Can Plug and Play and DIY LFP batteries be paralled?

1holaguy

Solar Enthusiast
I was not sure if this should be posted under Plug and Play or DIY batteries so if a monitor feels it should be posted elsewhere please let me know or simply move it. I really have been thinking about building a diy battery bank for my RV but I am running out of time for an up coming long trip and so I am thinking of buying 2- 100 amp plug and play , like battleborn, SOK or Tess batteries and then adding a DIY battery made with 280 Amp cells to bring my total to 480 amps. I know you can not mix wet cell and LFP but can I mix and match LFP if they are the same voltage (in this case 12vdc) Also, do they need to be the same capacity size or is it OK to mix (as in 2 at 100 amp and one at 280 amp in parallel)? If this is an issue please explain the details.
 

Terrapin

Solar Enthusiast
Check with manufacturers for the plug and play ones and the BMS Specs on your own build for info on those capabilities. Everything is different.
 

GXMnow

Photon Sorcerer
As long as the cells are the same basic chemistry it should actually work just fine. Just keep in mind that your maximum current will not totally add up. The two matching 100 amp batteries (1C rate 100 AH) will share pretty well, so you might be able to pull 180 amps from the pair without a problem. The 280 AH cell are probably good to 0.5C (maybe more) that is 140 amps. But if it does not share perfectly, you might only add another 100 amps. So that brings you up to 280 amps from your battery bank as a safe estimate. If you need more than 2,000 watts on a 12 volt system, then you will need to be very careful about balancing the loads across the different batteries. If you will be running under 100 amps (1,000 watts) then you don't have to worry about it. As flat as the discharge voltage curve is on LFP cells, there is still a constant curve slope. So let's assume you start with all of the cells near 50% as the sun comes up to charge your batteries. If you get 30 amps of charge current, it will start raising the voltage of all 3 4S battery strings. The cells that are at the lowest state of charge, will have the lowest internal cell voltage. That will cause that battery to pull more current. The battery that is at the highest state of charge will be at a slightly higher internal cell voltage. That will cause it to pull less current. The end result is that they will self balance as they charge up. As the first battery comes close to full charge, the internal voltage will start to spike up. But the terminal voltage can't rise, so what happens is that battery will basically stop getting charge current until the other cells catch up. They will all naturally top balance like that. When it comes time to pull current, the reverse will happen. If the cables don't match, and the cells have different internal resistance etc., it is very likely the 3 different batteries will have different amounts of current coming from them. But again, they will self balance, as long as the maximum current of any one battery is not exceeded. This is why you can't just add up the current capacities. As the current increases, the highest state of charge battery, with the lowest resistance cables will provide most of the current. But that will cause that battery to start discharging the fastest. It won't take long at all for it to come down to where another battery starts to pull more current. They should all find their sweet spot and share part of the current. The current might differ by 20%, and the state of charges could spread a little in the flat part of the curve, but once again, if you do run them to the lower knee, the lowest state cells will drop out and stop supplying current until the others catch up.

Fuse each battery at or below it's constant rating, and make sure you never pull more than about 75% of the total, and you should be fine. Adding different batteries is great for getting more time, but don't expect to get more peak power. I have 2 matching strings, and I still use this rule. I have each string fused at 125 amps, and the two together fused at 175 amps. And I would also recommend always staying well between the upper and lower knee voltages. Maybe charge to the top knee once in a while to help top balance the cells from time to time.
 

1holaguy

Solar Enthusiast
Thanks GXMnow. Would I be able to reduce some of the complexity by changing my DIY design? For example, If I use 2 or 3 100-amp hr units instead of one at 280 amp hr. The cost will be possibly a bit higher but then the individual batteries would be more similar. For example if I pulled 3k watts from five 100 AHr batteries in parallel I could comfortably pull 60 amps from each and I could design the batteries with an 80 or 100 Amp BMS. In reality I might only need 2 DIY for a total of 400 amps. While this would be a bit more work, it might be the better way to go.
 

GXMnow

Photon Sorcerer
If you truly need 3,000 watts, you should be building higher than 12 volt. 300 amps is a crazy amount of current.
 

time2roll

Photon Sorcerer
Thanks GXMnow. Would I be able to reduce some of the complexity by changing my DIY design? For example, If I use 2 or 3 100-amp hr units instead of one at 280 amp hr. The cost will be possibly a bit higher but then the individual batteries would be more similar. For example if I pulled 3k watts from five 100 AHr batteries in parallel I could comfortably pull 60 amps from each and I could design the batteries with an 80 or 100 Amp BMS. In reality I might only need 2 DIY for a total of 400 amps. While this would be a bit more work, it might be the better way to go.
Different capacity in parallel does not hurt anything. Each will contribute power in the relative ratio by capacity.
+1 for going 24+ volts if you will have a 3000 watt inverter. But then the parallel is not working the same.
 

1holaguy

Solar Enthusiast
time2roll, I am a bit confused by the "+1 for going 24+ volts". Since the I/C is 12v/120v it would seem that going to a 24+ battery bank would just complicate things. In general I would not be pulling more than 1500-1800 watts but went with 3K I/C to give me some room if needed.
 

time2roll

Photon Sorcerer
I would have recommended a 2000 watt inverter for 1500-1800 watts and stay with 12 volts.
Unless stated I would assume you will be running well over 2000 watts and may even need surge performance over 3000 watts if you are installing a 3000 watt inverter.
Sorry for the misunderstanding.
 

1holaguy

Solar Enthusiast
No worries. The RV came with a 2500 watt inverter that gave up the ghost after 15 years. I plan to eventually replace the RV fridge with a residential model so when it came time to change the I/C I stepped it up to 3K. So the move to 3K was more about future proofing the system rather than seeking peak efficiencies. Most of the time I am plugged in.
 
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