[TheForge] Battery technology (was Cordless drills)

[Erik Gutfeldt]

On Jan 26, 2005, at 8:54 PM, RICK KORINEK wrote:

> The newer nickle-metal hydride batteries are a better choice
> than the nickle-cadmium.  They don't have the memory that
> requires that they be completely discharged before putting
> them back on the charger.


Hmmm, I've researched this a lot a couple of years ago. "Memory" with 
NiCad batteries, requiring 'complete discharge' is an urban legend. It 
is false. DON'T believe it. WAY more battery packs have been destroyed 
by over-discharging than any other cause, even dropping them ;)

If you attempt to 'completely' discharge multi-cell battery packs 
(anything over 1.2V is multi cell) here is what will happen: One of the 
cells will completely discharge before the others. That cell will get a 
reverse current from the remaining cells. This destroys the cell. With 
a dead cell, the battery pack is dead.

Battery memory is only seen with very predictable, precisely repeated 
charge-discharge cycles. It was first encountered in solar powered 
satellites. The satellite would go in and out of the earth's shadow in 
a steady repetitive way. Eventually the batteries seemed to become 
discharged earlier and earlier. But, these satellites were fixed by 
changing the trigger point of where the batteries were considered 
discharged. As it turns out the voltage output of the batteries was 
reduced in the discharge cycle, but the power capacity (number of 
amp-hours) remained nearly unchanged.

Lab experiments showed that even varying the charge discharge cycle a 
bit would prevent the memory phenomena from occurring.

Note that this type of memory exists both with NiCad and NiMH. In fact 
these batteries technologies are quite similar. NiMH have about twice 
the amp-our capacity.


Overcharging will also destroy batteries. Faster chargers are less 
likely to over-charge. That's because fast charging MUST use 
sophisticated charge termination to keep batteries from exploding (not 
that that wouldn't be cool :) So 1-3 hour charge times are going to be 
good chargers. Most worthwhile power tool chargers are of this type.

The worst chargers are in the range of 5-10 hour charge time. This 
charge rate is sill capable of damaging batteries that are fully 
charged. If you insist on using (or are stuck with) this class of 
charger, check often to see that the battery pack isn't getting warm. 
If it's warm, it's done.

the 12+ hour 'trickle' chargers are again in the range of not damaging 
a charged battery. This is the way most cordless phones work.

Erik