David Cook thinking about robots David Cook
ROBOT ROOM™		 a robot
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Nickel-Metal Hydride N-Size Rechargeable Battery

NOTE FROM DAVID: This page is obsolete. None of the items seem available anymore.

Batteries sizes (left to right): N, AAA, AA, 9 V
Batteries sizes (left to right): N, AAA, AA, 9 V

I ran across a pair of N-size rechargeable batteries that I think are wonderful for very small robots.

The battery is nickel-metal hydride (NiMH), not nickel-cadmium, so it's:
  • Safer for the environment
  • Has a higher capacity than NiCd
  • Doesn't have significant memory effect
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, to settle at 1.2 V.


Capacity

The capacity is 360 mAh. 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. A AAA-size NiMH battery can supply slightly less than double (700 mAh) the N-size capacity at the same voltage.


Size

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 "create" the electricity and less to the container packaging and electrical delivery mechanism.

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

Where To Buy

The battery was available online at RadioShack. Part number 23-521. I paid $6.99 for a package of two. Apparently, you're lucky if you can find them now.

N-size battery holder, part #BH-2N from All Electronics

A holder for a pair of N-size batteries was available at All Electronics. Part number BH-2N. Two holders for a $1.00.

Two N-size to AA adapters (one with N cell installed)

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. It's $2 for a pack of 4, plus $2.50 for shipping. Again, apparently, you're lucky if you can order them now.

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. However, a spring might work in a fixed-contact recharger, as long as the spring doesn't go flying out!

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