The PX625 Mercury Battery Problem

by Alex

Mercury batteries provide a pretty consistent voltage over the course of their life, so this was quite an ideal choice when designing a camera back in the day. Light meters require an solid voltage to be accurate, so using mercury batteries was a pretty good way to skirt the added cost of a voltage stabaliser in the design. This was a brilliant idea at the time, until the environment concern of all this mercury being thrown into the garbage was realised…mercury batteries were banned, and now quite unavailable for good reason.
The most common of these batteries used in cameras was the PX625 battery, also known as MR9, 1124MP, PX625 or EPX625. A mercury based cell that delivered a firm voltage of 1.35v, and frequently found use in older cameras like Canonet rangefinders and Practika SLR’s. If you buy enough old cameras you’ll find one that requires this battery.
Now the replacement cells, both alkaline (A625PX) and silver (S625PX), deliver 1.5v. As such you’ll need to get your camera re-calibrated to make up for this voltage difference, which can be done by any competent repairman. You can also use a Zinc-Air battery (Z625PX), which works on the reaction of Zinc with air (duh), and deliver 1.4 volts. You could safely use these without recalibration I’d expect, but Zinc-Air batteries have a rather short life span. Wein Cell batteries, working on the same technology deliver an accurate 1.35v – available from [Microtools] and, as with anything, ebay.
Something else I’ve ran across recently though is the [C.R.I.S mercury battery adapter]. These take a 1.5v S386 Silver-Oxide battery, and incorporate a diode to step the voltage down to 1.35 volts. Basically a perfect replacement without any recalibration, and 386 batteries are quite common, being used in many watches.
The price, however, is inhibitive. $33USD for one adapter, but they’re reusable of course.

Links:
[photobattery.com] – Loads of batteries, selling all alternatives mentioned above.
[C.R.I.S.] – Mercury battery adapters.
[Microtools] – Sells an array of batteries, and other cool stuff