reverse biased Ni-Cad cells

A

aurgathor

Guest
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
 
aurgathor wrote:
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.
Click to expand...

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
Wescott Design Services
http://www.wescottdesign.com
 
"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:10sevt3ngkb4s4b@corp.supernews.com...
c) 0.9V/cell.
Click to expand...
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.
Click to expand...
Other then faster discharge, is there any definite advantage of
a constant current discharge?

TIA
 
"w_tom" <w_tom1@hotmail.com> wrote in message
news:41C7A581.299EACB5@hotmail.com...
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.
Click to expand...
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.
Click to expand...
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 you *must* discharge NiCd packs for some reason, don't go below 1.0
volts per cell.
Click to expand...
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.
Click to expand...
It does erase the "memory effect", but that's hardly worth doing, in most
cell types.
All it really does is spend cell life.
 
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