Multiple LFP batteries in parallel

[s/v Jedi]

I have been promoting to build or buy batteries that only have cells in series, not parallel. This is to prevent all sorts of balancing problems because you enforce that every cell gets the exact same charge current.

Many have been wary of this, not just because of the higher cost when you need two batteries instead of just one, but also because they fear that the batteries will be unbalanced, not charged correctly, not share loads etc.

The higher cost is not something that can be fixed, but I don’t see this as a problem because you also gain redundancy. I recommend to have two batteries that each would be enough to make do in case the other fails, while having luxury when both are online.

For the questions about unbalanced batteries I have explained that they will balance automatically, even when they are of different capacity. Finally someone actually made a video of testing and demonstrating this: four batteries in parallel at 20%, 40%, 60% and 80% SOC and of different capacities.

https://www.youtube.com/watch?v=bhCSccNsQOw

[fxykty]

Very interesting, thanks for this Jedi.

So, I’m about to (finally) connect our new 4S 700Ah battery in parallel with our existing 4S 700Ah battery.

The video talked about all the batteries being close in voltage and no problems with connecting them in parallel. However, our new battery is at 14.4V (3.6V per cell due to completing a top balancing on the 4 cells in parallel) while the existing battery will be up to 1.00V lower than that.

Should I charge the existing battery so that it is nearer to 14.4V before connecting it in parallel, or is it OK to have a relatively large voltage difference? How much current could a 1V difference generate?

[HankOnthewater]

Sorry I do not know the answer to above (fxty) question, but if the internal resistance is one milli Ohm then the current will be 1000 Amp, at least initially, However I think the various connections, relay, switches etc, will have a combined resistance of 10-100 milli Ohm, giving a current of 100-10 Amps, acceptable to me.

See the specs of your batteries, that should/could show you the internal resistance. Hmmm just checked mine, it only shows that external resistance (relays, switches, connectors) should not exceed 10 milli Ohm.

[s/v Jedi]

Originally Posted by fxykty

Very interesting, thanks for this Jedi.

So, I’m about to (finally) connect our new 4S 700Ah battery in parallel with our existing 4S 700Ah battery.

The video talked about all the batteries being close in voltage and no problems with connecting them in parallel. However, our new battery is at 14.4V (3.6V per cell due to completing a top balancing on the 4 cells in parallel) while the existing battery will be up to 1.00V lower than that.

Should I charge the existing battery so that it is nearer to 14.4V before connecting it in parallel, or is it OK to have a relatively large voltage difference? How much current could a 1V difference generate?

The video shows what happens when you parallel a 20% SOC battery with an 80% SOC battery.

Instead of measuring the resistance in all your wires, switches etc. after which you can calculate the current for a 1V potential difference, charging the lowest battery, or discharging the highest battery somewhat is a logical thing to do when experimenting isn’t at the too of the list

[smac999]

I don't think anyone denied that batteries would not balance when connected.

the problem is the inrush current between batteries might be too high and shut off the bms. so best pratice they should be the simular soc when connecting them.

plus no marine battery is gonig to have a switch on it. so that inrush current starts to happen while you are trying to connect the jumper wires. with fun sparks

I also call BS that the 20% and 60% battery are the exactly same voltage. those soc meters are probably wrong.

[s/v Jedi]

Originally Posted by smac999

I don't think anyone denied that batteries would not balance when connected.

the problem is the inrush current between batteries might be too high and shut off the bms. so best pratice they should be the simular soc when connecting them.

plus no marine battery is gonig to have a switch on it. so that inrush current starts to happen while you are trying to connect the jumper wires. with fun sparks

I also call BS that the 20% and 60% battery are the exactly same voltage. those soc meters are probably wrong.

The video clearly shows that inrush current is not a problem, at least not for the BMS’s he uses, which are a number of different ones.