Battery type PX25 — Compatible cameras

The PX25 (also V25PX) is a mercury-containing stacked battery with a nominal voltage of 3.9 V. Internally, it consists of three stacked mercury cells of 1.35 V each and was produced from the 1960s by Mallory, Eveready, and Varta mainly for CdS light meters in small film cameras. Dimensions: about 16.5 mm diameter × 21 mm height — a small cylinder, not to be confused with the much flatter 1.35 V PX625 button cell.

The PX25 is a niche format and is found almost exclusively in the Super-8 cameras of the early 1970s Honeywell-Elmo family — namely the Honeywell Elmo Dual Filmatic (identical to the Elmo C200) and the Honeywell Elmo Tri Filmatic (identical to the Elmo C300). In both models, the PX25 powers only the CdS light meter; the film transport motor runs separately on four AA cells. The full list can be found in the table at the top of this page.

Mercury-containing batteries like the PX25 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.

There is no 1:1 replacement — the following four paths are established in the community, each with its own strengths and weaknesses:

• Exell A25PX (Alkaline, 4.5 V): Dimensionally identical commercial drop-in replacement, easily available through photo specialty dealers. Disadvantage: about 0.6 V higher than the original voltage — the light meter tends to show about half to a full stop underexposure, plus the typical declining discharge curve of alkaline batteries over their lifespan.
• Three zinc-air hearing aid batteries Type 675 in the adapter tube: Delivers about 4.0 to 4.2 V under load — very close to the original voltage of the mercury PX25. Voltage remains constant over the entire discharge. Disadvantage: lifespan after opening only about three to six weeks, plus it requires a suitable mechanical adapter.
• Three silver oxide button cells SR44 in the adapter tube, optionally with Schottky diode: Three stacked SR44 cells provide about 4.65 V; a Schottky diode connected in series (e.g., BAT43) lowers the voltage by about 0.3–0.4 V toward the target. Advantage: standard button cells available everywhere, very long lifespan. Disadvantage: camera battery check does not always work correctly, the adapter is a DIY or small-series product.
• ISO compensation on the light meter: Those using the dimensionally fitting Exell A25PX (4.5 V) can permanently compensate for the measurement deviation by adjusting the ISO dial by half to a full stop — the cost-neutral option without an adapter, but requiring discipline.

Yes, but the direction and severity depend on the camera, film, and chosen replacement battery. Higher voltage than the 3.9 V of the original PX25 (e.g., 4.5 V alkaline in Exell A25PX or 4.65 V in SR44 stack) causes the light meter to accept too much light — the camera closes the aperture further, resulting in about half to a full stop 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 cells like the Exell A25PX continuously lose their voltage and deliver fluctuating readings over their lifetime — this exact aspect made them problematic for analog light meters. Mercury cells, on the other hand, kept their voltage constant over about 95% of the discharge — the historical reason for their success. Zinc-air (Type 675) and silver oxide (SR44) behave similarly consistently and are therefore the technically cleaner replacement options.