Actually, all Spotmatic models had one sort or another of bridge metering circuit, and are semi-independent of battery voltage. What you'll see is that as the battery voltage drops, meter needle deflection gets smaller, but the null (which is a simple comparison of metering cell resistance against a resistance selected by the ASA/speed dial) remains accurate (barring age-related problems with the dual CdS cells). I've used mine with silver oxide batteries for years, never a problem (though I don't shoot many slides) until coming up with an apparent shorted CdS cell recently.
GMA's advice is correct for nearly every other battery operated meter of the day, however, whether installed in a camera or hand held -- most of them used mercury cells because of their long life and flat voltage curve as they discharged, and depend on the cell to provide a reference voltage; those require expensive and unreliable adapters, or short-lived zinc-air cells to give accurate results.
Do watch polarity, though my 1967 SP only runs the meter in reverse (needle goes to minus with too much light) if I get the battery in backward. You can't reverse the original cell, but the modern ones, in an adapter, can be installed backward with a little effort...
