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Water level in NiCd batteries

Water level in NiCd batteries

(OP)
I never gave it much thought until a colleague of mine asked me why the water level increases.

To refill water in a NiCd battery that has a low water level I have to unscrew the cap. Then gas is released to atmosphere. During this release, before I totally remove the cap, the water level seen through the semi-transparent battery container is increased, for example from 36 cm to 37-38 cm.
To be more clear, our batteries have 2 cells in the same battery container. If for example water level of the "left" cell is lower than that of the "right" cell, and I unscrew the cap, then both water levels increase a little, but they don't end at the same height. So it is not a matter of just leveling the water in the battery.

When I remove the cap there are no bubbles (or foam) that may give the false indication of increasing the water level.

I was expecting that dissolved gas would be removed from the electrolyte, leaving the liquid volume constant or slightly smaller (since a small gas quantity has been removed from the liquid). But I was not expecting the level to increase.


Any ideas?

RE: Water level in NiCd batteries

When you remove the cap and lower the pressure the case walls move back in and push the water higher.

RE: Water level in NiCd batteries

(OP)
Thanks 3DDave. I'll check it carefully.

RE: Water level in NiCd batteries

Yep, that's what happens as the waist level of the batteries is where all the displacement occurs. So envisioning that can help you see why the adjacent battery levels may be vaguely related but not exactly.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Water level in NiCd batteries

I have only seen "dry" (moist) NiCd batteries, never a water-filled NiCd, but lead-acid batteries are always "water" (acid) filled.

RE: Water level in NiCd batteries

(OP)
Oh yes, there are

... . and by carefully looking at the battery container, it is just what 3DDave said.

RE: Water level in NiCd batteries

Thanks... didn't know that ni-cd batteries had liquid...

Dik

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