Tipo de batería PX21: cámaras compatibles

La PX21 es una batería cilíndrica alargada con tres pilas conectadas en serie. Dimensiones: aproximadamente 16,8 mm de diámetro × unos 50 mm de longitud; no es una pila de botón, sino una varilla alargada. Originalmente se fabricaba desde finales de los años 60 como pila de mercurio 3MR50 (3 × 1,35 V = 4,05 V), principalmente por Mallory (TR-133) y Eveready (E133). A mediados de los años 90 se cambió a alcalina de manganeso. Su sucesora se llama 3LR50 y hoy se vende con denominaciones como V21PX, A21PX, V4034PX o RX-2, con una tensión nominal de 4,5 V. A diferencia de la PX625, existe una sucesora directa y legalmente disponible con el mismo formato.

La PX21 alimentaba principalmente los modelos SLR Praktica con control electrónico de los años 70 y principios de los 80 procedentes de Dresde, es decir, la serie L con montura «Electric»: Praktica LLC, PLC, PLC2, PLC3, VLC, VLC2, VLC3, EE, EE2, EE3, LTL 2, DTL, DTL2, DTL3 y Electronic 24×36. Fuera de Praktica, también la utilizaban los obturadores centrales electrónicos Compur de los tamaños 0, 1, 3, 5 y 5 FS (fabricados por Deckel para cámaras de formato medio y grande), la Voigtländer Vitessa 126 S electronic y la Paillard Bolex H16. La lista completa con los 23 modelos de cámara se encuentra en la tabla consultable de la parte superior de esta página.

The PX21 dates from a time when every battery manufacturer assigned its own type number and there was not yet a uniform IEC standard for this form factor. 3MR50 was the original mercury type (three Mn cells, ~50 mm long), while 3LR50 is the current alkaline IEC designation. Manufacturer-specific names also circulate: Mallory TR-133, Eveready E133, Varta V21PX or V4034PX, 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 cylindrical battery.

With the PX21, the situation is more relaxed 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-ready 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 its service life, which can cause a slight drift in exposure values with some camera models. The price per battery 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 just a few euros, while the adapter is a one-time purchase and durable. Identical in voltage to the original 3LR50. Available as an 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 more suitable for the light meter. Disadvantage: SR44 cells are somewhat more expensive than LR44 cells and harder to find in drugstores.

Most PX21 cameras, particularly the Praktica L series (LLC, PLC, VLC), were already specified for the 4.5 V alkaline version when they were manufactured 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 nominal value would in principle result in underexposure. The light meter assumes there is 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 thanks to the bridge circuit.

More important than the absolute voltage is its consistency throughout the entire discharge curve. Alkaline cells (LR44, 3LR50) steadily lose voltage and provide 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 soundest choice; for negative film, which can easily tolerate one to two stops of overexposure or underexposure, the LR44 version is entirely sufficient in practice.