In article , hasan wrote:

I've had the same set of NIMH 2100 maH Energizer cells in my 259B for
over 3 years. I have had them on the internal charger 24x7, except when
in use. I have had no trouble with the batteries overcharging with the
internal charger in the 259B. (obviously, or I would have cooked them a
long time ago).

Good to know, thanks! Possibly the trickle charge is of a low enough
rate that the cells do not overcharge much and overheat. Heat is the
enemy of service life.

According to the battery-manufacturer literature I've read, NiMH cells
tend to have poor "charge acceptance" when trickle-charged at very low
rates. If you try to charge them at a rate of, say, .01C (20 mA, for
a 2000 mAh cell), almost none of this energy actually recharges the
cell - essentially all of it turns into heat.

One manufacturer's writeup I've read says that if it's necessary to
trickle-charge the cells to combat self-discharge, it's best done
through a periodic pulse-charging technique. Pulses of current in the
range of .05C through 1C, lasting for .1 to 60 seconds, are suggested,
with the time between pulses set so that the average rate of charge
delivered is around .02C per day. This is enough to combat
self-discharge, without overcharging.

One thing I've learned about NIMH batteries...they don't sit well. They
do wonderfully if kept trickle charged, but if let to sit, they will
self-discharge in less than two weeks. (at least for digital camera
purposes)I've observed this on three different "sets" of NIMH AA
batteries. All act the same way. Two weeks of sitting and they will only
operate the camera a very short time. Left in the charger and put
directly into service I can shoot well over 75 pictures in a row with no
difficulty.

High self-discharge rates are indeed an issue with the older NiMH
formulas (which account for most of the higher-capacity cells on the
market).

There seems to be a very real tradeoff between capacity and
self-discharge rate... it's a result of the different metal-hydride
alloy formulas used.

The new-generation "ultra-low self discharge" batteries are quite a
bit better in this regard. The Sanyo Eneloops and similar types are
billed as losing no more than around 10% of their total charge after a
month of storage, and still retaining as much as 60% after a year.
The price you pay for the longer holding time, is a reduced maximum
capacity (typically 2000-2100 mAh for an AA, as opposed to 2500-2700
for the ultra-high-capacity type).

Many reports indicate that new NiMH cells, and older ones which have
been sitting around unused for some time will exhibit reduced
capacity, until they've been fully charged and discharged once or
twice.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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