The meter system does not rely on the voltage of the battery by itself, but compares voltages between two parts of a circuit. The first part has a set resistance and the second part has the CdS cell (photo-resistor). Once this has been calibrated for the resitnace of the CdS cell, the input volatge is not so important (within certain limits).
The Spotmatics (and the K1000) used a 0-meter or a needle that goes to the 0 position when balanced.
I have used silver batteries in the spotmatic for ages, in fact my dad did so in the mid 80s when mercury batteries became expensive. The only think I've found is that fresh Silver oxide batteries tend to make the needle overreact to changes, after a month or so the movement is a lot smoother and works like that for years. (a 395 cell should last 2+ years)
After all this hoollaballo of replacing with zinc-air batteries started about a couple of years ago, I bit the hook and spent a lot of money on those batteries... they last a month or two. So I checked with a Minolta SpotMeter and a 395 battery... it was within 1/3 stop for bright sky, neutral wall and shady spots so I decided to save me some money and let the camera run as it should.
I'd reccomend joining the Spotmatic list (http://groups.yahoo.com/group/Spotmatic/), where there are documents on replacing batteries as well. Diagrams of the metering circuits can be found there as well
Originally Posted by cao