Following on from lowtimer's comments, one can indeed use NiMH rechargeable batteries in Garmin hand-held GPS units.
The Pilot III (for example) includes a menu option to tell the unit that you are using rechargeable batteries, presumably so that it knows to expect 1.2V, rather than 1.5V it would get from an alkaline cell and give "low battery" warnings accordingly.
Given that a charger and 4 AA NiMH 1800mA/H cells is available so cheaply I personally wouldn't bother with an alkaline battery charger.
Also note that depending on the sophistication of your battery tester, it may not give particularly accurate readings. Inexpensive battery testers don't place the battery under any load and this gives misleading results.