Battery type PX21 — Compatible cameras

The PX21 is a cylindrical rod battery with three cells connected in series. Dimensions: about 16.8 mm diameter × approximately 50 mm length — so not a button cell, but a long rod. Originally, from the late 1960s, it was made as a 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 — the successor is called 3LR50 and is now sold under names like V21PX, A21PX, V4034PX, or RX-2, with a nominal voltage of 4.5 V. Unlike the PX625, there is thus a direct, legally available successor in the same form factor.

The PX21 mainly powered the electronically controlled Praktica SLR models from the 1970s and early 80s made in Dresden — that is, the L-series with the "Electric" bayonet: Praktica LLC, PLC, PLC2, PLC3, VLC, VLC2, VLC3, EE, EE2, EE3, LTL 2, DTL, DTL2, DTL3, and Electronic 24×36. Outside of Praktica, they were also used in the electronic Compur central shutters 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 full list with all 23 camera models in the searchable table at the top of this page.

The PX21 comes from a time when every battery manufacturer assigned its own type number and there was no unified IEC standard for this form factor. 3MR50 was the original mercury type (three Mn cells, ~50 mm long), 3LR50 is today’s alkaline IEC designation. In addition, there are manufacturer-specific names: Mallory TR-133, Eveready E133, Varta V21PX or V4034PX, Duracell/Exell A21PX, Kodak RX-2. When researching or ordering, it’s worth trying several of these synonyms — they all mean the same cylindrical battery measuring 16.8 × 50 mm.

The situation with the PX21 is more relaxed than with the PX625 because a modern successor in the original format remains available. Still, each option has its own strengths and weaknesses:

• Alkaline original 3LR50 / V21PX / A21PX (4.5 V): The direct, factory-ready successor. Fits without an adapter, available online as Varta, Exell, or Duracell variants. 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. Price per piece is now usually 10–15 €.
• Ausgeknipst PX21 adapter + 3× LR44: 3D-printed sleeve adapter that converts three LR44 button cells (3 × 1.5 V = 4.5 V) to the PX21 format. Advantage: LR44 batteries are available in any supermarket or drugstore for a few euros, the adapter is a one-time purchase and durable. Identical in voltage to the original 3LR50. Available as 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 have a significantly flatter discharge curve than LR44 and maintain voltage almost constant over most of their lifespan — technically cleaner for the light meter. Disadvantage: SR44 are somewhat more expensive than LR44 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 variant and use a bridge circuit in the light meter. This circuit measures voltage ratios instead of absolute values and is therefore largely tolerant of the difference between 4.05 V (mercury) and 4.5 V (alkaline). A higher voltage than the target value would basically cause underexposure — the light meter assumes more light than is actually present and closes the aperture or shortens the exposure time. In most PX21 cameras, however, this effect is barely measurable due to the bridge circuit.

More important than the absolute voltage value is the consistency over the entire discharge curve. Alkaline cells (LR44, 3LR50) continuously lose voltage and deliver fluctuating readings over their lifespan. Silver oxide cells (SR44), on the other hand, maintain their voltage almost constant over about 90–95% of the discharge — very similar to the behavior of the original mercury cells. For demanding applications with slide film, the SR44 variant is therefore the technically cleanest choice; for negative film, which easily tolerates one to two stops of over- or underexposure, the LR44 variant is perfectly sufficient in practice.