Last Updated 98/12/16 1700PST
Button Batteries
The EverReady EPX 625 and the Mallory PX13 were identical in every way, 1.35 volts. KEH Camera Brokers carries a mercury replacement in stock.
The Wein zinc-air cell replacement is model# MRB625. These can be acquired from Porter's Camera Store @ $5.95 ea.
Some of the older cameras and light meters used a smaller mercury button cell, EverReady EPX 675. This is also a 1.35 volt battery, but smaller in diameter than the EPX625. The Wein zinc-air replacement for this is MR675.
A less expensive alternative to the Wein zinc-air cells are packaged and marketed as "hearing aid" batteries. Radio Shack sells a zinc-air hearing aid battery in packages of 4, currently on sale for $1.49. Model designation is RS675HP. The package is marked "1.4v," but I tested with a digital multimeter and got 1.357 volts - close enough. This battery is the same size and can be used as a direct replacement for the EverReady EPX675.
This Radio Shack RS675HP can also be used as a replacement for Mallory PX13 or EverReady EPX625, which are same voltage but larger in diameter. From a hardware/plumbing suppy store one can acquire a rubber O-ring type fawcet washer which fits around the battery, and will hold it in place inside the larger 625 sized chamber.
I have tested the Radio Shack RS675HP in a hand-held light meter calling for the EPX675. It fits perfectly, and gives accurate readings. The zinc-air cells may require about 1/2 hour to activate after removing the peel-off air seal, and come up to full voltage. So long as the air seal remains in place, shelf life is claimed to be 10 years.
By the way, Radio Shack isn't the only source for those hearing aid batteries. Eveready, Duracell, and perhaps all the other battery manufacturers produce them as well - so they can be found at Wal-mart, K-mart or any drug store. So long as the number "675" appears somewhere in the model designation, they should work fine.