Originally Posted by
unterhausen
the problem is that if you wait for the battery to get low, you may actually over-stress the external battery. Most batteries have higher capacities of they are discharged slowly
Over stress?
Most "portable USB chargers" charge at 0.5 to 2.0 amps (2 amps for the bigger ones.) Your GPS probably accepts around 0.5 amps (2.0 amps is more for charging your iPad) and it'll charge at that rate when nearly empty all the way up to when it has 80-90% charge, at which point the charge rate will taper off as it gets close to 100%.
Even if you're charging your iPad at 2.0 amps, these charges generally discharge the battery at less than a 1C rate (a 1C rate = it will only work for an hour, 2C = 30 minutes, 0.5C = 2 hours, etc.), and the battery probably can provide a few times that before it starts to lose more than a minuscule amount of capacity to an excessive discharge rate. And your GPS will accept power a lot slower than the iPad, even if fully discharged.
You will not stress it, unless it's one of those tiny units that's like 1 inch by 1 inch, and then it'll just charge slower rather than let itself be stressed. If you're using one of those chargers that takes a single AA battery (alkaline batteries definitely do work better with low discharge rates), then maybe, but anything better than that ... no.
Originally Posted by
njkayaker
Based on what, exactly? The device is consuming power as you are charging it. It might be better not to wait until it is "nearly dead" and charge it while the sun is shining (that is, it could be raining when it's "nearly dead" and you might not want to have to charge it then).
Based on the typical portable charging having that'll output up to one amp or so but they'll still use power even with no load.
So you can charge your GPS at 1/2 amp or so and power the battery light while it's on, or you can continue to charge (just replace power as used) your already full GPS at 0.04 amps ... and power the battery light while it's on.
If you're worried about not being able to charge it when you need to, charge it when it's 40% full, and stop when it's 90% full. My point is ... don't leave it charging all the time, or its own status lights will probably drain more power than the GPS does.