Probably belongs in Travel Technology

But the short answer is that the limit is in watt-hours (abbreviated Wh), which is a unit of energy, not mAh, which is a unit of charge. The FAA limit is 100 Wh per Li-ion battery in the cabin, and you can carry more than one.

To get from mAh to Wh, you need to know the battery voltage. For a typical USB charger, this probably 3.7V. The capacity in Wh is then 3.7*(capacity in mAh)/1000.