Follow-up on the subject of portable charging...
On the logic that the lightest charger is the one you're carrying already, I molished this adaptor for my e-Werk:
The idea being that if you set the output to 2.8V with an appropriate current limit
[1], it can safely be used to charge a pair of NiMH cells in series. Positive to positive (red wire on the battery clip to white wire on the e-Werk output cable), for those playing along at home.
Plus points: It works, is compact and weighs very little, and potentially doubles as a convenient way to carry spare batteries. Relatively unlikely to break from rattling around in your bag, unlike anything with a solidly-attached USB A plug on the end, and the batteries don't fall out.
Gotchas: By stopping at 1.4V (which is necessary to avoid damaging the cells, this isn't measuring delta-V or temperature), the battery only gets to about 80% charged. It's also fairly slow - I measured a little over 300mA into a pair of cells depleted to the point where the eTrex moans about low battery. The current tails off as the voltage rises.
And obviously you need to power your e-Werk. Riding your dynamo-equipped bike is traditional
[2], but it's actually specified for both AC and DC input across a wide range of input voltages. You could, for example, connect it to a 12V solar panel. Or an e-bike battery. And, interestingly, USB's 5VDC is just barely sufficient for this 2.8V charging, although the peak current is limited if there's any drop-out at all.
(This direct-connection-to-a-battery approach will work for other combinations of cells and chemistries, check
the manual for what's safe.)
Is it worth it? Possibly. I reckon you'd get better results with 18650 Lithium-ion cells.
[1] Keep it below 1C, which in practice means you can turn it all the way up for high-capacity AAs, but you might want to limit to 0.5A or so for AAAs.
[2] One advantage of directly charging batteries is they don't give a stuff about stop-start charging.