🚀 go-pugleaf

The reversal of a cell in a battery (battery = many cells) of NiCd cells
happens when 1 of the cells is weaker than the others.  The high forward
current causes a voltage drop across that cell that, if higher than the
charged level of the cell if it has any charge left, will appear to be
in the opposite direction of ordinary charging.

The stated figures for the lowest voltage to discharge a battery to are
some attempts to prevent a cell reversal.  If your battery is SUPPOSED
to have N*1.2 volts, then discharging it NOT LOWER than (N-1)*1.2 volts
will assure you of not actually reversing a cell since the weak cell
will have to pass 0 before becoming reversed, and at that point the
voltage of the bettery must be (N-1)*1.2 volts or less.

I have a radio that can operate on voltages from 6 to 15 volts.  I have
a bettery pack that is 11 cells and rated at 13.2 volts.  I can run this
pack until the radio dies at about 6 volts.  By this time, if I am going
to be reversing any cells, I may have ALREADY done so.

Many electrical devices manage to function over a significant range of
voltage.  Alkaline batteries have a gradual voltage drop, so many devices
are designed intentionally to work under these conditions.  The normal
capacity of alkaline cells is rated in terms of how long for their voltage
to drop to 1.0 volts (from 1.5).

So it may be moot to even attempt to discharge your NiCd battery DOWN to
(N-1)*1.2 volts since it may in fact already be BELOW that level.  So what
can you do?

I have for a long time suspected that the "memory effect" is caused by the
internal discharging of a cell.  This is a chemical reaction that is very
different than the reactions to chanrge or normally discharge a cell.
This reaction apparently increases the internal resistance.  Think of a
single cell as an array of cells in PARALLEL.  The chemical surfaces of
the cell are electrically in parallel, so this is valid.  When the cell
is PARTIALLY discharged in normal use, half of the cell is used up, and
half (the weaker half) remains charged.  The first time around this is
not a distinct boundary.  The cell is then recharged.  As this cycling is
repeated, part of the cell is constantly discharged and recharged while
the rest of the cell is NOT.

When this abused cell is used for longer than the cycles it is used to,
it is forced to start discharging from the lesser used portion since the
rest of the cell is discharged.  This portion, having a higher resistance,
conducts less current and appears to have a significant voltage drop.
This may even be so severe as to prevent the device to function, and the
capacity of the cell will apparently be diminished.  I also believe that
higher discharge current bring this condition on faster.

Also, cells are not manufactured equal in capacity.  Typically you might
see as much as a 10% variation in capacity and/or internal resistance.
So your battery may already have a weak cell fresh out of the box before
any abuse gets to happen.

Given that it is not really practical to discharge battery packs down to
the (N-1)*1.2 volt level, and that cells can get weaker if they are
allowed to only be partially discharged, and that weak cells can become
reversed in discharging UNDER NORMAL USE, my practice is to TOTALLY
discharge my battery packs to ZERO volts EACH AND EVERY cycle they are
used.  After I am done with a battery pack, I set it up to be fully
discharged AT A LOW CURRENT for 24 hours or so, then put it on charge.
The idea is to prevent a cell from becoming noticeably weaker than the
other cells in the pack.  What I am doing is keeping the battery pack
BALANCED.

I have been doing this for over 120 cycles so far on 3 packs, and a 4th
was added a few months ago which has had about 40 such cycles.  Ordinarily
NiCd's last much longer than 120 cycles, so my testing period is not over
yet, but neither has this caused an untimely death.  One pack was
inadvertenly subject to sitting in a very hot car for a day a couple
of weeks ago, but it seems to have survived the ordeal just fine.
I also have carried and used the packs in deeply freezing (below -15 C
or +5 F) weather many times this past winter, without any apparent
problems.

--Phil Howard, KA9WGN-- | Individual CHOICE is fundamental to a free society
<phil@ux1.cso.uiuc.edu> | no matter what the particular issue is all about.