Cons of building a 240Ah/12v bank out of 120Ah cells?

[Ghillie]

I have a solar generator I am building/upgrading. I am putting a 2000 watt pure sine inverter, 40 amp MPPT charge controller, and one 4S 12volt 120Ah battery bank inside one pelican case. This will work well for my truck camper, 4H events and similar activities.

I would like to make another battery bank to have more capacity. What would be the cons of building one box with 2P4S 12volt 120Ah cells (total of 240Ah in one box).

I've considered going to a 280Ah cell and just building one bank of 4 cells.

I could wire the generator so the 120Ah bank (inside the pelican case) is not used and we could use the 280 Ah bank exclusively.

I am specifically wondering how this will wear on the 240Ah bank (with 120Ah cells) if I use it in parallel with the internal 120Ah bank.

 

[snoobler]

Welcome to the forum.

Small disadvantage due to the possible uneven wear. Not really a big deal as long as you wire it to balance load as much as possible. That could be eliminated by making 2 120Ah batteries each with their own BMS and parallel the batteries themselves, but that adds cause.

Likely a greater $/kWh vs. 280ah cells as they seem to be the sweet spot in that regard.

 

[Ghillie]

Welcome to the forum.

Small disadvantage due to the possible uneven wear. Not really a big deal as long as you wire it to balance load as much as possible. That could be eliminated by making 2 120Ah batteries each with their own BMS and parallel the batteries themselves, but that adds cause.

Likely a greater $/kWh vs. 280ah cells as they seem to be the sweet spot in that regard.

Thank you for the welcome.

I had originally thought about two separate batteries. Harbor Freight has a case that 8 of these cells fit into nicely though. That's what got me thinking about building them 2p 4s (am I describing that right?).

I guess I could also divide those cases to carry one battery bank and other accessories (progressive dynamic charger for lousy days, dc to dc charger, jumper cables, etc).

Edit: Is it wrong to think that having 12 of the 120Ah cells total is a redundancy that may help if I have a cell or two get damaged or just go bad?

 

[snoobler]

2P4S is the correct terminology.

Cells in parallel with other cells that fail take their parallel cells down with them - no redundancy.

Batteries in parallel will do the same with a failed cell; however, it's easy to have a disconnect between two batteries and isolate them for redundancy.

 

[Ghillie]

Cells in parallel with other cells that fail take their parallel cells down with them - no redundancy.

Batteries in parallel will do the same with a failed cell; however, it's easy to have a disconnect between two batteries and isolate them for redundancy.

My apologies. The redundancy I was referring to is still having good parts to put together a working bank, albeit at a lower capacity. So if I start with 3 banks of 4S, somehow 2 cells get damaged (run over, dropped, etc), I can still put together 2 banks fo 4S so a boon docking trip isn't without power, just rationed power.

Or does a bad cell permanently damage the rest of the cells in the bank, typically?

Which, if that is the case, I would still be left with 2 banks 4S. So, I like am now leaning toward building two banks 4S instead of one bank 2P4S. the cost of an extra case and BMS is minor compared to losing 2/3 total capacity.

 

[snoobler]

Ah. I just ordered 9 cells for a 8 cell 24V pack...

If you have single string batteries, replacing cells is viable. If you have cells in parallel, a failing cell will take out its parallel cells; HOWEVER, if this happens gradually, and you catch it soon enough, it may not damage the cells.

There's always the issue of new vs. old capacity, but what can you do? A working battery that's a little wonky is better than no battery.

 

[HRTKD]

I started down the 2p4s route, but the eggheads here on the forum finally beat it into my brain that a 4s2p configuration was better. The finer granularity of the 4s2p configuration allows for better cell monitoring. If I need to take one battery offline, the other battery is still there.

I did go with the 280Ah cells, but considered the 100Ah cells initially. I figured I might as well get the biggest bang for the buck and fill the space available.

With regard to putting the cells in cases, the best practice of compressing the pack of cells may make it difficult to put the cells in a case.

My 560Ah of LiFePO4 cells are in an RV trailer.

 

[Ghillie]

With regard to putting the cells in cases, the best practice of compressing the pack of cells may make it difficult to put the cells in a case.

My 560Ah of LiFePO4 cells are in an RV trailer.

Once I figure out where the cooling fans will be located, along with the new inverter, I should be fine. I’ll have to build some sort of structure to get around the ”wheel wells” anyway. I also need space for the heating pads to keep the battery warm in winter. This LiFePo bank will be going where the 2 SLA 35Ah batteries are now.

Overview of the case it’s going in.

I started down the 2p4s route, but the eggheads here on the forum finally beat it into my brain that a 4s2p configuration was better. The finer granularity of the 4s2p configuration allows for better cell monitoring. If I need to take one battery offline, the other battery is still there.

I think I’ve been talked out of 2P4S also. I can see two banks of 4S being plenty of capacity most of the time. Having the 3rd bank as a ”spare” is fine with me.