In article ,
Geoffrey S. Mendelson wrote:

In effect what they have done is traded off how long a charge lasts
versus hom huch that charge is. Unless you have the guts of a battery
hidden in a black hole, there is only so much room in the case.

Yup. I understand that it's an issue of the specific metal alloy and
structure that they use to create the hydride. Alloy structures which
bind the hydrogen more firmly have a lower self-discharge rate, but
less capacity per volume, and vice versa.

Changes in the plate separators also seem to be part of what they've
done to reduce the self-discharga rate.

Putting the guts in a black hole seems impractical... you just can't
get enough of the stored energy back out in Hawking radiation when you
really need it :-)

That depends upon what power level you run the radio. I found that out in
the mid 1990's with the Ray-O-Vac rechargable alkelines. Everything I had
as except flashlights only worked for one or two charges before they
could not put out enough current to be of any use.

In general, alkalines seem to be a poor choice for high-discharge
applications. Digicam owners have discovered this independently...
most digital cameras will "kill" alkalines quite quickly. I believe
I've read that the rechargeable alkalines were even worse in this
regard... their ESR rose significantly after a few charge/discharge
cycles. So, I don't find your experience at all surprising.

There were just some places that 1 watt would not open the local repeaters
even with an better quality rubber duck than the one that came with the
HT. Switching to higher power drew too much current. :-(

I've found the low-discharge NiMH cells to work quite well in my HT
even at high power.

Somebody over in one of the rec.radio.amateur newsgroups (I think it
was Roy but it might have been Cecil) posted an interesting point
about high-current usage recently. He noted that NiMH cells have a
reputation as being worse than NiCd, when high discharge currents are
required... presumably due to higher internal resistance.

His point was true that the comparison works out this way *only* if
you're doing the comparison on the basis of discharge rates in terms
of the battery's total capacity... say, how much energy is lost from
the battery at a C/2 or C/5 discharge rate. Measured in this way,
NiMH cells do come up looking rather worse than NiCd.

In many situations, though, this isn't the right question to ask. The
better question to ask is how much of the battery's total energy is
wasted, at a specific discharge rate measured in amperes (e.g. running
a given radio at high power, or taking photos with a digital camera).

When compared on this basis, modern NiMH cells come out looking about
the same as NiCd in terms of loss.

The reason for the difference, of course, is that the NiMH cells have
a much higher capacity (2x or so) than the corresponding NiCd cells.
They'd look worse in the first sort of comparison, even if their
internal resistance was identical with that of a NiCd, because they
were being tested at a higher absolute current level.

--
Dave Platt AE6EO
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