Charging 3.7V lithium-ion battery?

Terry, the "normal" voltage range of Lithium-Ion cells is 4v2 at full charge down to cutoff at typically 3v0. Oddly enough, three times what you'd sensibly attribute to a NiMH cell, 1v4 down to 1v0. I'd try three NiMh in a temporary configuration to see how it panned out.

As far as charging is concerned, any built-in charging won't match the proper regimes for NiMH. There are straightforward non-destructive ways to determine what the charger does. Altenratively, put two diodes in series with four cells and that way you get a better voltage match without any charging in-situ.

I think that you are mis-understanding what Terry is asking. I think that what he is saying is that he would like to have a sort of 'battery belt' arrangement fabricated from dry cells or NiMH or whatever, totalling 6v, to effectively form a long duration 'field replacement' for the home-based mains power charger, and plugging into the GPS via the same plug as the home charger. If that is indeed what he is asking, then there should be no problem with this. It's basically the pedestrian equivalent of running it in the car, via a 12v car adapter, which I'm sure is probably available. In fact, this might be another possibility. Look to see if a car adapter is available (there's a good place in the UK called globalpositioningsystems.co.uk) and if so, you could hook it to a small 12v gel battery, that would be no heavier to carry around than a bunch of dry cells. That way, your GPS will be powered, and the internal battery will remain 'topped up'. I do a similar thing when I take my GPS out in the wife's car, the cradle being normally in my car.

Arfa

Thanks both, very helpful. An hour or so ago I had a recommendation to try this:

Duly ordered!