Safely balancing batteries with different SoC

[kolek]

I'm seeing the 18 awg method theory now, let us know the voltages and the length of 18 awg and let's do some math and then if you monitor it for fire maybe we can learn something.

4 cells at 3.34V
12 cells at 3.29V

Edit: forgot 18 awg wire lengths: 113mm (4.5 inches)

 

[hwy17]

.4 feet x 18 awg is apparently 0.003 ohms, roughly.

3.34-3.29= .05 volts difference

.05v / .003 ohms into an ohms law calculator gives 16.666 amps.

310.17 ampacities of single insulated conductors in free air gives 18 awg 90c wire an 18 amp ampacity.

So, based on my inaccurate, potentially flawed, math that may contain fatal errors, it seems viable that you could parallel them with 18 awg wire and observe the wire for overheating and melting.

The spark would be a 16 amp spark, so may be as bad as plugging in a running vacuum cleaner. It would be ideal if you could connect two wires and then using an intermediate mating method so that the sparking is just damaging disposable connectors, rather than the terminal studs themselves. But it doesn't seem like it will blow up on you if you just yolo a ring terminal straight onto the cell terminal.

 

[kolek]

.4 feet x 18 awg is apparently 0.003 ohms, roughly.

3.34-3.29= .05 volts difference

.05v / .003 ohms into an ohms law calculator gives 16.666 amps.

310.17 ampacities of single insulated conductors in free air gives 18 awg 90c wire an 18 amp ampacity.

So, based on my inaccurate, potentially flawed, math that may contain fatal errors, it seems viable that you could parallel them with 18 awg wire and observe the wire for overheating and melting.

The spark would be a 16 amp spark, so may be as bad as plugging in a running vacuum cleaner. It would be ideal if you could connect two wires and then using an intermediate mating method so that the sparking is just damaging disposable connectors, rather than the terminal studs themselves. But it doesn't seem like it will blow up on you if you just yolo a ring terminal straight onto the cell terminal.

@hwy17 Thanks for taking the time to run those calculations.

I am already terrified of these big batteries (umm, sorry, I should say "cells" otherwise the nomenclature police will call me out for misuse of terms), there is no way I'm going to do anything that involves "observing the wire for overheating." lol. I assumed that with big copper wire or busbars for that matter, I could see a 100amp spark, which is worse than a 16amp spark, no? My reasoning for using thinner wire was mitigating risk to the batteries, it seems like it's actually increasing the risk.

 

[hwy17]

@hwy17 Thanks for taking the time to run those calculations.

I am already terrified of these big batteries (umm, sorry, I should say "cells" otherwise the nomenclature police will call me out for misuse of terms), there is no way I'm going to do anything that involves "observing the wire for overheating." lol. I assumed that with big copper wire or busbars for that matter, I could see a 100amp spark, which is worse than a 16amp spark, no? My reasoning for using thinner wire was mitigating risk to the batteries, it seems like it's actually increasing the risk.

You could parallel 12, take them up to 3.35 while charging with the power supply, parallel them with bus bars while all are at 3.35, and then remove the charge and let the balancing amps flow.

 

[kolek]

You could parallel 12, take them up to 3.35 while charging with the power supply, parallel them with bus bars while all are at 3.35, and then remove the charge and let the balancing amps flow.

Right, that would be easiest, but as I said, I don't want to charge up the pack right now because it may be a couple weeks before I assemble the battery and I don't want it sitting with a high SoC for that long.

 

[hwy17]

Right, that would be easiest, but as I said, I don't want to charge up the pack right now because it may be a couple weeks before I assemble the battery and I don't want it sitting with a high SoC for that long.

I think that if you put the power supply on them set to 3.35 you will find that they get up to 3.35 quickly and enter absorption (constant voltage mode) quickly, well before they have absorbed a high state of charge, and maybe within 15 or 30 minutes of applying 3.35 you will read 3.35 on the bus bars, at which point they can be paralleled and then the charger released and then the 12 can suck the power out of the 4 leaving them all at some middling state of charge.

 

[hwy17]

And a random reminder not to forget to take any metal rings or jewelry off when handling raw cells and bus bars directly.

 

[kolek]

I think that if you put the power supply on them set to 3.35 you will find that they get up to 3.35 quickly and enter absorption (constant voltage mode) quickly, well before they have absorbed a high state of charge, and maybe within 15 or 30 minutes of applying 3.35 you will read 3.35 on the bus bars, at which point they can be paralleled and then the charger released and then the 12 can suck the power out of the 4 leaving them all at some middling state of charge.

Thanks again for your help.
That's an interesting approach. That might be my best option. I gather you didn't think much of my Nichrome wire idea?

 

[time2roll]

@hwy17 Thanks for taking the time to run those calculations.

I am already terrified of these big batteries (umm, sorry, I should say "cells" otherwise the nomenclature police will call me out for misuse of terms), there is no way I'm going to do anything that involves "observing the wire for overheating." lol. I assumed that with big copper wire or busbars for that matter, I could see a 100amp spark, which is worse than a 16amp spark, no? My reasoning for using thinner wire was mitigating risk to the batteries, it seems like it's actually increasing the risk.

Use 10' of wire and let it run. String the wire out to dissipate heat.

 

[kolek]

@time2roll Nice! That's a creative idea I didn't even consider.

 

[ericfx1984]

It's really not that big of a deal... Just attach them in parallel and let them sit for a little bit and they will balance to each other

I actually have a 460 amp hour (230ah, 2p4s) in parallel with a 1,216 amp hour (304ah 4p4s) battery... Oftentimes the smaller battery will finished charging long before the larger battery does... Have you even had many occasions with a larger battery does not finish churching at all and then after the sun has gone down for a few minutes power will get siphoned off of the smaller battery over to the larger battery

The opposite effect happens in the early morning before the sun comes up... The smaller battery basically stops supplying charge while the larger battery handles the charge...

Because they are in parallel they will naturally achieve a similar state of charge due to the voltage differential between the two battery packs