Highfields Amateur Radio Club
Glossary of Radio Terms.

Appendix:
Battery Details.

The replacement Alkaline cell will fit perfectly but, as with all Alkaline cells the voltage is not stable, it drifts down from 1.5 volts towards 1.35 volts (the voltage of the original Mercury cell) and then lower still. If you use the cell in a device that needs a steady reference voltage avoid the Alkaline and select either the higher voltage Silver Oxide or the direct replacement Zinc Air.

Another option if you need an exact voltage replacement for the MR9 producing exactly 1.35 volts, is the MR-9 Adapter, Voltage Reducing Converter from the Small Battery Company.
Possibly the best option of replacements this adapter, fitted with a Silver Oxide 386 cell, fully replaces the MR9 in shape and size. It has a recess into which a 1.55 volts Silver Oxide 386 cell can be dropped and and contains voltage regulating electronics to drop the voltage to match the 1.35 volts of the original Mercury cell. The MR-9 adapter is a little expensive, but it can be reused time and time again with the inexpensive Silver Oxide 386 cells.
Note: Although the MR-9 adaptor is more expensive than the Zinc Air replacement cell the Silver Oxide battery supplied lasts significantly longer than the Zinc Air. Also replacement of the Silver Oxide 386 is typically a quarter of the cost of the Zinc Air cell.

Equivalents:

Image not to scale.	Common Name.	Dimensions. Length × Diameter.	Chemical Composition.	Nominal Voltage.
	PX13.	6 mm × 15.7 mm Contacts: Case and flat base Positive Slightly raised area Negitive.	Alkaline Silver Oxide (Ag2O), Zinc-air.	1.5 volts, 1.55 volts (Ag2O) 1.35 volts (Zinc Air) (1.35 volts for the discontinued mercury types)