> For over 30 years RC model airplane guys have known that NiCads require
> special care, and yet to this day most less-expensive electronic equipment
> with rechrageable batteries comes with NiCads.
That's because NiCads are still cheaper.
> When you leave a NiCad on
> charge (even a small trickle) all the time (like a cordless
> phone) it loses its charge capacity.
That's not necessarily true, it's easy enough to build a proper charger that
shuts off the flow of current once the battery is charged. But again, it's
cheaper to build a charger that doesn't do that, so many cheap devices
don't. Generally, these are the ones that have instructions buried in the
fine print somewhere to not leave batteries on charge more than 8 or 10
hours.
> If you regularly use the (phone, radio, flashlight,
> etc.) just a little while and then recharge it, the battery develops a
> memory and "thinks" that that's all the capacity it has.
The much bally-hooed "memory effect" does not affect the vast majority of
devices. What happens is that below the "memory point", the output voltage
drops a bit. That's not an issue, unless you've built a $100 million
satellite that shuts down when the voltage drops below a threshold that's
too high. Most cheap devices have no such shutdown mechanism, and so would
never notice that the voltage is a bit low. And to achieve the memory
effect, you have to charge and discharge the battery to the exact same point
each time ... not hard to do in a satellite in low orbit, but not likely to
happen in any other sort of service.
Also, all it takes to overcome the memory effect is to charge the cells
properly ... again a good charger will do this (but that famous satellite
didn't until it was reprogrammed).
> On the other hand,
> if you discharge a NiCad battery pack all the way, the first cells to drop
> to zero voltage then become charged in reverse (because of the
> current from
> the other cells flowing through them). A few reverse-charge cycles and
> those weaker cells begin to break down.
Actually takes only once ... reverse voltage causes the cell to develop
internal shorts which will then cause it to self-discharge even after being
normally charged. If you can access the single cell, frequently they can be
recovered by applying a large forward current/voltage surge, but the
capacity will be reduced.
> Therefore, a phone with NiMH or LiIon cells would be far preferable.
Or just a proper charger and shutdown circuit.
Randall Young
Senior Software Engineer
NavCom Technology, Inc
A John Deere Company
- who build some very expensive electronic equipment that uses NiCads,
because in some applications they are better than NiMH or Li-ion.
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