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Battery Information
Battery Details

Do not confuse with 505 22.5 volt battery, also numbered 221 (& A221).

This battery and it's equivalents have been discontinued. The only thing I can find that comes close is the purpose made 388 Adapter or the non voltage reducing 329 - 388 converter, both from the Small Battery Company.

If you know differently, let me know.

Equivalents:

211, 221, 388, 503, 1157M, 9910, 10R10, Accucell-2,

Image not to scale.	Common Name.	Dimensions. Diameter × Thickness.	Chemical Composition.	Nominal Voltage.
	388.	8.9 mm × 3.3 mm Contacts: Case and flat base Positive. Raised area Negative	Mercury.	1.35 volts(Ag2O).