Which battery replaces the PX640?

The PX640 is a 1.35-volt mercury cell (mercury oxide–zinc chemistry) manufactured from the 1960s onward by Mallory, Varta, and Eveready, mainly for light meters and drive systems in analog and movie cameras. It is the significantly taller sibling of the better-known PX625: dimensions of approximately 15.6–16.0 × 11.2 mm, making it almost twice as tall as the PX625 with the same diameter, often described as a “fat button” or “barrel cell.” Equivalent designations: HM-N (Mallory), H-N / E640 (Eveready), NR52 (National), MR52 / LR52 (IEC), V640PX (Varta).

The PX640 was used in a range of rangefinder and cine cameras from the late 1960s and 1970s: Yashica Electro 35 FC and Electro 35 GX, Olympus 35 EC / EC2 / ECR / ED, Minolta Hi-Matic 5, Hi-Matic E, Hi-Matic F and CS, Fuji/Fujica Date and GA / GE / GER, Canon Canodate E, Canon Cine Canonet 8, Canon Date-Matic, Canon Motor Zoom 8 EEE, Chinon 35 EE-II, Ricoh Elnica 35 / 35M / F, Petri Computor II (requires two cells) and Petri ES Auto, Miranda Sensoret, as well as light meters such as the Pentax/Honeywell Spotmeter series and the Sekonic L-88 Microlite. You can find the complete searchable list of all 41 models at the top of this page.

Mercury-containing button cells such as the PX640 were banned in the EU under the Batteries Directive 2006/66/EC. The last legal stocks were used up around 2009. In the USA, 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 pollution caused by mercury when it is disposed of improperly. Since the PX640 was used in only a limited number of camera models anyway, no manufacturer ever produced a direct 1:1 replacement, unlike the popular PX625.

There is no single “best” replacement; each of the following options has its own strengths and weaknesses:

• Type 675 hearing-aid batteries + PX640 adapter: Type 675 zinc-air cell with a spacer ring to fit the larger PX640 dimensions. Correct 1.35–1.4 V; these cells are widely available as inexpensive standard hearing-aid batteries. Before inserting, remove the protective sticker and leave the cell exposed to air for 5 minutes so the voltage can stabilize. At Ausgeknipst as the 675 → PX640 adapter set.
• Wein-Cell (MRB640): Zinc-air cell in the original PX640 size with the correct 1.35 V. Relatively expensive, available only from specialist photography shops, and after activation it lasts only around three to six weeks, like all zinc-air cells.
• Rechargeable NiMH battery in PX640 form factor: NiMH cells provide approximately 1.2–1.3 V, slightly below the target value, and are rarely available on the market in the appropriate barrel geometry; for the PX640, there is no established standard solution like the V80H set for the PX625.
• Adapter with a Schottky diode for SR44/LR44: Regulates a standard silver-oxide SR44 down to around 1.35 V. Advantage: standard button cells are available everywhere and have a long shelf life. Disadvantage: the camera's battery check often does not work correctly, the adapters are expensive, and they are complicated to make for the tall PX640 form factor. Currently offered mainly by Kanto Kamera (Japan); an Ausgeknipst version is in development.

That depends on the camera, film, and replacement battery selected. A higher voltage than 1.35 V (alkaline 1.5 V, silver oxide 1.55 V) causes the light meter to assume there is too much light → approximately one to one and a half stops of underexposure. A lower voltage leads conversely to overexposure. 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 of two evils.

However, more important than the absolute voltage is its consistency across the entire discharge curve: alkaline and standard silver-oxide cells continuously lose voltage and deliver fluctuating readings throughout their service life. This made them particularly unsuitable for analog light meters. Mercury cells, by contrast, maintained a constant 1.35 V for around 95% of their discharge, which was precisely the reason for their historical success. Zinc-air cells (Wein-Cell and 675 hearing-aid batteries) and NiMH behave similarly consistently and are therefore the technically cleaner replacement options.