Archive for February 26th, 2009
Ever wonder why rechargeable AA cells don’t quite fit in older flashlights & gizmos? Somewhat to my surprise, the dimension specs for alkaline and rechargeable cells aren’t quite the same.
At the bottom of the Wikipedia AA battery page, we find “brand-neutral” drawings (allegedly) based on ANSI specs:
- Alkaline: 14.0 ± 0.5 dia x 49.85 ± 0.65
- Rechargeable: 14.1 ± 0.6 dia x 48.9 ± 1.6
A rechargeable cell can thus be 0.2 mm larger in diameter, but should have the same maximum length.
Based on my collection, alkalines seem to be near their nominal and NiMH cells near their maximum. Across a four-cell layer, the difference adds up to 1 mm or so, which is enough to strain the plastic.
Hint: Put some paper on the negative terminal when you measure the cell length. Steel calipers are pretty good conductors and the short-circuit ratings (even for alkalines) are surprisingly high …
When I make up NiMH packs for our bike radios, I lash the cells in place with cable ties. It’s not pretty, but the plastic cases don’t split.