PX21 Battery: Compatible Cameras

The PX21 is a cylindrical stick battery made up of three cells wired in series. It measures roughly 16.8 mm in diameter and about 50 mm in length, so it isn’t a button cell but a slim, elongated stick. It was originally made from the late 1960s onwards as the 3MR50 mercury battery (3 × 1.35 V = 4.05 V), mainly by Mallory (TR-133) and Eveready (E133). In the mid-1990s it was switched over to alkaline manganese chemistry. Its successor is the 3LR50, sold today under names such as V21PX, A21PX or RX-2, now with a nominal voltage of 4.5 V. So unlike the PX625, there’s a direct, legally available successor in the same form factor.

The PX21 mainly powered the electronically controlled Praktica SLRs built in Dresden in the 1970s and early ’80s, namely the L series with the “Electric” bayonet mount: Praktica LLC, PLC, PLC2, PLC3, VLC, VLC2, VLC3, EE, EE2, EE3, LTL 2, DTL, DTL2, DTL3 and Electronic 24×36. Beyond Praktica, it was also used in the 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’ll find the full list of all 23 camera models in the searchable table further up this page.

The PX21 dates from a time when every battery maker assigned its own type number and there was no unified IEC standard for this format yet. 3MR50 was the original mercury type (three mercury cells, ~50 mm long), while 3LR50 is today’s IEC designation for the alkaline version. On top of that, you’ll come across various manufacturer-specific names: Mallory TR-133, Eveready E133, Varta V21PX, Duracell/Exell A21PX, Kodak RX-2. When you’re searching for one or placing an order, it’s worth trying several of these names. They all refer to the same 16.8 × 50 mm stick battery.

With the PX21, things are more relaxed than with the PX625, because a modern successor is still available in the original format. Even so, each option has its own strengths and weaknesses:

• Alkaline original 3LR50 / V21PX / A21PX (4.5 V): The direct, factory-made successor. It fits without an adapter and is available online from Varta, Exell or Duracell. The downside is the typical alkaline discharge curve: the voltage drops steadily over the battery’s life, which on some camera models causes the exposure readings to drift slightly. These days they usually cost €10–15 each.
• Ausgeknipst PX21 adapter + 3× LR44: A sleeve adapter that brings three LR44 button cells (3 × 1.5 V = 4.5 V) up to the PX21 format. The advantage: LR44s are sold in every supermarket and drugstore for a few euros, and the adapter is a one-time purchase that lasts. Its voltage is identical to the original 3LR50. Available as the PX21 adapter with three LR44 cells.
• Ausgeknipst PX21 adapter + 3× SR44 (silver oxide): The same sleeve, but loaded with silver oxide button cells (3 × 1.55 V = 4.65 V). The advantage: SR44s have a much flatter discharge curve than LR44s and hold their voltage almost constant for most of their life, which is technically cleaner for the light meter. The downside: SR44s cost a little more than LR44s and are harder to find in a drugstore.

Most PX21 cameras, especially the Praktica L series (LLC, PLC, VLC), were already specified for the 4.5 V alkaline version when they were built, and their light meters use a bridge circuit. This circuit measures voltage ratios rather than absolute values, so it is largely tolerant of the difference between 4.05 V (mercury) and 4.5 V (alkaline). In principle, a voltage higher than the nominal value would cause underexposure: the meter assumes there is more light than there actually is, and stops down the aperture or shortens the shutter speed. On most PX21 cameras, though, the bridge circuit keeps this effect barely measurable.

What matters more than the absolute voltage is how consistent it stays across the entire discharge curve. Alkaline cells (LR44, 3LR50) lose voltage steadily and give fluctuating readings over their life. Silver oxide cells (SR44), on the other hand, hold their voltage almost constant for around 90–95% of their discharge, very much like the original mercury cells. For demanding work with slide film, the SR44 version is therefore the technically cleanest choice; for negative film, which easily forgives one to two stops of over- or underexposure, the LR44 version is perfectly adequate in practice.