Heard Will the other night discussing combining individual cells to make a large combined battery. The advice was to parallel the cells first, then put them in series. The problem I see with that is when using a BMS on each paralleled group, you can't have the BMS disconnect that group or else your .......let's say 24 V combined battery would become approx. 3.2 volts lower ( with one group disconnected by the BMS ). With a series then parallel arrangement, if one group gets disconnected, the combined battery voltage stays the same, but your capacity goes down. Only way around this issue for a parallel then series combined battery that I can see is to hook up a high amperage disconnect relay between the inverter / charger and the combined battery bank. Comments ?
Combining cells to make a large battery bank
- Thread starter Shazzam
- Start date
Not sure I've seen this in Will's videos. I could be mistaken but he always seems to connect the cells up in series first (you see him swap the cell orientation from cell-to-cell, connecting +ive to -ive) so the voltages add up 3.2V + 3.2V..., then, to double the capacity, add another string in parallel to it.
I did see one video where he created a single 24V battery with 8 cells in series but this needed an 8-cell BMS, which are not that common.
I did see one video where he created a single 24V battery with 8 cells in series but this needed an 8-cell BMS, which are not that common.
I'll have to find that video and post it..............but thinking more about this..............the main problem I was seeing is that I assumed you'd need a BMS for each of your cells that are being paralleled, to protect each cell. But if you connect lets say 4 - 3.2 volt LiFePO4 cells together in parallel, they should automatically equalize their voltage, so really you could use just one BMS on a pack like below for a larger 12 volt battery ( i.e. one BMS lead will accurately report the voltage of all cells in a parallel pack since they are synchronized ). With this configuration the BMS will shut off the whole battery pack if one 4 cell pack gets low or high volts or over current.
4-parallel-pack (series) 4-parallel-pack (series) 4-parallel-pack ( series ) 4-parallel-pack = 12 volts
|________________________|________________________|_________________________|
|
One BMS lead for each 4 cell pack
|
To inverter / charger / loads
4-parallel-pack (series) 4-parallel-pack (series) 4-parallel-pack ( series ) 4-parallel-pack = 12 volts
|________________________|________________________|_________________________|
|
One BMS lead for each 4 cell pack
|
To inverter / charger / loads
D
Deleted member 4428
Guest
If the system were 48 volt in series and you lost a series pack you may be able to re-program your settings down to get buy. Did this with Trace Engineering (Xantrex) inverters and Lead Acid batteries in the past, but it was only 2 volts.
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Yes, There are a couple of ways to configure cells, Series-then-Parallel or Parallel-then series
Neither are 'wrong' There are pros and cons to both.
With Parallel first you can have a single high amperage BMSs. With Series first you can use multiple low-amperage BMSs
With Parallel first everything cleanly shuts down on any error. With series first if one string shuts down you can have continuing operation at lower amperage and capacity.
Edited to point out reduced capacity on failure with series first configurations.
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Not quite! With series first if one pack shuts down your voltage will stay the same but your capacity will be cut in half.
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Yes, I was not complete in my comment. I will update the post
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
One more advantage of series first is that it can be easier to find a bad cell.
