Isolation between cells needed?

Batteries, Chargers, and Battery Management Systems.
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endrew   10 W

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Isolation between cells needed?

Post by endrew » Apr 09 2021 7:49am

I just built my first pack, 16Ah, 72V nominal. I glued the cells together with hot glue before welding and incased with heat shrink.
After the build I saw some videos saying that it is important to add isolation between the cell groups. Is this an important safety issue? Should I be warried?


Thanks!

john61ct   100 GW

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Re: Isolation between cells needed?

Post by john61ct » Apr 09 2021 9:27am

If by group you mean (as is standard) cells in parallel only, at the "lowest" level in the layout at 1S voltage,

then those (cell) groups connected together to create one series string

then No

there is no current flow possible between those groups, they do not affect each other even if at different voltages aka unbalanced.

The overall pack capacity will be governed by the lowest (weakest) group

which also will wear out fastest compared to the other groups.

Staying on top of your State of Health, observing the balancing process periodically testing relative capacity and resistance levels,

are important to ensure "no surprises" and proactive planning of replacement packs before wear symptoms become obvious

endrew   10 W

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Re: Isolation between cells needed?

Post by endrew » Apr 09 2021 9:44am

No, I mean that the first row of cells (0 to 3.6V) is directly touching the second row of cells(3.6V-7.2V) and so on. if for some reason the wrapping on the cells is damaged, it is possible that the cells will short circuit. people add another layer of isolation like kapton tape or cardboard, separating the cell rows.

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spinningmagnets   100 GW

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Re: Isolation between cells needed?

Post by spinningmagnets » Apr 09 2021 9:46am

The cells in the center of the pack will run hotter than the cells at the edge, which can passively shed "some" heat. This means the cells that are not on the edge will degrade sooner, and the pack will have reduced capacity when those cell degrade.

As a general rule, I am not a fan of hot-gluing.

I understand the desire to pack as many cells as possible into the limited space of the frame. If the cells can put out much more current than the controller/motor/throttle demand from them, then...the cells will not get "hot", but perhaps just warm.

As far as electrical isolation, the negative ends and the entire sides of the 18650 cells are one single piece of conductive metal, forming a tall and thin cup. The dangerous point is the gap between the shoulder of that shell, and the positive nipple in the center.

Added electrical insulation between cells is not a bad idea.

john61ct   100 GW

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Re: Isolation between cells needed?

Post by john61ct » Apr 09 2021 12:33pm

Aha I thought you meant electrical isolation, like diodes or circuit protection between groups

rather than physical protection, insulation

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