Paul wrote:
> I just purchased an M.15V control pack 1 which came with a fresh battery
pack that needed to be installed. I followed the instructions at Scott
Gomez's site and the FAQ, and everything went pretty smooth. I closed up the
pack and plugged in the charger for an overnight charge, as recommended.
>
> In the morning I detached the charger and pressed the check button and got
a steady red glow. Sounds good, right? When I attached the pack to my new MD2
it fired about 10 shots (on an OM-2n) before it started to slow noticeably.
after about 25 shots it was dead, just groaning and not firing.
>
> After a 4 hr charge the battery shows about 14.6 volts on my DVM. When it
stops firing its about 6v.
>
> Any suggestions?
>
> Paul Schings
Paul,
It usually takes multiple overchargings and/or high rates to damage
cells but long strings of cells (like in the MD) are much less reliable than
the 2 to 4 cells used in many smaller consumer devices. The reason, is that
in a battery of cells, the cells should be very well matched or else on deep
discharge the cell with lowest capacity gets discharged first and thereafter
it is reverse charged by the normal forward current through it from the
remaining cells. It is much harder to have 10 well matched cells than 2 to 4!
The common "wisdom" of completely discharging battery packs of NiCd's has
probably killed more battery packs than any amount of "memory" effect or
"voltage depression" that has been avoided. In fact, it is quite difficult to
demonstrate any cell "memory effects" unless you do well controlled
repetitive discharges. Deep discharges guarantees reverse charging weaker
cells which rapidly kills them especially at high current. Cell conditioners
that discharge cells in the pack individually so that no reverse charging
can occur are safe, but that would require bringing out 2 wires from each
cell to the conditioning device, impractical here.
What can you do? Ideally replace all the cells with a "matched set"
from the same manufacturing lot. When you buy individual loose cells you
actually have no guarantee they are from the same lot and have any chance of
having matched capacity/discharge curves. Battery pack manufacturers try to
make packs from same date code cells and the cell manufacturer trys to make
cells with consistent capacity at least within the batch. Some battery
vendors measure cell capacity and then choose matched sets. In future avoid
deep discharges and don't run the pack way down despite the "memory effect"
mythology. Usually you want to discharge to an endpoint voltage of greater
than 0.8V/cell but in a long string of cells you can't work out whether an
individual cell is below this level or not.
This is obviously expensive so as an alternative you could check your
exsisting pack and weed out weak/damaged cells and try replacing those. The
cells will be poorly matched but if you then avoid deep discharges by not
trying to get too much from the pack you might get a reasonable cycle life
this way.
To avoid damaging the cells by overcharging you can use a timer, as is
often recommended on the list, but this is not very good since you don't
really know how deeply discharged the cells are with a partial discharge and,
the Olympus charger provides a high charge rate that is not well controlled.
A better way is to charge the cells at a slower rate than the Olympus
charger. The cells can then more easily take extended overcharge with no
damage. You could add a series resistor to the Olympus charger output line
and if you put a shorting switch accross the resistor you can select either a
"fast" or slow charge.
You could even make your own slow charger with a wall wart, a
suitable 1Watt current limit resistor and the Olympus remote motor cable
which has spring loaded terminals you can hook wire to. The wall wart needs
to provide 20VDC or more (not so common). I have charged battery packs this
way with an external lab supply.
Regards,
Tim Hughes
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