Watch out, the mAh ratings of those things are often speced as the mAh capacity of the thing's internal 3.7V battery rather than the number of mAh that you'll get out of it at 5V.
Does the Garmin need 5V? How do you know how much capacity the unit will provide at 5V?
@hippy
Does the Garmin need 5V? How do you know how much capacity the unit will provide at 5V?