What is this current?

[ltosolini]

This is a Wina 50Ah cell.
I noticed that the aluminium cage is at ~+3.2 Volt with respect to the anode. So I wanted to measure how much current can flow from the case to the anode getting to figures between 0.2 to 4 mA.
What is this current? Can be somehow be related to self discharge?
If there is a current between the aluminium cage and the anode, it means that aluminium anions are flowing from the cage to the anode slowly dissolving the cage and at the same time plating the anode with aluminium! Which doesn't sound good to me.....

Any idea?

 

[Sunder]

I'm keen to find out as well, as I've been warned that the cells I am buying have the same issue, meaning that you need to separate each cell using shrink-wrap or some other means. Sounds like if the shrink wrap is damaged, you could have a bit of a fireball.

Sorry I can't help more than that, except to say that it's obviously not a manufacturing defect, if it's a "known issue" with completely different cells (Mine are Toshiba SCIB copies)

 

[heathyoung]

Current flow and potential difference are two different things. A current only exists when there is something bridging the potential difference (ie. your meter).

You are seeing a weak current leakage current setting the case at the potential of the cathode, which you are measuring against the anode.

What it probably means is that the insulation between the battery internals and case is poor. In a multi-cell pack, the case to case differential WILL result in self-discharge (and other issues).

Most prismatic cells are plastic cased for a reason. I've never been a fan of metal cased cells.

 

[Sunder]

Makes sense. I think I will use automotive grade vinyl as the first insulator, then double sided tape as a spacer.

If the current flow isn't enough to cause a short and heat, then that should be safe enough.