Battery balancing before connecting in parallel

sgrevskott

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Mar 21, 2019
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Hi,

I have sorted my nissan leaf cells, and are more or less ready to connect the parallell bus bar. I have batteries from 3 cars, where 2 of them are 3.98v now and the last 3.79v pr cell. Is this too high difference to connect? Should I charge up/drain to get them more equal? How much diff would be ok? 0.1? 0.05v?

Much appreciated!
 

Wolf

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Sep 25, 2018
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If you were talking about 18650 I would say .05V difference would be OK to connect in parallel.
I would even go as far as 0.1V as the amount of current flow would not be substantial at that level.
You would get an initial spike as the voltages quickly equalise and then a slow transfer of electrons till final ballance.
I'm sure the leaf cells would react pretty much the same but I do not have any experience with them so do not take my word for it.
I do know though if the voltage differential potential is >0.5 you will get a serious inrush of current into the lower voltage battery. Enough to cause an issue? Probably not.
That being said as you mentioned you have 2 at 3.98v and 1 at 3.79v I would bring that one up to as close to 3.98v as possible and do your connection.

Wolf
 

sgrevskott

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Mar 21, 2019
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Hi, i actually remebered a little wrong: they are 3.88 and 3.98. (and 7.76 and 7.97 across the 2+2 leaf setup. So its within 0.1v
 

Roland W

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Oct 9, 2017
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Shouldn't be a problem. Take an Amp-meter when you connect the first group and see how much of a spike you cause.
 

daromer

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The difference at the nominal voltage is more critical than during the end points. So at 4V+ its not a problem with 0.2v difference to be honest but at 3.6-3.7v it need to be lower and this is due to the amount of energy stored at that potential.

0.1v is not a huge thing. You can connect them with the thinner wire that acts as a resistor potentially and lowers the current somewhat :)
 

sgrevskott

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Mar 21, 2019
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Connected now, everything went fine! Thanks for all the support :)
 

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sgrevskott

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Mar 21, 2019
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Don't forget to take the screwdriver out of the bottom rack! :D
Yeah, had a nut that broke the inside plastic, so screw and nut is just turning freely. Screw is well inside the nut, so i will make a copper wedge to ensure good contact. This was just to verify the connection :)
 
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