Battery type PX25: Compatible cameras

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

The PX25 is a niche format found almost exclusively in early-1970s Honeywell Elmo Super 8 cameras, 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. You can find the complete list in the table at the top of this page.

Mercury-containing batteries such as the PX25 were banned in the EU under the Batteries Directive 2006/66/EC. The last legal remaining stocks were used up around 2009. 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 impact of mercury when improperly disposed of.

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

• Exell A25PX (alkaline, 4.5 V): Commercial drop-in replacement with the same dimensions, readily available from specialist photo retailers. Drawback: around 0.6 V higher than the original voltage. The light meter tends to show about half to one stop of underexposure, plus the voltage drop over its lifespan typical of alkaline batteries.
• Three zinc-air hearing-aid batteries, type 675, in an adapter tube: Delivers around 4.0 to 4.2 V under load, very close to the original voltage of the mercury PX25. The voltage remains constant throughout the discharge. Disadvantage: once activated, they last only about three to six weeks, and a suitable mechanical adapter is also required.
• Three silver oxide button cells, SR44, in an adapter tube, optionally with a Schottky diode: Three stacked SR44s produce about 4.65 V; a Schottky diode connected in series (e.g. BAT43) lowers the voltage by around 0.3–0.4 V toward the target. Advantage: standard button cells are available everywhere and have a very long shelf life. Disadvantage: the camera's battery check does not always work correctly, and the adapter is DIY or small-batch merchandise.
• ISO compensation on the light meter: Anyone using the dimensionally compatible Exell A25PX (4.5 V) can permanently compensate for the measurement deviation by adjusting the ISO dial by half to one stop. The cost-neutral option without an adapter, but it requires discipline.

Yes, but the direction and severity depend on the camera, film, and replacement battery chosen. A higher voltage than the original PX25's 3.9 V (e.g. 4.5 V alkaline with an Exell A25PX or 4.65 V with an SR44 stack) makes the light meter assume there is too much light. The camera closes the aperture further, resulting in about half to one stop of underexposure. Lower voltage leads to overexposure, conversely. With slide film, any exposure error is critical; color negative film tolerates one or two stops and is noticeably more forgiving of overexposure than underexposure, so a slight undervoltage is generally the lesser evil.

More important than the absolute voltage, however, is its consistency across the entire discharge curve: alkaline cells such as the Exell A25PX continuously lose voltage and deliver fluctuating readings over their service life—this is precisely what made them problematic for analog light meters. Mercury cells, by contrast, maintained a constant voltage over around 95% of their discharge. That was 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.