Battery type PX625 — Compatible cameras

The PX625 is a 1.35-volt mercury battery based on the zinc-mercury oxide system (Zn/HgO — anode zinc, cathode mercury(II) oxide), produced from the 1960s by Mallory, Varta, and Duracell mainly for light meters in analog cameras. Its special feature was an extremely stable voltage curve throughout discharge — a property essential for the correct function of analog light meters. Dimensions: 15.6 × 6.3 mm.

The PX625 was used in a wide range of popular cameras from the 60s to the 80s: Olympus OM-1 and OM-1n, Leica M5 and Leica CL, Canon F-1 and Canonet QL17, Minolta SR-T 101 and SR-T 303, Pentax Spotmatic F, Yashica Mat-124G, Rollei 35 (early versions), Asahi Pentax K1000 (early series), and many more. The complete searchable compatibility list with over 200 camera models can be found at the top of this page.

Mercury-containing button cells like the PX625 were regulated in the EU by Battery Directive 2006/66/EC (in force since 26.09.2008); button cells with up to 2% mercury were initially exempt. The complete ban on mercury-containing button cells only followed with the amendment directive 2013/56/EU on 01.10.2015. In the USA, a comparable ban has been in place since 1996 under the Mercury-Containing and Rechargeable Battery Management Act. The reason is not the battery itself, but the environmental impact of mercury in improper disposal.

There is no single "best" replacement — each of the following options has its own strengths and weaknesses:

• Wein-Cell (MRB625): Zinc-air cell with correct 1.35 V. Relatively expensive, only available in photo specialty stores, lasts only three to six weeks after activation.
• Zinc-air hearing aid batteries + adapter: Chemically the same system as Wein-Cell, but as an affordable standard hearing aid battery type 675 with a matching adapter ring. Available at Ausgeknipst as a PX625 adapter plus a zinc-air refill pack.
• Rechargeable battery (Varta V80H) with charger: NiMH cell with cutoff charge just above 1.3 V — close enough to the target value for most light meters. Higher initial cost, but reusable with no follow-up costs. Available at Ausgeknipst as a USB-C charging set with V80H.
• MR-9 adapter with Schottky diode for SR43/386: Regulates a silver oxide SR43 (type 386) from 1.55 V down to 1.35 V (the common C.R.I.S. MR-9 adapter is designed for SR43, not the larger SR44 format). Advantage: standard silver oxide cells, long lifespan. Disadvantage: camera battery check often does not work correctly, and the adapters are expensive. Currently mainly offered by Kanto Kamera (Japan) — an Ausgeknipst version is in development.

It depends on the camera, film, and chosen replacement battery. Higher voltage than 1.35 V (alkaline 1.5 V, silver oxide 1.55 V) causes the light meter to accept too much light → about one to 1.5 stops underexposure. Lower voltage, conversely, leads to overexposure. With slide film, any misexposure is critical; color negative film forgives one to two stops and reacts much more forgivingly to overexposure than underexposure — so slight undervoltage is generally the lesser evil.

Even more important than the absolute voltage value is the consistency over the entire discharge curve: alkaline and standard silver oxide cells (LR9, PX625A, V625PX) continuously lose their voltage and deliver fluctuating readings over their lifetime — this made them particularly unsuitable for analog light meters. Mercury cells, on the other hand, kept their voltage constant at 1.35 V over about 95% of the discharge — exactly why they were historically successful. Zinc-air (Wein-Cell and 675 hearing aid batteries) as well as NiMH (Varta V80H) behave similarly consistently and are therefore the technically cleaner replacement options.