I was fairly sure that a USB OTG cable was about connecting using a tablet's mini USB socket as if it was a full size one (so irrelevant on this like Garmin computers that don't control USB devices) and nothing to do with charging but several of you seem to be saying it is so I'm doubting myself now!
If you want to charge a Garmin 500 then you definitely need an OTG USB cable if you want to charge and record at the same time.
Plugging in a standard USB cable and connecting to a power bank when the Garmin 500 is recording results in the Garmin 500 entering charge mode only. Remove the cable and it powers off.
I've seen this topic discussed several times on various websites and forums and this particular 'feature' does seem to be restricted to the Garmin 500. I've heard many people offerring the 'useful' info that a standard cable works with their Garmin - which might be a 530/800/830 or whatever other version they have.