You are reading a single comment by @Nick.Earthloop and its replies.
Click here to read the full conversation.
London Fixed Gear and Single-Speed is a community of fixed gear and single-speed cyclists.
@Nick.Earthloop
Some specify it, or you measure it roughly and use that number in an amazon review to troll the manufacturer into revealing the actual number. In the case of my 15000mAh Anker, that's 10500mAh. Amazon.co.uk: nick6483's review of Anker® Astro E5 15000mAh Portable Charger ...
Typically the converters are about 70 to 80% efficient I think. They output 5V and a fully charged Garmin only wants 4.2V to keep the battery topped up, so I expect it just discards the extra 0.8V, which is another 16% loss. It seems unlikely that they'd put in extra electronics to make slightly more efficient use of external power, but this is a guess.
So, the cost of using a USB cable to connect the external battery pack to the Garmin (rather than removing the Garmin's internal battery and macgyvering a direct connection to a battery taken from an external battery pack) is about 1/3 of the battery capacity.