richard oleson , Jul 04, 2005; 06:31 p.m.
The standard replacement battery is the 625A Alkaline, which is readily available. The Alkaline battery has an initial voltage of 1.5 instead of the correct 1.35 volts. In addition, the voltage of the Alkaline battery drops off gradually while that of the Mercury cell maintained a very constant voltage over the life of the battery.
The effect of this is that, with a fresh 625A battery, the OM1 meter will read high and you'll get some underexposure. As the battery ages, the voltage will drop and you'll get good exposures for a fairly long period of time, and then as it continues to drop you will begin to see overexposure. If you aren't paying attention, this can eventually become significant - it can reach as much as 2-3 stops.
There are alternatives. One is the CRIS adapter, which lets you use a 1.5 volt silver cell and corrects the voltage for you; another is to just put a $1, 1.4 volt #675 hearing aid battery in the camera. Either of these will give acceptable results. It's also possible to have the camera modified for a 1.5 volt cell, but since these cells are not the same size as the 625 the CRIS adapter seems to me the logical way to use a silver cell in an OM1.
Personally, I use the hearing aid batteries. Here's everything I know on the subject.