Highland Ham wrote:
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I had the same experience with my ancient Garmin GPS 40 ;alkaline batts held
longer than NiMHs.
Possible reason is that alkalines cells are 1.5 V and NiMHs 1.2 V
It is well possible that although the NiMHs are only considered discharged
when the voltage has dropped to below 1 Volt (per cell) this voltage (or its
multiple) is too low for the GPS to operate, whereas alkalines hold a higher
voltage for a longer period.

That's a common myth. Neither type has a constant voltage during
discharge, but the NiMH is much more constant than alkaline. Try this
experiment: Put a resistor across an alkaline cell to simulate roughly
the load a GPS receiver would present. With a DVM, check the voltage
every half hour or so, and continue until it reaches 0.8 - 0.9 volts,
which is where it'll be when nearly all its energy is gone. You'll find
that the "1.5" volt alkaline cell voltage will reach 1.25 volts (about
the voltage of a NiMH cell for most of its discharge period) at a point
where half or more of the cell's energy still remains. That is, the cell
voltage will be below 1.25 volts for half or more of its life. If you
repeat the test with a NiMH cell, you'll find that the cell voltage is
1.2 - 1.25 volts for nearly the whole discharge cycle. So for about half
the time, the NiMH cell will have a higher voltage than the alkaline.

Or, you could do it the easy way and look at the curves supplied by the
manufacturers. They're readily available on the web.

If a device quits working at a cell voltage of 1.0 volts, you'll get
nearly all the energy from a NiMH cell, but you'll be throwing away an
alkaline cell that still contains a significant amount of energy.

I noticed that when the GPS switched itself off when using NiMH batteries
,the cells apparently were not fully discharged.
Using an automatic NiCad /NiMH charger operating with an initial discharge
period as part of the overall cycle,
it took quite a while ,before the NiMH cells were discharged before charging
commenced.

Sounds like something was wrong with the GPS. If there was signficant
energy left in the NiMH cells, it quit at a voltage greater than 1.0
volt/cell. And if it did that when using alkaline cells, you'd be
throwing away an even bigger fraction of its total energy than with a
NiMH cell.

There are several reasons why NiMH cells might not do as well as they
should. One is that chargers will often shut down well before the cells
are fully charged. This is particularly common with new NiMH cells,
until they've been cycled a half dozen times or so. Another potential
problem is voltage depression, mistakenly called "memory". This can be
cured by a full discharge to 1.0 volt/cell then recharge. Cells also
have less capacity after long storage or a history of light use and
recharge. Several cycles are necessary to restore full capacity. And of
course, modern NiMH cells at 2500 mAh or more have a lot more capacity
than earlier ones which were as little as 1300 mAh or so -- if you did a
comparison some time ago, things have changed since. A number of fast
chargers don't charge cells to their full capacity. Finally, a
substantial portion of some cells' capacity originates in the marketing
department -- tests I've run show the capacity of some to be pretty
badly inflated (Lenmar is a common brand that comes to mind). They do
take some care and feeding -- if it's too much of a hassle, alkaline
cells are a solution.

Roy Lewallen, W7EL