Which battery replaces the PX675?

The PX675 (IEC designation MR44, also known as EPX675, RM675, H-C, and V675PX) is a 1.35-volt mercury cell based on zinc–mercury oxide (Zn/HgO), manufactured from the 1960s onward by Mallory, Varta, and Duracell for light meters in analog cameras. Measuring 11.6 × 5.4 mm, it is significantly smaller than the related PX625 (15.6 × 5.95 mm) and was used in particularly compact camera bodies. Like all mercury cells, its voltage remains extremely stable throughout discharge, a feature essential for the correct operation of analog light meters.

The PX675 was used in numerous compact rangefinder and SLR cameras from the 1960s to the 1970s: Konica C35 and C35 family (Automatic, EF, FD, Flashmatic), Konica Auto S3, the complete Konica Autoreflex T series, Olympus Pen D3, Minolta Hi-Matic G and G2, Petri Color 35 and Petri FT series, Chinon 35 series, Cosina 35 and Hi-Lite series, Ricoh 500 series, Vivitar 220/SL through 420/SL, Miranda Sensomat and Sensorex II. You can find the complete searchable compatibility list at the top of this page.

Mercury-containing button cells such as the PX675 were gradually banned in the EU. The original Battery Directive 2006/66/EC initially still included an exception for button cells (up to 2% mercury by weight was permitted). Only Amendment Directive 2013/56/EU closed this loophole; as of October 1, 2015, button cells may no longer contain significant amounts of mercury either. In Germany and the United Kingdom, however, the import of PX675 had effectively already been illegal since September 18, 1992, under the predecessor Directive 91/157/EEC. In the United States, a comparable ban has been in effect since 1996 under the Mercury-Containing and Rechargeable Battery Management Act. The reason is not the battery itself, but the environmental contamination caused by mercury when it is disposed of improperly.

The PX675 has a technical peculiarity that makes replacement considerably easier than with the larger PX625: Modern type 675 zinc-air hearing-aid batteries (IEC designation PR44) have exactly the same form factor as the old PX675 (11.6 × 5.4 mm) and are therefore a direct, mechanically compatible replacement that fits most cameras without any adapter. Nevertheless, each option has its own advantages and disadvantages:

• Type 675 zinc-air hearing-aid batteries (direct replacement): The most affordable and, for most PX675 cameras, simplest solution. It fits directly into the battery compartment and provides 1.4 V (close enough to the mercury reference value of 1.35 V) with a flat discharge curve. Disadvantage: Once activated (after removing the protective foil), the cell lasts only about three to six weeks because it continuously consumes oxygen from the air. Available from Ausgeknipst as a type 675 zinc-air refill pack.
• Wein-Cell MRB675: Zinc-air cell specially manufactured for PX675 cameras, with reduced air supply and therefore a longer service life than standard hearing-aid batteries. Relatively expensive and available only from specialist photography shops.
• Rechargeable battery (Varta V80H) with charger: NiMH cell with a nominal voltage of 1.2 V. This is approximately 0.15 V below the mercury reference value of 1.35 V and results in slight systematic overexposure of around half a stop (not critical with color-negative film, but already visible with slide film). The advantage is the flat discharge curve and reusability. Note: The V80H primarily corresponds to the PX625 in terms of form factor; a simple spacer ring or adapter may be required for PX675 compartments. Available from Ausgeknipst as a USB-C charging set with V80H.
• Adapter with Schottky diode for SR44: Regulates a standard silver-oxide SR44 down to 1.35 V. Advantage: Standard batteries are available everywhere and have a long shelf life. Disadvantage: The camera's battery check often does not work correctly, and the adapters are expensive. Currently offered mainly by Kanto Kamera (Japan); an Ausgeknipst version is in development.

That depends on the camera, film, and replacement battery selected. A voltage higher than 1.35 V (alkaline 1.5 V, silver oxide 1.55 V) makes the light meter assume that there is too much light → the camera closes the aperture or shortens the exposure time → approximately one to 1.5 stops of underexposure. A lower voltage leads to overexposure, conversely. With slide film, every exposure error is critical; color-negative film tolerates one to two stops and is significantly more forgiving of overexposure than underexposure, so slight undervoltage is generally the lesser evil.

However, even more important than the absolute voltage is its consistency across the entire discharge curve. Alkaline cells (LR44, V625PX) continuously lose voltage and provide fluctuating readings over their lifetime. They are particularly unsuitable for analog light meters. Mercury cells such as the PX675, by contrast, maintained a constant 1.35 V over around 95% of their discharge, which was precisely the reason for their historical success. Zinc-air batteries (675 hearing-aid batteries, Wein-Cell) match the mercury target voltage very closely at 1.4 V and also have a flat discharge curve. NiMH (Varta V80H) has the same flat discharge curve, but at 1.2 V it is around 0.15 V below the target voltage; as a result, the V80H battery causes slight overexposure (~½ stop), which is uncritical with color-negative film. Silver-oxide cells (SR44) are also voltage-stable, but at 1.55 V they are too high and therefore require the diode adapter.