PX400 Battery: Compatible Cameras

The PX400 is a 1.35-volt zinc-mercury oxide button cell (Zn-HgO) measuring 11.2 × 3.6 mm. From the 1960s onward, Mallory (as the RM400), Eveready (EPX400) and other manufacturers made it mainly to power the light meters of smaller SLRs. Like all mercury cells, it delivered extremely stable voltage across roughly 95% of its discharge, which is why it caught on for analog light meters. IEC designation: MR42.

The PX400 was used mainly in the Pentax Spotmatic family: the Pentax Spotmatic SP, SP-II, SP-IIA, SP500 and SP1000, plus their sister models sold in the US under the Honeywell Pentax label. It also powered the early Fujica SLRs, the ST601 and ST701. You’ll find the complete, searchable list of all compatible cameras at the top of this page.

Pure mercury oxide cells like the PX400 were effectively banned from 2000 onward under the EU amending directive 98/101/EC, because their mercury content was well above the permitted limit. The Battery Directive 2006/66/EC later confirmed the ban. Worldwide production had already wound down by the late 1990s, and new PX400 cells haven’t been legally available since. The problem isn’t the battery itself but the environmental damage mercury causes when batteries are disposed of improperly.

First, an important point: at 11.2 × 3.6 mm, the battery compartment in the Spotmatic family is very shallow. Unlike with the larger PX625, an LR44 or SR44 (5.4 mm tall) simply won’t fit. The sensible options are:

• P312 zinc-air hearing aid battery + adapter: At about 3.5 mm tall, the P312 is thin enough for the PX400 compartment. Under load it delivers a steady 1.35 V, exactly the original voltage. Available from Ausgeknipst as an adapter set with P312 batteries or as a P312 refill pack. Downside: once you peel off the protective tab, the cell lasts only about 4-8 weeks, even if you don’t use it. Recommended for all PX400 cameras, and essential for the Fujica ST601/ST701.
• Wein Cell-equivalent button cells: Specialty zinc-air cells sold by photo retailers, chemically identical to the P312 but more expensive and harder to find.
• Alkaline/silver oxide cell of the right height (e.g. 387S, 392): These fit physically but deliver 1.5-1.55 V instead of 1.35 V. That’s not a problem for the Pentax/Honeywell Spotmatic family (see the next question), but they are NOT recommended for the Fujica ST601/ST701, whose light meter is voltage-sensitive and gives noticeably skewed readings at 1.5 V.

On this point, the Spotmatic and the Fujica are two completely different worlds and shouldn’t be lumped together:

• Pentax/Honeywell Spotmatic family (SP, SP-II, SP-IIA, SP500, SP1000): The light meter is designed as a Wheatstone bridge circuit, which is insensitive to the supply voltage when balanced. A Spotmatic meters equally accurately with 1.35 V (mercury/zinc-air), 1.5 V (alkaline) or 1.55 V (silver oxide). The only bottleneck here is mechanical: matching the 11.2 × 3.6 mm form factor.
• Fujica ST601 and ST701: According to Camera-wiki, these models do NOT use a bridge circuit but a silicon photocell with an FET circuit. The light meter is explicitly voltage-sensitive and gives significantly wrong readings with modern 1.5 V alkaline or 1.55 V silver oxide cells. The ST601/ST701 must have 1.35 V, either from a zinc-air adapter (P312) or through a Schottky diode modification to the camera.

As a general voltage rule for other voltage-sensitive cameras: a higher voltage than specified makes a voltage-dependent light meter think there’s more light than there actually is. The camera stops down or picks a faster shutter speed, and the result is underexposed. Lower voltage, in turn, leads to overexposure. Negative film tolerates overexposure much better than underexposure, so a slight undervoltage tends to be the lesser evil. Even more important than the absolute value is stability throughout the discharge: alkaline and standard silver oxide cells lose voltage steadily and skew the readings over their lifespan, whereas mercury, zinc-air and NiMH cells hold their voltage largely constant.