Type: Silver Oxide. Voltage: 1.55 V.
Diameter: 11.2 mm (9.6mm without collar). Height: 3.6 mm.
Price: £5.50 each. £5.20 each if you buy two or more. Postage and packing included.
1.55 V Replacement for: PX400 / EPX400 / RM400 / H-B / V400PX / E400N / PM400R / KX400 / T400N / 387.
The S400PX (or 387S) is a replacement for the once popular Mercuric Oxide RM400 and PX400 coin type cell that was commonly used in cameras and is now banned because of toxicity and environmental fears.
It has a distinctive appearance appearance with its plastic collar.
These S400PX cells produce 1.55V which is different from the 1.35V produced by the original PX400 mercury cell.
Because the S400PX silver oxide cell produces a steady 1.55V, you are able to compensate for the higher voltage by a fixed number of stops or by adjusting the ASA setting. There may be as many as two f-stops underexposure by a camera that uses 1.55v instead of the original 1.35v, but the compensation you require is constant.
You may find alkaline equivalents that fit perfectly but, as with all alkaline cells the voltage is not stable, it drifts down from 1.5V towards 1.35V (the voltage of the original Mercury cell) and then lower still. As the voltage is not steady throughout the useful life of the cells, you must constantly adjust how you compensate.
Another PX400 replacement is a zinc air battery. The WeinCELL MRB400 is a custom zinc/air battery designed to replace banned mercury batteries. WeinCELLs deliver the same 1.35 V voltage and stability of output as did the mercury PX400 batteries so there is no need to adjust your exposure. The WeinCELL MRB400 batteries last much longer than hearing aid batteries, some times up to a year. To achieve this longer life, the MRB400 used a proprietary electrolyte and has only two small air holes, instead of the 7 larger holes found in standard hearing aid batteries.
Possibly the 'Rolls Royce' of RM400 replacements is the H-B adapter fitted with a silver oxide 377 cell. The H-B adapter is specifically for the Spotmatic range of cameras and replaces the battery compartment cover. It has a recess into which a 1.55V silver oxide 377 cell can be dropped and and contains voltage regulating electronics to drop the voltage to match the 1.35V of the original mercury cell. The H-B adapter is expensive, but it can be reused time and time again with inexpensive silver oxide 377 cells. Over time you may find the H-B adapter will actually save you money over the other options. The drawback is replacing the original battery compartment cover with a replacement and reducing the authenticity of the camera.
The final option, an exact voltage replacement, is suitable for watches such as the Accutron Tuning Fork watches and is known as the Accucell 1.
* Actual manufacturer may vary depending upon availability
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