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Would connecting 2 batteries in different SOC in parallel cause damage?

sunrise

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Jul 16, 2020
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Hi all,
I am almost done with my DIY LiFePo4 batteries (105ah x 2), and they will be connected in parallel, the question is, if I connect them in parallel, and if one of them are fully charged and one is fully discharged, would that cause too high of current flow from one to the other and damage both of them? I am also wondering if it's necessary to have add a inline 100A fuse to the wire between 2 positive terminals? I already have a 100A breaker from batteries to positive bus bar.
Thanks!
 
First, I assume the discharged battery has been top balanced.

It depends on how long they've sat and how close they are, and if a BMS is installed, and what it's limits are, etc.

In a typical low resistance LFP battery, a 0.1V difference would result in an instantaneous surge of about 250A for a microsecond with a very rapid taper to much much lower current.

A recent recommendation (@DThames ?) is to have one cable with a power resistor in it that can slow the current to something reasonable and allow the voltages to equalize at a controlled rate. You'd need to limit it to about 50A to ensure you don't exceed the 0.5C charge rate of the discharged battery, and the voltages should stabilize quickly.

Alternatively, hitting the low battery with a charge to raise its voltage would be helpful.

The bigger concern is that even once they're paralleled, they will be at a different state of charge - even at the same voltage, and it may take a week to stabilize. I recently posted this:


They will still need to be top balanced as a set.
 
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What @snoobler said is valuable insight. I have done individual cells with seemingly no spark or surge. It is wise to be more careful with higher volts.
 
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First, I assume the discharged battery has been top balanced.

It depends on how long they've sat and how close they are, and if a BMS is installed, and what it's limits are, etc.

In a typical low resistance LFP battery, a 0.1V difference would result in an instantaneous surge of about 250A for a microsecond with a very rapid taper to much much lower current.

A recent recommendation is to have one cable with a power resistor in it that can slow the current to something reasonable and allow the voltages to equalize at a controlled rate. You'd need to limit it to about 50A to ensure you don't exceed the 0.5C charge rate of the discharged battery, and the voltages should stabilize quickly.

Alternatively, hitting the low battery with a charge to raise its voltage would be helpful.

The bigger concern is that even once they're paralleled, they will be at a different state of charge - even at the same voltage, and it may take a week to stabilize. I recently posted this:


They will still need to be top balanced as a set.
Thank you @snoobler, that's an insightful post.

"In a typical low resistance LFP battery, a 0.1V difference would result in an instantaneous surge of about 250A for a microsecond with a very rapid taper to much much lower current."

I am not sure how to read this though - did you mean even a 0.1v diff would result in a huge surge, and could damage the battery? or a microsecond is short enough that no concerns as result?

In your experiment, the the diff between the 2 was > 1v, so did you add resistor or it was fine without one? from the conclusion, if it made no difference in terms of SOC, then there should be no sustained current flow from the high to low one right?

BTW I have 2 BMSes for these 2 banks
 
It's a risk you take. I'm just describing what happens.

Busy post, so you probably missed it:

1608231964800.png

Discharged cell bounced to 3.124V after sitting for about 16 hours.
Full cell settled to 3.34 after sitting for about 16 hours.

Delta was only 0.1V. Reason I mentioned "how long they've sat."

Additionally, I don't care about these cells. I woulda slapped them together at a 2V difference. I have about 80 of them, and I'm not doing anything with them. They've all been stressed by use in a gen2 Prius plug-in-hybrid conversion pack in the Phoenix area with no active cooling. Most of the 40Ah cells only test to 25-30.
 
@sunrise I will be doing the same (adding another battery pack in parallel to the existing) in the coming weeks, and I am planning to get the voltages as close as possible and then just connect the outputs from the two BMS and the positives. A bit of spark, but should be ok?
 
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