What is a PX640 battery?
The PX640 is a 1.35-volt mercury cell (mercury oxide-zinc chemistry) that was mainly produced from the 1960s by Mallory, Varta, and Eveready for light meters and drive systems in analog and super 8 cameras. It is the significantly taller sister of the better-known PX625: dimensions about 15.6–16.0 × 11.2 mm — almost twice as tall as the PX625 with the same diameter, often described as "fat button" or "barrel cell." Equivalent designations: HM-N (Mallory), H-N / E640 (Eveready), NR52 (National), MR52 / LR52 (IEC), V640PX (Varta).
Which cameras need a PX640 battery?
PX640 was used in a number of rangefinder cameras and Super 8 cameras from the late 60s and 70s: 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 as well as 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 like the Pentax/Honeywell Spotmeter series and the Sekonic L-88 Microlite. The complete searchable list with all 41 models can be found at the top of this page.
Why is the PX640 no longer available for purchase?
Mercury-containing button cells like the PX640 were banned in the EU by Battery 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 impact of mercury in improper disposal. Since the PX640 was only used in a limited number of camera models, no manufacturer ever produced a direct 1:1 replica — unlike the popular PX625.
What options are there for PX640 replacements, and what are their respective pros and cons?
There is no single "best" replacement — each of the following options has its own strengths and weaknesses:
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Hearing aid batteries type 675 + PX640 adapter: Zinc-air cell type 675 with spacer ring to the larger PX640 geometry. Correct 1.35–1.4 V, cells are widely available as inexpensive standard hearing aid batteries. Remove the protective sticker before inserting and let it sit in the air for 5 minutes to stabilize the voltage. Available from Ausgeknipst as adapter set 675 → PX640.
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Wein-Cell (MRB640): Zinc-air cell in PX640 original size with correct 1.35 V. Relatively expensive, only available in specialty photo stores, lasts only about three to six weeks after activation like all zinc-air cells.
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Rechargeable NiMH battery in PX640 form: NiMH cells are slightly below the target voltage at about 1.2–1.3 V and are rarely available on the market in the matching can geometry — for the PX640 no established standard solution like the V80H set for the PX625.
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Adapter with Schottky diode for SR44/LR44: Regulates a normal silver oxide SR44 down to about 1.35 V. Advantage: standard button cells available everywhere, long shelf life. Disadvantage: camera battery check often does not work correctly, adapters are expensive and complex for the large PX640 form factor. Currently mainly offered by Kanto Kamera (Japan) — an Ausgeknipst version is in development.
Does the voltage difference in the replacement really make a difference in exposure?
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 leads conversely to overexposure. With slide film, any misexposure is critical; color negative film forgives one to two stops and reacts much more forgivingly to over- than underexposure — a slight undervoltage is therefore 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 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 — this was exactly the reason for their historical success. Zinc-air (Wein-Cell and 675 hearing aid batteries) as well as NiMH behave similarly consistently and are therefore the technically cleaner replacement options.
