reverse biased Ni-Cad cells

a) Probably not.

b) Think "trashed battery".

c) 0.9V/cell.

A better discharger would that takes a constant current until the battery voltage reaches the discharge point, then stops.

--

Tim Wescott
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OK, that's 2 diodes + current limiting R

Other then faster discharge, is there any definite advantage of a constant current discharge?

TIA

The pack was composed of cells from 2 different manufacturer.

I took it apart and recharged the reversed ones individually. No problem as yet, but I'll be replacing all of them soon; until then I just keep an eye on them.

A voltage reversed NiCd is typically due to a weaker cell being in a stack of other stronger cells when the entire battery pack is too discharged. Sometimes a negative cell could be corrected by hitting the cell with higher current in the reverse direction. On older cells, this would (theoretically) burn out the reverse section. Then that cell would be recharged. This technique has not been very successful for me with newer technology NiCds. But then you would only be restoring that reversed cell so that operation can continue until a new battery pack is obtained.

Aga> A quick check on a 6 pack containing D size Ni-Cad

Once some older tech NiCds would suffer a 'memory' problem. Solution was to complete discharge and then recharge them. Newer technology NiCds no longer have that problem. No reason to fully discharge a NiCd other than to put all to a same discharge state before recharging.

And yes, cells from different manufacturers could have completely different characteristics. For example, some NiCd are for high load, short term use. Others are for l>...

I don't think you can protect individual cells against this with a simple circuit. Reverse-biasing tends to occur as a result of overdischarging a battery pack... one cell drains to zero before the others, and the others continue to push current through the drained cell, in effect "charging it backwards". You can't prevent this with a simple diode protector before some amount of damage occurs, I fear.

The best thing to do is stop discharging the pack when the voltage drops to a certain threshold. I've heard that 1.0 volts per cell is a really good place to stop... there's very little useful charge remaining in the pack at this point, and this is usually high enough to stop the discharge before any one cell reaches 0 volts.

I understand that it causes the growth of dendrites (thin fingers of metal) inside the cell, through the separators. In effect, the cell develops internal short-circuits, which can cause the cell to self-discharge quite rapidly each time you recharge it.

If you *must* discharge NiCd packs for some reason, don't go below 1.0 volts per cell.

My understanding is that full discharge of NiCds is not actually very useful at all. Doing it is beneficial only under fairly specialized conditions, and doing it improperly is more likely to damage the cells that it is to help.

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Both Nicad and NIMH have this problem, but it's been way overblown, and confused with other issues. I've seen it specifically in Sanyo HR-AUC cells where I ran 10-30 cycles to the same discharge voltage at C/2, then recharged normally at 1C. The result is a slight voltage droop during the subsequent full discharge at the same voltage point.

Absolutely they do! Nicads work better at low temperatures, and high discharge rates, than NIMH's do.

It makes for easy diagnosis of the battery by plotting the time/voltage curve. Power out is then easy to calculate, as well as joule output.

Not a stellar idea. All the cells should not only be of the same exact type, but they should also be the same age, both in calendar terms, and "experience".

If they are put back into a pack together, you'll have the same thing happen. The reversed ones are permanently damaged now, to some degree, and will have less capacity than before, making this even more likely.

0.9 is what the makers reccomend typically, but there's not much run-time difference between the two.

It does erase the "memory effect", but that's hardly worth doing, in most cell types. All it really does is spend cell life.

No confusion here.