Battery type PX21: Compatible cameras

The PX21 is a cylindrical battery made up of three cells connected in series. Dimensions: approximately 16.8 mm in diameter × about 50 mm long—so it is not a button cell, but a long, slim battery. It was originally produced from the late 1960s as the mercury cell 3MR50 (3 × 1.35 V = 4.05 V), mainly by Mallory (TR-133) and Eveready (E133). In the mid-1990s, it was switched to alkaline-manganese chemistry. Its successor is called 3LR50 and is now sold under designations such as V21PX, A21PX, or RX-2, with a nominal voltage of 4.5 V. Unlike the PX625, there is therefore a direct, legally available successor in the same form factor.

The PX21 primarily powered electronically controlled Praktica SLR models from Dresden from the 1970s and early 1980s, namely the L series with the “Electric” mount: Praktica LLC, PLC, PLC2, PLC3, VLC, VLC2, VLC3, EE, EE2, EE3, LTL 2, DTL, DTL2, DTL3, and Electronic 24×36. Outside Praktica, it was also used by electronic Compur leaf shutters in sizes 0, 1, 3, 5, and 5 FS (made by Deckel for medium- and large-format cameras), the Voigtländer Vitessa 126 S electronic, and the Paillard Bolex H16. You can find the complete list of all 23 camera models in the searchable table at the top of this page.

The PX21 dates back to a time when every battery manufacturer assigned its own type number and there was no uniform IEC standard for this form factor. 3MR50 was the original mercury type (three mercury cells, ~50 mm long), while 3LR50 is today’s alkaline IEC designation. Other manufacturer-specific names are also used: Mallory TR-133, Eveready E133, Varta V21PX, Duracell/Exell A21PX, and Kodak RX-2. When researching or ordering, it is worth trying several of these synonyms. They all refer to the same 16.8 × 50 mm stick battery.

With the PX21, the situation is less difficult than with the PX625 because a modern successor in the original format remains available. Nevertheless, each option has its own strengths and weaknesses:

• Alkaline original 3LR50 / V21PX / A21PX (4.5 V): The direct, factory-made successor. Fits without an adapter and is available online in Varta, Exell, or Duracell versions. Disadvantage: typical alkaline discharge curve. The voltage continuously drops over the battery’s lifetime, which can cause slight exposure-value drift in some camera models. The price per cell is now usually €10–15.
• Ausgeknipst PX21 adapter + 3× LR44: Sleeve adapter that converts three LR44 button cells (3 × 1.5 V = 4.5 V) to the PX21 format. Advantage: LR44 cells are available in any supermarket or drugstore for a few euros, while the adapter is a one-time purchase and built to last. Identical in voltage to the original 3LR50. Available as a PX21 adapter with three LR44 cells.
• Ausgeknipst PX21 adapter + 3× SR44 (silver oxide): Same sleeve, but filled with silver-oxide button cells (3 × 1.55 V = 4.65 V). Advantage: SR44 cells have a significantly flatter discharge curve than LR44 cells and maintain an almost constant voltage over most of their service life, making them technically cleaner for the light meter. Disadvantage: SR44 cells are slightly more expensive than LR44 cells and harder to find in drugstores.

Most PX21 cameras, especially the Praktica L series (LLC, PLC, VLC), were already specified at the time of manufacture for the 4.5 V alkaline version and use a bridge circuit in the light meter. This circuit measures voltage ratios rather than absolute values and is therefore largely tolerant of the difference between 4.05 V (mercury) and 4.5 V (alkaline). A voltage higher than the specified value would fundamentally result in underexposure. The light meter assumes there is more light than is actually present and closes the aperture or shortens the shutter speed. In most PX21 cameras, however, this effect is barely measurable because of the bridge circuit.

More important than the absolute voltage is its consistency across the entire discharge curve. Alkaline cells (LR44, 3LR50) decrease in voltage continuously and deliver fluctuating readings over their service life. Silver-oxide cells (SR44), by contrast, maintain an almost constant voltage over around 90–95% of their discharge, very similar to the behavior of the original mercury cells. For demanding applications with slide film, the SR44 version is therefore the technically cleanest choice; for negative film, which can easily tolerate one to two stops of over- or underexposure, the LR44 version is perfectly adequate in practice.