Parallel batteries + SOC (1 Viewer)

May 17, 2016
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I had a bit of an upgrade to my batteries a few weeks ago. 2 x 125Ah KS energy lithiums, one under each seat wired in parallel.

Pretty impressed so far, all seems to be working exactly as I wanted it.

Of course, like many others, I maybe spend too much time looking at the bluetooth supplied read outs and I notice that the batteries never seem to be at the same state of charge. (via the KS app one always seems to be about 8% higher).

The voltages seem to be the same but I think I read that I can't take too much from that with lithiums so my questions are, why is the SOC so different? Should they not equalize after a certain amount of time? And when I am on hook up and charging, if one battery is still not at 100% could this not lead to the other battery being overcharged by still pumping amps in?

I'll be emailing the fitter later today anyway but just wanted to get any info from here first.
 

eddievanbitz

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Oct 4, 2007
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Disconnect the higher one to allow the other to 100% charge then reconnect the other and monitor again

If a disparagey reoccurs you'll need to look at the wiring of the charging arrangement IE is one battery connected to the charge sources and two tails connect the other battery under the other seat, which is understandable due to practicality but undesirable from a technical point of view
 

Hoovie

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May 16, 2021
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I had a bit of an upgrade to my batteries a few weeks ago. 2 x 125Ah KS energy lithiums, one under each seat wired in parallel.

Pretty impressed so far, all seems to be working exactly as I wanted it.

Of course, like many others, I maybe spend too much time looking at the bluetooth supplied read outs and I notice that the batteries never seem to be at the same state of charge. (via the KS app one always seems to be about 8% higher).

The voltages seem to be the same but I think I read that I can't take too much from that with lithiums so my questions are, why is the SOC so different? Should they not equalize after a certain amount of time? And when I am on hook up and charging, if one battery is still not at 100% could this not lead to the other battery being overcharged by still pumping amps in?

I'll be emailing the fitter later today anyway but just wanted to get any info from here first.
The voltages would be the same if one battery was at 100% SOC and the other was at 1% SOC.
Nothing to do with the way Lithiums are, but all to do with you having connected the +ves together and the -ves together to parallel the batteries.

Many Bluetooth enabled batteries also have a switch on the battery to turn off/enter standby mode. Try turning one off whilst charger plugged in, then once no more current going to one left on, turn the first back on and the other off and monitor charge again. Then when no more charge in, turn them both on and they should be equalised at 100% for both.

Of course how long they stay like that is another matter. They are different batteries and react differently. And if not cabled up correctly, one will discharge more than the other anyway.
The best way to keep the batteries in sync is (as well as correct cabling) to connect the BMSes together with the data interface cable, but most cheap Lithiums don't have that option.

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OP
OP
monzer
May 17, 2016
3,389
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Aberdeenshire
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2nd base
Disconnect the higher one to allow the other to 100% charge then reconnect the other and monitor again

If a disparagey reoccurs you'll need to look at the wiring of the charging arrangement IE is one battery connected to the charge sources and two tails connect the other battery under the other seat, which is understandable due to practicality but undesirable from a technical point of view
Thanks for that Eddie. I haven't looked at their rewiring but I'd be surprised if they haven't fitted it the way you mention - charger to one battery then 2 tails.
 
OP
OP
monzer
May 17, 2016
3,389
7,224
Aberdeenshire
Funster No
43,137
MH
B524
Exp
2nd base
The voltages would be the same if one battery was at 100% SOC and the other was at 1% SOC.
Nothing to do with the way Lithiums are, but all to do with you having connected the +ves together and the -ves together to parallel the batteries.

Many Bluetooth enabled batteries also have a switch on the battery to turn off/enter standby mode. Try turning one off whilst charger plugged in, then once no more current going to one left on, turn the first back on and the other off and monitor charge again. Then when no more charge in, turn them both on and they should be equalised at 100% for both.

Of course how long they stay like that is another matter. They are different batteries and react differently. And if not cabled up correctly, one will discharge more than the other anyway.
The best way to keep the batteries in sync is (as well as correct cabling) to connect the BMSes together with the data interface cable, but most cheap Lithiums don't have that option.
Thanks for the info. I'll need to have a look under the seats to see if their is a physical switch on the batteries themselves. I don't think there is looking on the website.
 
Dec 2, 2019
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I agree with Eddy, there is a imbalance in the parallel cables and one will always work harder, while the other will assis but lagging. You will not overcharge but over work.

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bigtwin

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Oct 29, 2009
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Agreed, I’d be checking if they both supply the same discharge current too. If not, then it’s likely a cabling/connections issue.

Ian
 
Dec 2, 2019
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I don’t think it’s cabling loose. As example: A 16mm2 cable at 1.5m long has about 1.6-1.7 mohms resistance. If first battery is on 0.5m and the second on 1m you will have double the resistance on the path of the second battery. This may only be 0.5mohm difference, but with cycling the drift cumulates and the longer it’s cycled the bigger the drift. To correct this is strive for equal resistance for both paths, as in equal cable, equal connection points etc.
 

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