But there is something odd happens with matched sets of NiMH batteries
in some digicam applications. I have several sets for a Pentax istD
digicam. What I have noticed is that one cell always appears weaker
than all the others by a significant margin and the camera fails early
with "low battery" when NiMH cells are used ending up with 3 good ones
and one flat. The first one to go flat kills it. The odd thing is that
I suspect the flat is almost always in the same battery slot - as if
there is some extra burden on that specific cell. Using flash heavily
or excessive cold generally tips it over the edge more quickly.
I've noticed that as well, but it's to be expected when the
batteries are used too long between recharges. Cells in a matched
set don't all have the same capacity. They're just placed in sets
that have tighter tolerances, but one of them will *always* be have
less capacity than the rest, and this 'weaker' cell will be the
first to die.
More curious still single use Duracells with higher terminal voltage
which I use when the NiMH have all died always seem to discharge
evenly. I would be interested if anyone can explain this.
Alkaline and NiMH batteries may have large differences in their
measured voltage curves as they're used, but both really discharge
fairly evenly. The big difference is that alkalines don't have
voltages that decline precipitously when they're nearly exhausted.
If you measure individual voltages of alkaline cells as they're
used, there may be differences, but they're fairly close, and they
remain usable at much lower voltages than rechargeables. If
measured, you'll probably find voltages clustered around 1.1v, 0.9v,
0.7v and 0.5v. Using the last as an example, with 4 AA cells you
might measure 0.53v, 0.50v, 0.48v and 0.47v under load. Cameras are
different in that they tend to require slightly more than 1.0 or 1.1
volts per cell to keep operating. When cameras exhaust their AA
alkaline cells, they're far from being really exhausted, will
probably show more than 1.2 volts if measured without a load, and
can continue to be used for hours, sometimes many dozens of hours in
devices that don't require high loads, such as analog radios (at low
or medium volume), LED lights, etc.
NiMH cells also discharge uniformly, but at the point were they're
nearly exhausted (somewhere between 1.1v and 1.0v) the first to
become completely exhausted will show a very rapid drop in voltage
as it plunges from 1.0v to 0.0volts. The remaining 3 NiMH cells
will still be pumping out more than 3.0 volts and you'd really want
the device they're powering to shut down at this point, because if
it doesn't, the depleted cell will start to become reverse charged,
damaging or killing it. This is true even though the 3 remaining
cells may have less than 1% of their capacity remaining. Most
digital cameras require more than 3.0volts to operate, so they won't
tend to kill NiMH cells, but if you don't remove them for recharging
until the camera shuts down, one of the cells is pretty much
guaranteed to appear near dead in a battery tester. The harm in
doing this repeatedly depends on the individual camera, ie, at how
low the voltage of the complete battery set has to go before it
shuts down. At the point that digital cameras shut down when
alkalines are used, they'll have a good deal of unused capacity, and
can probably provide a little more than 1.1 or 1.2 volts under light
loads to other devices for a long time.
It's possible that one or two cells may be loaded more heavily in
certain slots, and if that's the case it might be to provide a lower
voltage for clock or memory chips at very low currents. In this
case the effect would be noticeable only if the devices are used
very intermittently, where the batteries would last quite a while
before needing to be replaced, depending on design anywhere from a
couple of months to a year or more. This can be ruled out if you
open the case and check the battery compartment to see if it has no
more than two wires connecting the device to the battery pack.
