Battery type PX27: Compatible cameras

The PX27 is a 5.6-volt mercury battery (nominal voltage; 5.4 V under load) made up of four stacked NR43 mercury cells. Dimensions: around 13 mm diameter × 20.5 mm height. It was manufactured from the 1970s onward by Mallory, Varta and Duracell for compact camera bodies that simply had no room for a wider PX625. Common cross-reference designations: V27PX, 4NR43, H-4D, TR-164.

PX27 was used mainly in the Minox 35 family of the late 70s and 80s: Minox 35 EL, GL, GT, PL, PE and AL, as well as in the Minox subminiature cameras C, CLX, EC, ECX, LX and TLX, the Minox light meter and the FL4 flash. The late Minox 35 models from 1988 onward (GT-E, GT-E II, GS-E, GT-S, GT-X and the Goldknopf edition), on the other hand, don't take a PX27 but 2x CR 1/3N in the built-in battery holder. Several Rollei compact models also use PX27: Rollei 35 LED, 35SE, 35TE, A110 and E110. The Agfa Optima 335 and Optima Sensor Flash as well as the Hanimex VEF Zoom are among them too. You'll find the complete searchable list at the top of this page.

Like all mercury button cells, the PX27 was banned in the EU under the Battery Directive 2006/66/EC (and its amendment 2013/56/EU). The last remaining stocks were used up around 2009 to 2015. The reason is not the battery itself but the environmental damage caused by mercury when it is disposed of improperly. Compared with the PX625, this makes the PX27 doubly hard to replace: it is not only rarer in remaining stock but also unusual in its construction (4 stacked cells instead of a single button cell).

There is no single "best" replacement, each option has its own strengths and weaknesses:

• Four stacked SR44 (silver oxide) with adapter sleeve: Total voltage 6.2 V. Standard button cells available everywhere, long shelf life, constant voltage across the entire discharge curve. Available at Ausgeknipst as Minox 35 battery with adapter. The sleeve positions four SR44 correctly in the battery compartment and ensures electrical contact.
• Four stacked LR44 (alkaline) with the same sleeve: Voltage also around 6 V, considerably cheaper than SR44, but alkaline cells lose voltage continuously and thus skew the exposure metering over their lifetime. Not recommended for serious slide film photography.
• Exell S27PX (silver oxide) or Golden Power PX27A (alkaline): Both are direct replacement cells in the PX27 format that are still being produced, 6 V, usable without an adapter. Advantage: plug and play. Disadvantage: only available from specialist photo shops, expensive at around 15 to 20 € per cell and with lower capacity than the 4-cell solution. The silver oxide version (Exell) holds its voltage more constantly across the discharge curve and is therefore preferable for slide film; the alkaline version (Golden Power) is cheaper but electrically less stable.
• Hearing aid batteries type 675 with adapter (zinc-air): Closest to the original in terms of voltage (1.4 V per cell, four of them ≈ 5.6 V), available at Ausgeknipst as zinc-air refill pack. For the PX27 not currently available as a ready-made adapter set; an Ausgeknipst version with a Schottky diode for full exposure accuracy is in development.

Yes, but usually less dramatically than often feared. The classic Minox 35 models (EL, GL, GT, PL) have a bridge circuit in the light meter and are therefore relatively tolerant of voltage; electrically, the 6.2 V solution with four SR44 is uncritical. In practice, however, these models may show slight underexposure of about ½ to 1.5 EV, because the light meter assumes "more light" at higher voltage. You can compensate by setting the ISO dial half a stop to a full stop lower (e.g. ISO 200 film at ISO 100 or 125).

More important than the absolute voltage, however, is the consistency across the discharge curve: alkaline cells (LR44) lose voltage continuously and deliver drifting readings over their lifetime. Silver oxide (SR44), zinc-air (type 675) and mercury, by contrast, keep their voltage constant over most of the discharge, which is exactly what makes them the technically cleaner replacement options. For slide film, which reveals every exposure error, SR44 or the zinc-air version is therefore worth it; colour negative film easily forgives one to two stops of over- or underexposure.