I ran across N-size rechargeable batteries that I think are wonderful for small robots. For example, a pair are installed in XSBoost.
Batteries sizes (left to right): N, AAA, AA, 9 V
This N-size battery is nickel-metal hydride (NiMH), not nickel-cadmium, so it’s:
The nominal voltage is 1.2 V, but I find a factory-fresh battery hovers at around 1.35 V after recharge. This will likely decline after multiple recharges.
Alkaline N-cells are also available, but are not rechargeable.
The RadioShack NiMH N cell’s capacity is 360 mAh (milliamp hours).
It suggests that the battery could supply 360 mA for an hour, but batteries rarely achieve their rating if drained as quickly as an hour. More likely it indicates the battery could supply 36 mA for ten hours, or somewhat less than 360 mA for an hour.
Compare this to a 9 V rechargeable battery that can supply 150 mAh (but has 6× the voltage). A AAA-size NiMH battery can supply slightly less than double (700 mAh) the N-size capacity at the same voltage.
The battery is N size, which is a little taller than half the height of a AA. (You can’t stack two N-size in place of one AA.) The battery diameter is more than a AAA but less than a AA.
You'll find that total battery power capacity (watt hours) improves with size, because more of the battery’s dimensions and mass is dedicated to the chemicals that “pump” the electricity and less to the container packaging.
So, the N size is appropriate where you must save space and mass, at the expense of capacity per mass or volume.
Pair of N-size rechargeable batteries from RadioShack, part #23-521
The battery is available online at RadioShack as “Model: N-360NM-GP, Catalog #: 55030811” for $8.99 each. Originally, it was part number 23-521 for a package of two for $6.99.
A quick Google search for “N cell NiMH” or “N cell battery NiMH” located many rechargeable N-cell batteries. The cost is much lower ($2) than RadioShack, and the rated capacity (400 mAh or 500 mAh) is higher.
N-size battery holder, part #BH-2N from All Electronics
A holder for a single N-cell is available from All Electronics, #BH-50, for $0.50. The original holder (pictured above) was for a pair of N-size batteries. It came from All Electronics, #BH-2N; Two holders for a $1.00.
A recharging adapter was available from RadioShack, part number RSU #11286218 Cat No. 23-137, which converts the N-size to AA-size for recharging.
Thanks to Warren Jones for letting me know I could special order this from RadioShack several years ago. It was $2 for a pack of 4, plus $2.50 for shipping. But, now the part number shows something completely different. (RadioShack reuses part numbers?)
Using a circuit-board spacer to recharge N-size battery in AA/AAA Energizer recharger
Before I got the adapter, I’d been recharging the batteries in a standard AA-size NiMH recharger with metal circuit-board spacers to fill the gaps. This trick works because the Energizer-brand NiMH battery recharger has a sliding contact.
In a fixed-contact recharger, you could use: