What would happen if batteries with a voltage difference of 0.1v are paralleled?

t_tberg · Sep 5, 2018

I have two 20s batteries, one 10ah and the other is 12ah both are multistar batteries. I would like to occasionally fully utilize my 18 FET controller without stressing the batteries too much. I plan on drawing about 70 amps when paralleled. My question is what would happen if there is a voltage difference of 0.1v, also what would happen if the voltage difference was larger, say 0.5v? I know the outcome wouldn't be good, I would just like to get an idea. Both batteries are individually fused with a 40 and 50 amp fuse respectively.

amberwolf · Sep 5, 2018

Depends on the resistance of the batteries, wires, connectors, etc. You can use ohm's law to figure out the current that will flow at any difference in voltage.

Current = Voltage (difference) divided by Resistance.

Then you can multiply the current times the voltage difference to determine how many watts are being dissipated by that total resistance along the line.

t_tberg · Sep 5, 2018

So 5 or 10 feet of 10 gauge wire should be able to dissipate a watt or two of heat without an issue?

Chalo · Sep 5, 2018

t_tberg said:
So 5 or 10 feet of 10 gauge wire should be able to dissipate a watt or two of heat without an issue?

Better a longer piece of thinner wire. Use a cheap extension cord or speaker wire as a limiting resistor. Limiting the current flow rate is gentler on the batteries too.

t_tberg · Sep 5, 2018

Chalo said:
t_tberg said:

So 5 or 10 feet of 10 gauge wire should be able to dissipate a watt or two of heat without an issue?

Click to expand...

Better a longer piece of thinner wire. Use a cheap extension cord or speaker wire as a limiting resistor. Limiting the current flow rate is gentler on the batteries too.

I'm using a 5amp charger with a 22ah battery, the controller will be limited to 70 battery amps. Wouldn't both of these limitations be the overall charge and discharge current limiters? Or do you mean on the initial paralleling of the batteries that the longer thinner wire would be beneficial?

Chalo · Sep 6, 2018

t_tberg said:
Or do you mean on the initial paralleling of the batteries that the longer thinner wire would be beneficial?

That's what I mean. If the internal resistance of the batteries is very low, and the wires and connections have very low impedance, then the batteries can see a big rush of current even if the voltage potential between them is small.
Once they're leveled out with each other, there's no need to worry about it anymore.

BVH · Sep 7, 2018

With my 2 Paralleled 52 Volt, 12 ah packs, I regularly charge them separately, then let them sit a few hours and by that time, they are between .1 and .2 Volts apart. I have had no issues connecting them at this point for the past 4 years. No special wires, just turn the ignition switch of each pack to connect them. I probably would not do this at anything over .3 Volts.

docw009 · Sep 7, 2018

When you connect two in parallel, the higher one will try to charge the lower one. It's going to depend on the voltage of the lowest battery as well as the relative voltage difference. As a battery gets closer to full charge, it will want less current, and the current it wants will depend on how many cells in parallel.

I've got 36V 10S-2P batteries that I often connect in parallel. I used to make sure my voltages were exact when connecting in parallel, but after measuring the currents, I believe a 0.3V difference was negligible when both are near full charge. I just re-took these measurements, using my y-connector and a Tenergy wattmeter on the higher battery.

-Two 10S-2P batteries at 42.0V and 41.7V, a .30v delta. Connected in parallel, I'm seeing .25 amps flow into the lower one.
-Two batteries at 41.9V and 40.8V, a 1.1V delta. In parallel, I am seeing 1.0 amps flow into the lower battery.

If the lower battery had been a 10S-20P, it would draw 10X the current of a 10S-2P. Hence 20A at a 1 volt differential. Not good at all.

You'll have to consider carefully what you do. I always check voltages before I do this. You never know what you might have on a battery.

John in CR · Sep 7, 2018

What would happen if batteries with a voltage difference of 0.1v are paralleled?

Click to expand...

Nothing other than the batteries will eventually reach an identical voltage. I've done it many times with batteries up to a 1V difference. The voltage differential is so small that the current flow between them will be quite low while trying to reach equilibrium.

craigsj · Sep 10, 2018

docw009 said:
If the lower battery had been a 10S-20P, it would draw 10X the current of a 10S-2P. Hence 20A at a 1 volt differential. Not good at all.

This is not right.

Your example batteries each have an IR of about 600 mOhms ((.3/.25) / 2). If your receiving battery becomes a 10s20p, then it's IR drops to about 60 mOhms while the other stays the same. The current would then be 1 / (600 + 60) or about 1.5A at 1V differential. Your source battery in this instance cannot even achieve 2A for a 1V differential even into a dead short.

Now, if both batteries are 20p then the current is much higher, over 8A. That likely wouldn't be excessive for a 20p though.

BVH · Sep 12, 2018

docw009 said:
When you connect two in parallel, the higher one will try to charge the lower one. It's going to depend on the voltage of the lowest battery as well as the relative voltage difference. As a battery gets closer to full charge, it will want less current, and the current it wants will depend on how many cells in parallel.

I've got 36V 10S-2P batteries that I often connect in parallel. I used to make sure my voltages were exact when connecting in parallel, but after measuring the currents, I believe a 0.3V difference was negligible when both are near full charge. I just re-took these measurements, using my y-connector and a Tenergy wattmeter on the higher battery.

-Two 10S-2P batteries at 42.0V and 41.7V, a .30v delta. Connected in parallel, I'm seeing .25 amps flow into the lower one.
-Two batteries at 41.9V and 40.8V, a 1.1V delta. In parallel, I am seeing 1.0 amps flow into the lower battery.

If the lower battery had been a 10S-20P, it would draw 10X the current of a 10S-2P. Hence 20A at a 1 volt differential. Not good at all.

You'll have to consider carefully what you do. I always check voltages before I do this. You never know what you might have on a battery.

Good to see some actual current flow numbers. Thanks!

What would happen if batteries with a voltage difference of 0.1v are paralleled?

t_tberg

100 W

amberwolf

Administrator

t_tberg

100 W

Chalo

100 TW

t_tberg

100 W

Chalo

100 TW

BVH

1 kW

docw009

1 MW

John in CR

100 TW

craigsj

10 mW

BVH

1 kW

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