Battery type PX14 — Compatible cameras

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. Inside, it is a stacked battery made of two MR9 mercury cells connected in series (each 1.35 V Hg-Mn chemistry, identical to the inside of a PX625) in a common cylindrical metal casing. Dimensions: approx. 16.8 × 16.0 mm. Like its smaller sibling the PX625, it delivered a very constant voltage over most of the discharge — a prerequisite for precisely working light meters in cine cameras.

PX14 was used almost exclusively in 8mm and Super-8 film cameras from the late 60s and 70s, each for the built-in light meter. Typical models are 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), as well as numerous Cosina Dart and Super-Eight models. The complete searchable list with 60 cameras from 8 brands can be found at the top of this page.

The PX14 fell under the same mercury bans as the PX625: in the EU with Battery Directive 2006/66/EC, in the USA since 1996 with the Mercury-Containing and Rechargeable Battery Management Act. The last legal stocks were used up around 2009. Today, the name "PX14" is still sold by third-party manufacturers (e.g., Exell as A14PX, V14PXA, TR112) as an alkaline variant — but this delivers 3.0 V instead of the original 2.7 V and causes misreadings in unregulated light meters.

Since the PX14 is a double cell, all serious replacement solutions are based on two single cells connected in series. There is no universally "best" way — each option has its own strengths and weaknesses:

• Zinc-air hearing aid batteries type 675 + adapter: Two cells type 675 together deliver exactly 2.7 V under load (2 × 1.35 V). Chemically similarly stable as the original mercury cell. We sell this solution as PX14 adapter with a 6-pack of zinc-air cells. Disadvantage: Zinc-air cells are only usable for a few weeks to a few months after activation.
• Wein-Cell MRB625 (stacked): Two pieces of the zinc-air photo cell MRB625 from Wein stacked also yield 2.7 V. Wein-Cell does not offer a dedicated PX14 variant — the stack must be built yourself. More expensive and less available than the 675 solution, but somewhat 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. Advantage: standard batteries, long shelf life, no decay from air contact. Disadvantage: complex adapter, camera 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 causes about half to a full stop of underexposure depending on the model. Only sensible as an emergency solution or if the exposure measurement is adjusted anyway.

For cine cameras, it depends on what the PX14 actually powers in the respective camera. In most Single-8 and Super-8 models, it only powers the light meter or the servo drive of the automatic aperture — the film transport runs mechanically or via separate AA cells. The same rule applies here as with still cameras: a voltage higher than 2.7 V (e.g., alkaline 3.0 V or unregulated 2× SR44 with 3.1 V) causes the light meter to accept too much light → the automatic closes the aperture → slight underexposure. Lower voltage leads to overexposure. Negative film forgives overexposure much better than underexposure, so slight undervoltage is generally the lesser evil.

Even more important than the absolute voltage value is the consistency over the discharge curve: alkaline and standard silver oxide cells continuously lose their voltage and thus deliver fluctuating readings over their lifetime. Mercury cells, on the other hand, kept their voltage constant at about 1.35 V per cell over about 95 percent of the discharge — exactly 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 in a particular camera only powers one motor, the voltage deviation is less critical — it primarily changes the frame rate there, not the exposure.