Prismatic Aluminum Skin Voltage?

EcoRV

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I've had a couple of 200ah 12v DIY banks for a while now which have always been inside a compressed plastic case without issue. Today i pulled them out, disassembled / reassemble on the workbench and then wrapped with Kapton tape for the very 1st time. Everything checked on on voltmeter, BMS etc. Then when i picked up the the taped / assembled cells form the bottom i got a tingle from the aluminum. Voltmeter showed 9.97v. Then as i disassembled the cells 1 by 1 the bare aluminum voltage on the bottom of the cells reduced. I then tested the second pack with plastic in between the cells and it was reading 10v from cell 1-4 and reduces by 3.3v when measure between cells 1-3 and then 1-2.

Like i said the batteries before today have always been in a case before and never handled like this so i just wanted to check if the volt meter should see voltage on the bare aluminum bottom while assembled.

Thanks for your help.
 

Short_Shot

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You probably will see voltage on the aluminum but it's a rather high resistance circuit. Avoid "shorting" the case.

As for getting a "tingle" I'm surprised you felt anything at all. I can (and often do) put your hand(s) across 24vdc + and - and feel nothing.
 

Supervstech

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Odd indeed you felt a tingle…
That implies you were in contact with both terminals. The case has direct electrolyte contact, so there is a voltage leak, but it should be all the same polarity, so shouldn’t generate voltage from cell to cell… unless you measured from positive, or negative to the case…
 

EcoRV

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Odd indeed you felt a tingle…
That implies you were in contact with both terminals. The case has direct electrolyte contact, so there is a voltage leak, but it should be all the same polarity, so shouldn’t generate voltage from cell to cell… unless you measured from positive, or negative to the case…
I just put the probes across the bottom of the cells (not on any terminals) and get ~ +3.3v as i move the probe further from one cell to the next.
 

EcoRV

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Resolved - I checked my Eve 280's and they register the same voltage. The deal is the Eve's have a glue on piece of plastic at the bottom so you dont get the tingle but if you peel the edge off and put a volt meter on in you will see it has around 10 volts just like my 200ah units are showing.
 

obd.tech

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So what could happen in RV mobile use, if the cell wrapping wore away with vibration on a conductive surface?
What's the purpose of people placing thin cutting board between cells 'to protect them', in some Youtube battery builds?
 

Short_Shot

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So what could happen in RV mobile use, if the cell wrapping wore away with vibration on a conductive surface?
What's the purpose of people placing thin cutting board between cells 'to protect them', in some Youtube battery builds?
Not sure how that's a question.

If you know the thin wrapper can wear through why wouldn't you add extra insulation between them and all around?
 

Supervstech

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The voltage can increase around one cell less than pack voltage… you can feel it with sweaty hands… but it isn’t a short issue, it is a battery drain issue. Like leaving a low wattage light on.
 

Horsefly

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I think I posted this here before, but I don't remember where.

I have 8 of the little 25Ah Topband prismatic cells. I had them wired into a 8S / 24V pack, and was reading comments about a voltage between the aluminum skin and one or both of the terminals. I checked and verified that it was there in my case, to both terminals (more to one - I can't remember which). I then checked it from the skin of cell one to the terminals of cell eight, and it was larger. I also checked it between the skin of one cell and the skin of another cell, and again it was there.

Just for grins I decided to see if this voltage was "real" as in a genuine potential that could deliver current. I shorted the skin of two cells, and there was no spark, no current. So I think this "skin" voltage is not real, in that it isn't tied to the potential of the actual LiFePO4 cell.

Now having said that, when I built up my 8S 230Ah pack, I did put some of the cutting board material between them. No harm in being extra careful. But I don't think people need to worry that a huge short is going to happen if the metal skin of the cells touch each other.
 

EcoRV

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So what could happen in RV mobile use, if the cell wrapping wore away with vibration on a conductive surface?
What's the purpose of people placing thin cutting board between cells 'to protect them', in some Youtube battery builds?
I think the purpose is to try and minimize the blue wrap wear. I put about 7000 miles on my RV this summer with 8-200AH cells moderately compressed wit the dollar store plastic cutting board material in between. Then when returning home i disassembled the pack and the cutting board material was scuffed pretty good.
 

S Davis

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I think the purpose is to try and minimize the blue wrap wear. I put about 7000 miles on my RV this summer with 8-200AH cells moderately compressed wit the dollar store plastic cutting board material in between. Then when returning home i disassembled the pack and the cutting board material was scuffed pretty good.
I am mounting my 280ah cells on vibration mounts to try and minimize cell movement, they are being installed on my truck.
 

S Davis

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I think the purpose is to try and minimize the blue wrap wear. I put about 7000 miles on my RV this summer with 8-200AH cells moderately compressed wit the dollar store plastic cutting board material in between. Then when returning home i disassembled the pack and the cutting board material was scuffed pretty good.
How were the cells compressed? Did you have any issues with the movement and your
Terminals, buss bars.

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EcoRV

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Nice setup! - I used 1ga welding cable on the cells in a U configuration. My 8 200ah cells were in a plastic tool box with dense foam so nothing on the scale you have done. I also had 4 -280ah cells in a purpose built 280 plastic battery case i bought from Basen in China. Those cells had much better compression and never moved. My biggest issue is total weight of my 1200ah and 1500 watts of solar in my Class B so im trying to find the right balance of cell longevity to weight.
 

S Davis

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Ok, thanks. I have the cells compression set to about 11.7 psi at 50% soc so hopefully they don’t shift and move around.
 
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