The application to multiple cells isn't missed (at least not by myself anyhow), however the entire essay is based on a major assumption, that the device is properly designed to handle a per-cell voltage down to 0.9 or thereabouts (which would be an ideal design for any consumer battery chemistry, I'd totally agree)... also that it's a moderate current draw (see below)
However, it appears in this case of the OP, the products in question are designed too close to the edge of the nominal voltage, and are not drawing a large current in operation (in fact I just looked up the Garmin specs for his model, at MOST it draws around 160mA (0.5 watts max per spec sheet, at 3.0 volts nominal). The spec sheet also mentions alkaline's poor cold weather performance and that it could cause the unit not to turn on, in which case you should use Lithiums instead... both items to me point that the device is NOT designed to handle much below nominal alkaline voltages (bad)... it might even be as high as a 2.5V cutoff (1.25V per cell)... IF that is the case (a blanket assumption), and assuming a reasonable 100-150mA draw under normal (not worst case) conditions (also reasonable from the product specs), then the device would perform EXACTLY as described by the original poster, who described roughly 3-4 hours of life on alkalines, and around 1 hour on NiMH (estimates from his numbers posted). The alkalines are cutting out around 1.25V per cell (from Energizer spec sheets,a reasonable extrapolation would put that at between 4-10 hours)... a NiMH would cutout shortly after getting into it's "flat" discharge area (the constant 1.25V section where it likes to operate)... so when it's at it's best, it's getting cutoff by the device. Basically the NiMH are only working in the "initial discharge" bend of their curve, and never getting to operate at their nominal region.
The NiMH/Alkaline crossover as described in the essay (and thank you again for that, I've written several things like that over time that I've decided not to post, you nailed it perfectly) fails to describe in this case the observations noted.
The problems I believe are the assumption of a well-designed device (doubtful in these cases, it appears they BOTH must cutoff somewhere above 1.1 to 1.2V per cell or even higher), and the assumption of a significant current draw, the NIMH/alkaline curves don't cross for a long time under light loads (between 100-200mA is the break-even point in duration to 1.1V from the manufacturer charts I've seen, typically around 185mA is where NiMH starts pulling ahead at a 1.1V cutoff (at 0.9V cutoff it's no contest, NIMH is WAY ahead even at lower currents).
Bottom line, after thinking, calculating, and looking at spec sheets more, I STRENUOUSLY recommend going to lithium primary cells (Energizer E2 Lithiums)... if these assumptions (and that is what they are, I'll be the first to admit) are correct, then you are actually not only getting bad rechargeable performance, but likely are throwing out more than 1/2 full alkaline batteries as well. You say you are getting roughly 3.5 hours more or less (12-16 sessions at 15 minutes each) on alkalines... if these assumptions above are all true, then you are using ONLY about 525mAh of capacity due to voltage issues of the Garmin... lithium E2 cells will NOT drop any significant voltage on such a device at those current levels, you will stay at or above 1.5V for nearly the entire cell capacity, meaning it will last for nearly the full 3000mAh of capacity, so instead of 3.5 hours, you'll get closer to 20 hours of life out of them. So although a bit pricey, each lithium will replace nearly 6 alkalines (so for your Garmin with 2 batteries, you can replace 12 alkalines for around $4 of lithiums. I can't find the specs on the Foxpro device, but if it's a similar issue, then 6 lithiums (at a cost of around $12) will replace potentially 36 alkalines. In cold conditions, this difference is MUCH greater, if you are hunting in freezing temps, each lithium will probably double or even triple it's advantage over alkalines... at that point alkalines are very EXPENSIVE compared to the lithium E2s.
2 things to test (at home for grins, or in the field, doesn't matter), after running the device to "cutoff" with alkalines, save the batteries... test them with a voltmeter if you have one, individually, or leave them in the device and probe the opposite ends of the "stack" if you can, to see what the net per-cell voltage is. (Best results would be to test right after the device dies before the cells can "bounce back"... or wait a bit, turn the device back on while testing the batteries for a little bit (it should come back on at least briefly after some rest) and get a measurement. If the batteries are over 1.1v, then neither alkaline nor rechargeables are a good solution in the device, use lithiums... or use an external pack solution as described, with one extra rechargeable to ensure staying above the device cutoff (although I wouldn't recommend this blindly on the 2-cell Garmin, the voltage might be TOO high with 3 rechargeables, probably not, but it's your risk on that...) the 6-cell device would definitely have no problem with a 7-cell rechargeable pack though (at 8.4V, still below the device nominal 9V, probably would be fine with 8 in fact at 9.6V).
I'd enjoy hearing your findings if you can test the 2 devices just to see where they are really cutting off at and to see if any of my assumptions are correct.
Have you read Curious_character's essay? You really should.
