After various issues with paper maps, I found that bike routing with a GPS unit is a fantastic experience. But for long trips with rare access to a power socket, recharging the batteries can be an issue. This post summarises my experience in the hope that others will find it useful.
First, a few words about the GPS unit: it is a Garmin eTrex 20x, powered by two AA batteries. The good thing about AA batteries is that they are very easy to find: if your charging strategy fails, buying spare batteries will keep you afloat. Autonomy is very reasonable: with 2 fully charged AA batteries of 2500 mAh each, I get 2 to 3 days of navigation (about 20h of use, say). The unit accepts both rechargeable and non-rechargeable batteries, which is great, but also has an important downside: for safety reasons, you cannot charge the batteries while they are inside the unit, even when it is connected (via USB) to a power source.
My goal was to be able to sustain the unit with my dynamo hub. There are essentially two ways to do that. The first one would be to keep the unit connected to an external source while in use, for instance a power bank or buffer battery that you would recharge with the hub. This way, the internal batteries are not used unless the buffer is flat. The other way is to have multiple pairs of AA batteries, and to recharge one pair while you use another one. This is what I have been using and it works… more or less.
I use a Busch & Müller E-WERK converter that takes the output of my Shimano X7 dynamo hub and converts it to a stable DC source. The good thing about this converter is that you can set the voltage and current as you please (unlike many converters which just give you a USB output). A word of caution about this setting: when using the voltage and current dials, make sure you check the DC output with a multimeter before plugging anything to them. On my device, the voltage dial is inverted, and I accidentally killed a USB device because of that.
I have been looking for a battery charger that I could feed with this converter but did not find anything satisfactory (most chargers take 230V AC or 12V DC as input), so I decided to build a simple charger.
The two batteries are directly connected to the output of the converter, with no charge electronics. The good thing is that the GPS unit is never directly connected to the current generated by the hub, which is a useful protection. The main downside is that I have no simple way to check the charging level. Using the charge indicator of the GPS unit is not reliable as batteries tend to look full when they have been charging just before. So, as a rule of thumb, I leave the batteries in the charger for one or two days. I have found that with 2.8V and 400 mA, two days of riding are sufficient to charge the batteries so that they last two days in the GPS unit. This is significantly higher than the recommended charge current (200 mA), but I have to compensate the fact that charging is interrupted by my breaks. If you have any suggestion or use another system yourself, I'd be very interested to hear about it.