The 18650 is larger than an AA. It will provide a nominal 3.7 V at max capacities circa ~3,500 mAh (there are lower capacities in the same size cell). After the initial drop from 4.2 V, the 18650s will provide a stable voltage until it drops off quickly when exhausted - unlike alkaline batteries' continual loss of voltage. The 18650 is very common in all sorts of BT speakers and other small devices so there's lots support. The battery number is its dimensions: 18 mm diameter x 65 mm long.Thanks. I have some N20s on order to try, and some 8-20mm motors with small gearboxes installed. There are so many things to try. The problem with small batteries is that, yes, they fit, but they store so little power.