B & M do two different models of chargers btw. The original is the e-Werk which doesn't have a cache battery built in but has the advantage of variable output so you can charge 9v devices like some cameras. The cheaper USBWerk only outputs USB friendly 5v. It has a very small cache battery in it which it charges first before supplying to the external device. Your first 5-10 minutes or so charges the cache battery. The cache battery is useful for devices where they pop an alert up on the screen every time you connect/disconnect a charger. I used to have a different charger without the cache and I'd be endlessly pressing OK on my phone screen at every junction or when pedalling slower than required to charge. The cache battery solves that issue. It still works even with a dead cache battery. I'm on my second USBWerk having killed the first in cold weather after about 3-4 years and found this out by experience. If you're just using the device to charge a power pack to charge your phone later, you don't need a cache battery. Be aware some external power packs do not charge unless you press a button to start charging. Some also allow you to charge the powerpack at the same time as discharging it; in effect acting as a big cache battery. If you do have a camera that needs a 9v charger, there are third party battery chargers that run off USB supplies. I have one for my Olympus camera. It's also smaller than the Olympus official charger which is a bonus. I don't have a Garmin as I just use my phone and Komoot but from what I gather with some Garmins they think they have been attached to a PC if you attach them to a powerpack that puts out a voltage over the two data pins (the middle two) in the USB socket. Many powerbanks do this because Apple stupidly decided that it wouldn't follow the USB charging standard. Apple USB chargers output different voltages over the data pins to tell the iPhone/iPod/iPad if the charger is a low of high power charger. In this case, get a cable which shorts the two middle data pins or has a resistor across them and the garmin will not see a voltage across the data pins and assume it is connected to a dumb charger. In theory, charging the device directly is the more efficient way of charging. If you're charging a powerpack you're going from 6v AC to 5v USB DC to 3.7v DC (in the cells in the powerpack) and then back to 5v USB when charging the device later which probably has a 3.7v battery in it. All those conversions suck efficiency. In practice, it probably doesn't matter over the course of a day's cycling. Personally, I find the USBWerk reliable and predictable. The one complaint might be that it never outputs more than 1A no matter how fast you cycle where some of the newer devices do. But, they're about £50 from Germany so quite a bit cheaper than the others.