On Fri, 30 Jul 1999, Ian Collier wrote:
> The memory effect is a bit controversial as to whether it exists or not
> and what effects it has. Ask different people and you will get different
> opinions.
That's for sure.
>
> Here is what I am given to understand about the memory effect. This is
> second-hand evidence, but it sounded credible at the time.
> It happens to NiCd batteries when they are overcharged. Overcharging causes
> the battery to drop its voltage by several millivolts, but this effect is
> reversable. If you drain a charge C from the battery and then recharge it
> then the effect will be partially reversed. This means that the battery now
> has its full voltage again and will function normally until the charge C has
> again been drained. At that time you start using the "old" stored charge
> which is still suffering from the memory effect, and the battery voltage
> drops by several millivolts. Now, if the equipment is well designed then
> it shouldn't mind this small voltage drop and should continue functioning
> normally. However, it is possible that the equipment will see the voltage
> drop as a sign that the battery has run out, and the remaining charge in the
> battery will be unusable.
>
> The fix for this is of course to drain the battery completely and then
> recharge it normally, taking care not to overcharge it.
I agree on the fix, but disagree on the cause. The cause is recharging
without fully discharging. This makes it almost inevitable on certain
applications - laptops, and poser phone batteries (mobile
phones). I'm no chemist, but find the concept of a battery
discriminating between new and old charge somewhat confusing. I have
had it explained as a chemical change in the plates which reduces their
capacity to store charge. In any case, The voltage when charged remains
similar, but the voltage when, say, a quarter of the nominal charge has
been drained off is lower with the memory effect. I'm sure I have
something really technical on this available, because I'm in
Electronics, but I imagine most listmembers have a poor stomach for
this technical discussion on batteries.
Regards,
Declan Moriarty
