S packs you have a few advantages by having less cells.
Ideally, top charging the cells to 3.60 and then top balancing in sets of four is the way to go.
Preferably, if you have a method to capacity test or test the Resistance & Impedance of the cells to match them up as close to each other as possible would be best. With 16 cells to make 4x4S packs you could probably manually match them up.

Another way is to capacity test each cell and match them up that way, many capacity testers also report the mOhm resistance values for the cells.


SOURCE: Pre-Balancing Cells | Orion Li-Ion Battery Management System
Thanks for the input. I have a resistance tester but I find it’s not very repeatable so (it does read SOC good) I think the best way for me to match is do some test cycles of them and match that way.
 
Thanks for the input. I have a resistance tester but I find it’s not very repeatable so (it does read SOC good) I think the best way for me to match is do some test cycles of them and match that way.
I am looking to do the same thing , did you get it going yet?
 
Question regarding charging.....

For a 16 x 100aH cell bank (400aH @13.2V nom.), are there any concerns or differences with charging @100A if it is a 4p4s or 4s4p configuration?
 
Question regarding charging.....

For a 16 x 100aH cell bank (400aH @13.2V nom.), are there any concerns or differences with charging @100A if it is a 4p4s or 4s4p configuration?

No difference.

I prefer the 4s4p to give me redundancy and a finer granularity of control over the cells.
 
No difference.

I prefer the 4s4p to give me redundancy and a finer granularity of control over the cells.
What I'm scratching my head over is this....

I have a 4p4s setup with 16 100aH cells, which is essentially 4 400aH @3.2V 'batteries' in series that create the 400aH @12.8V bank.

In my mind, I see my 100A @12V nom. charger current inputting into one of the 400aH @3.2V batteries first, and this irks me or doesn't feel intuitive. Is this not a lot of current or voltage not only for the 'battery' but for the 4 single cells in it?
 
What I'm scratching my head over is this....

I have a 4p4s setup with 16 100aH cells, which is essentially 4 400aH @3.2V 'batteries' in series that create the 400aH @12.8V bank.

In my mind, I see my 100A @12V nom. charger current inputting into one of the 400aH @3.2V batteries first, and this irks me or doesn't feel intuitive. Is this not a lot of current or voltage not only for the 'battery' but for the 4 single cells in it?

It depends on the C charge rate that your cell manufacturer specifies. I have two 280 Ah batteries and typically charge with 50 amps with a single solar charge controller. If I fire up every charger in my current system (two 50 amp solar charge controllers and one 55 amp converter) I would get 155 amps. Even with just one battery active, I'm comfortable with that charge rate. It's very unlikely that I would ever see that many charge amps. My batteries would have to be quite depleted for me to crank everything up at once.

Keep in mind that a good charger is going to throttle back the charge rate as the battery is getting full. So that 100 amps may be only 30 amps.
 
It depends on the C charge rate that your cell manufacturer specifies. I have two 280 Ah batteries and typically charge with 50 amps with a single solar charge controller. If I fire up every charger in my current system (two 50 amp solar charge controllers and one 55 amp converter) I would get 155 amps. Even with just one battery active, I'm comfortable with that charge rate. It's very unlikely that I would ever see that many charge amps. My batteries would have to be quite depleted for me to crank everything up at once.

Keep in mind that a good charger is going to throttle back the charge rate as the battery is getting full. So that 100 amps may be only 30 amps.
The Calb specs are Max. Charge Rate: 1C, Standard Charge Rate: 0.25C.

I can or do charge @100A MAX for the 400aH bank as a rule. But, now I am thinking that this may be too much for a 4p4s setup......
 
The Calb specs are Max. Charge Rate: 1C, Standard Charge Rate: 0.25C.

I can or do charge @100A MAX for the 400aH bank as a rule. But, now I am thinking that this may be too much for a 4p4s setup......

100 amp charge on a 400 Ah battery bank is 25 amps per battery, so .25C. With 4p4s you get 25 amps per cell or .25C. I don't see a problem.
 
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