Rechargeable USB "AA" Batteries. Has anyone used them?
Those "batteries" are actually a 3.7V¹ ~750mAh Lithium rechargeable cell with a 1.5V switching regulator and 5V charge circuit built in. 

In about the same space as one of those, if you throw away the charge circuit and switching regulator, you can get a single 18650 Lithium rechargeable cell at 3.7V¹ (nominal) and three times the capacity. Two of those plus a small 5V switching regulator² would give you more power capacity and longer run time in a smaller space than the equivalent voltage of these strange "batteries".



¹ Nominal

² Use a good one with a Li-Ion battery protection circuit and maybe a built in 2S charger too for convenience