Does a dead NiMH cell lead to overcharge of the others?

[xyster]

The nickel battery failures recently described here got me to thinking:

If a single cell in a pack of nickel batteries, all wired in series, is shorted, won't the other batteries become overcharged? Consider a 20-cell charger charging 19 cells because one went into reversal, or otherwise died closed-circuit.

And if a cell in that series instead fails open-circuit, won't the entire pack neither charge nor discharge?

I asked these questions in another thread, but didn't get any replies. Seems like if this is the case, each cell in a suspect pack should be tested before the next charge.

[Lowell]

Won't the charger still detect the voltage hump near end of charge?

[fechter]

Most Nimh and Nicd chargers are "constant current" until they detect the voltage dip at the end of charge. If a cell is shorted, the charging current would probably still be the same, depending on how smart the charger is.

[xyster]

If a cell is shorted, the charging current would probably still be the same, depending on how smart the charger is.

So if the charging voltage is unaffected, wouldn't a shorted cell lead to the others overcharging?

[Ypedal]

Actually it's the oposite i beleive...

When my pack had a reversed cell, it quit charging pre-maturely.. I do not know exactly why.. but i think it's either causing the pack to heat up from resistance and trigger the temp cut-off.. Or causing the voltage to peak during charge somehow...

Given the way the cells are made.. i can't see how a cell would short open-circuit.... SLA with 6 cells conected within a brick is another story.. but.

[fechter]

If a cell is shorted, the charging current would probably still be the same, depending on how smart the charger is.

So if the charging voltage is unaffected, wouldn't a shorted cell lead to the others overcharging?

Current, not voltage. If you have a constant current charger, which is common for Ni chemistry, if one cell in a string shorts, the voltage will be less, but the charger maintains the same current, so the other cells will still charge normally.

Dumb chargers just put out a voltage that ususally gives you the right current. If a cell shorts, then the current would be higher in all the cells and they might get cooked.

Use Knoxie's needle jabber probe idea to test the pack if you are in doubt. A shorted cell will be easy to find.

[Matt Gruber]

while a dumb fast charger could be dangerous
a dumb slow charger is so slow, like 70-100ma, that cells can short and not do significant damage, especially since cells often un-short during charge.

[Matt Gruber]

i rigged up a cv charger for 3 nicad sub-c 1.3AH. 2 amp charge, FAST! Tapered off fine as it charged
BUT
when i tried a pack with a shorted cell, it was 7 amps