Zinc Air Batteries. Two brands of zinc air cells are now available as replacements for mercury photo batteries. Zinc air cells are best known for powering hearing aids, however the common ones sold in most drug stores and major chains are physically too small to work in cameras.

Wein zinc air cells. These 1.35 volt batteries have been developed by Stan Weinberg and Bob Shell (editor of Shutterbug Magazine) specifically to provide an accurate replacement power source for older cameras. Wein cells are sold online by MyOldCamera.com and VintageBatteries.com and can also be found at many local independent camera stores. Retail price in Seattle is about $5. Available to replace the PX625, PX675 and RM400 / V400PX mercury batteries.

Although Wein cells come closest to matching the voltage output of mercury batteries they have shortcomings. First, they require air for the cell to produce electricity. This means if your battery cover doesn't have a small hole for ventilation, you may need to drill one. (Hint, think Dremel, not Black & Decker.) Other problems with zinc air cells are discussed below.

Exell zinc air cells. 1.4 volt Exell zinc air cells are sold online and by some local camera retailers. Although slightly higher in voltage than a Wein cell, they will work in most cameras, requiring minimal exposure compensation, especially when shooting color negative or black and white film. Available to replace the PX625, PX675, E400, E400N / RM400 mercury batteries.

Wein cells and all zinc air batteries have other disadvantages:
Besides requiring air to function, they won't reach full power output for up to 30 minutes after removing an adhesive sticker covering the tiny air holes, and they die in two to six months when the battery electrolyte dries out, depending on ambient temperature and humidity. Best advice, keep the adhesive sticker handy, cover the air holes after your shooting day is over, and always carry a spare because they have a shelf-life of many years until activated.

Visiting your camera repair technician. You can have the light meter permanently recalibrated to compensate for the higher voltage. This is a pricey solution, costing $70 or more depending on the camera model and only makes economical sense if your camera is being serviced for other problems. Additionally, the camera technician will adjust the meter to work at medium light levels, meaning the meter may be inaccurate at high and low light levels. This doesn't solve the problem of the declining voltage found in alkaline battery substitutes. Some well-known cameras such as the Canon F1 cannot be adjusted.

Do-It-Yourself Battery Adapter. Franz de Gruijter in the Netherlands offers do-it-yourself adapter kits for the PX-625 (aka PX-13) and hand-made adapters which substitute readily available SR44, 357 and S76 silver batteries and output 3.5 volts. The kits require a drill, soldering iron, volt meter, some tools and mechanical aptitude. For the genuine do-it-yourselfer, he provides detailed instructions on how to make your own mercury battery adapter.
 

The MR-9 Adapter is a sleeve which accepts the readily available 1.55 volt silver oxide 76 cell. The outside dimensions of the MR-9 approximately match the size of original PX13/625 mercury cells. (Because it's slightly thicker, the battery cover may not screw in as deeply as before.) Silver oxide cells share two characteristics with the banned mercury batteries: they yield consistent voltage throughout their lifespan and decline quickly when their life ends.

The adapter contains micro-circuitry which steps the voltage of the silver oxide 76 cell down to a consistent 1.35 volts.

The advantages offered by the MR-9 explain its popularity. The battery adapter is a one-time purchase and is reusable over and over again, your camera meter doesn't have to be recalibrated, no exposure compensation is necessary, and the silver 76 battery should last a year or more. In short, it's back to picture-taking as before.

The MR-9 Battery Adapter costs $29.95 (plus s&h) and can be purchased directly from the US importer, C.R.I.S. Camera in Chandler, Arizona. C.R.I.S. has an FAQ about the adapter on its web site and also lists more than 100 cameras and meters which can use the MR-9.

Now if someone would just come up with a solution to replace the 5.6 volt PX32 mercury battery required to operate my beautiful $300 Sekonic flash meter I'd be totally happy.

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