Most of the messages in this thread relate to charging the GPSMap 64 while it's powered off, and the general consensus appears to be that regardless of battery type, rechargeable batteries take longer than expected to reach full capacity, though they do eventually get there and the charging symbol changes to reflect that.
My question: Does the same apply to charging the batteries while actively using the unit? I see the same animated symbol when I briefly press the power button, but I have yet to see it indicating what I'd call a fully charged state. Unplugging the GPS after some time shows the batteries to be full, and indeed I get 3 or 4 hours use before the first bar disappears, but on reconnecting it just seems to charge forever.
My background: I purchased the GPSMap 64 to use on a multi-day trip, where I will be relying on an Anker PowerCore+ 26,800 USB battery pack to keep me going. The power pack will stay permanently connected to the GPSMap 64, which is fitted with a pair of Duracell Ultra 2500 rechargeable cells. Assuming a conservative 90% charging efficiency I reckon this will be good for at least 9 cycles, and so look forward to 120 hours of continued use as a minimum. Just would be good to not have to keep unplugging it whenever I think it's full.