reverse biased Ni-Cad cells

[aurgathor]

A quick check on a 6 pack containing D size Ni-Cad
revealed one with -0.4V, another with -0.1V. I know
that's not a very good thing, so here are my questions:

a) is there some simple circuit that could be used
to prevent this? I'm thinking of a parallel Schottky
diodes, but preferebly with a Vf under 0.1V.
How low can Schottkies go, BTW?
b) how much damage can reverse voltage cause,
and how quickly?
c) what should be terminal voltage when discharing
Ni-Cads? I plan to build a discharger from a battery
holder where they would discharge through some
series diodes and a resistor.

TIA

 

[Tim Wescott]

A quick check on a 6 pack containing D size Ni-Cad
revealed one with -0.4V, another with -0.1V. I know
that's not a very good thing, so here are my questions:

a) is there some simple circuit that could be used
to prevent this? I'm thinking of a parallel Schottky
diodes, but preferebly with a Vf under 0.1V.
How low can Schottkies go, BTW?
b) how much damage can reverse voltage cause,
and how quickly?
c) what should be terminal voltage when discharing
Ni-Cads? I plan to build a discharger from a battery
holder where they would discharge through some
series diodes and a resistor.

TIA

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.

 

[w_tom]

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.

Again, the most probably reason for such a destructive
failure - battery pack was permitted to discharge well beyond
what should have been its lowest limit causing its weakest
cell to become reverse charged.

 

[aurgathor]

OK, that's 2 diodes + current limiting R

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

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

TIA

 

[aurgathor]

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.

The pack was composed of cells from 2 different
manufacturer.

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.

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.

 

[Dave Platt]

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

revealed one with -0.4V, another with -0.1V. I know
that's not a very good thing, so here are my questions:

a) is there some simple circuit that could be used
to prevent this? I'm thinking of a parallel Schottky
diodes, but preferebly with a Vf under 0.1V.

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.

b) how much damage can reverse voltage cause,
and how quickly?

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.

c) what should be terminal voltage when discharing
Ni-Cads? I plan to build a discharger from a battery
holder where they would discharge through some
series diodes and a resistor.

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.

 

[w_tom]

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 long term,
slow discharge. Some have longer shelf life and therefore
will discharge faster. So you didn't know so much science
exists in a silly little battery? Check Panasonic for their
application notes on NiCds - if they still exist.

 

[Dave VanHorn]

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.

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.

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 long term,
slow discharge. Some have longer shelf life and therefore
will discharge faster. So you didn't know so much science
exists in a silly little battery? Check Panasonic for their
application notes on NiCds - if they still exist.

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.

 

[Dave VanHorn]

The pack was composed of cells from 2 different manufacturer.

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".

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.

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.

 

[Dave VanHorn]

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

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

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.

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

 

[w_tom]

Just to avoid confusion, my paragraph was discussing NiCds
only; not MiMHs. Some NiCds are for high power, short term
operation. Other NiCds are for long term, low power
operation. Mixing both types in a same stack can cause
problems such as premature NiCd failure. A stack of NiCds
(and NiMHs) should be same manufacturer, date, age, etc (as
another has posted) to avoid problems such as reverse
charging.

 

[Dave VanHorn]

Just to avoid confusion, my paragraph was discussing NiCds
only; not MiMHs.

No confusion here.