Which battery replaces the PX14?

The PX14 (IEC designation 2MR9, also marketed as Mallory PX14, Eveready EPX14, or Varta V14PX) is a 2.7-volt mercury battery from the 1960s and 1970s. Internally, it is a stacked battery consisting of two MR9 mercury cells connected in series (each with 1.35 V Hg-Mn chemistry, identical in construction to the interior of a PX625) in a shared cylindrical metal housing. Dimensions: approx. 16.8 × 16.0 mm. Like its smaller sibling, the PX625, it delivered a very constant voltage throughout most of its discharge. This was essential for accurately operating light meters in cine cameras.

The PX14 was installed almost exclusively in late-1960s and 1970s 8 mm and Super 8 film cameras, where it powered the built-in light meter. Typical models include the Fuji/Fujica Single-8 series (P1, P100, P300, Z1, Z2, Z400, C100, NP300), the Nikon Nikkorex 8 and 8F, the Konica Single-8 3-TL and 6-TL, the Bell & Howell Autoload series (306, 311, 430, 440, 490, 8432), the Chinon Pacific and Power Zoom models (310, 410, 471, 609-672), and numerous Cosina Dart and Super-Eight models. You can find the complete searchable list of 60 cameras from 8 brands at the top of this page.

The PX14 was subject to the same mercury bans as the PX625: in the EU under the Batteries Directive 2006/66/EC, and in the USA as early as 1996 under the Mercury-Containing and Rechargeable Battery Management Act. The last legal remaining stocks were exhausted around 2009. Today, third-party manufacturers still sell batteries labeled “PX14” (e.g. Exell as A14PX, V14PXA, and TR112) as alkaline variants. However, these deliver 3.0 V instead of the original 2.7 V and cause incorrect readings in unregulated light meters.

Because the PX14 is a dual cell, all serious replacement solutions are based on two individual cells connected in series. There is no universally “best” option; each has its own strengths and weaknesses.

• Zinc-air hearing-aid batteries type 675 + adapter: Two type 675 cells together deliver exactly 2.7 V under load (2 × 1.35 V). Their chemistry is similarly stable to the original mercury battery. We sell this solution as the PX14 adapter with a 6-pack of zinc-air cells. Disadvantage: once activated, zinc-air cells last only a few weeks to a few months.
• Wein-Cell MRB625 (stacked): Two Wein MRB625 zinc-air photo cells stacked together also provide 2.7 V. Wein-Cell does not offer a dedicated PX14 variant. The stack must be built yourself. More expensive and less readily available than the 675 solution, but slightly better sealed.
• Silver-oxide adapter with Schottky diode: Two SR44/357 silver-oxide cells (3.1 V open-circuit) are regulated to ~2.7 V via a Schottky diode. Advantages: standard batteries, long shelf life, and no degradation from air exposure. Disadvantages: more complex adapter, and the camera’s battery check often does not work reliably. Mainly offered by Kanto Kamera (Japan).
• Alkaline “replacement” A14PX / V14PXA / TR112: Sold under the PX14 name, but delivers 3.0 V instead of 2.7 V. In cameras without voltage regulation, this results in roughly half to one stop of underexposure, depending on the model. Useful only as an emergency solution or if the exposure metering is adjusted accordingly.

With cine cameras, it depends on what the PX14 actually powers in the particular camera. In most Single-8 and Super-8 models, it powers only the light meter or the servo drive of the automatic aperture. Film transport is mechanical or powered by separate AA cells. The same rule applies here as with still cameras: a higher voltage than 2.7 V (e.g. 3.0 V from an alkaline battery or 3.1 V from two unregulated SR44 cells) causes the light meter to assume that there is too much light → the automatic aperture closes down → slight underexposure. Lower voltage leads to overexposure, conversely. Negative film tolerates overexposure much better than underexposure, so slight undervoltage is generally the lesser evil.

More important than the absolute voltage is the consistency throughout the discharge curve: alkaline and standard silver-oxide cells continuously lose voltage, resulting in fluctuating readings over their service life. Mercury cells, by contrast, maintained a constant voltage of 1.35 V per cell for about 95 percent of their discharge, which was precisely why they were so successful in light meters. Zinc-air (type 675, Wein-Cell) provides a similarly consistent curve and is therefore the technically cleanest replacement option. If the PX14 powers only a motor in a particular camera, the voltage deviation is less critical. There, it primarily changes the frame rate, not the exposure.