Type: Mercuric Oxide. Voltage: 1.35 V.
Diameter: 11.2 mm (9.6mm without collar). Height: 3.6 mm.
Also known as: PX400 / EPX400 / V400PX / RM400 / H-B / E400N / PM400R / KX400 / T400N / 387.
The PX400 is is a discontinued Mercuric Oxide coin type cell that was commonly used in cameras and is now banned because of toxicity and environmental fears.
The original PX400 mercury cell produced 1.35V.
There are three possible replacements for this PX400 cell:
There are no alkaline cells manufactured of this size and shape.
The S400PX cells produce 1.55V which is different from the 1.35V produced by the original PX400 mercury cell. This doesn't normally cause a problem because today's films are very tolerant to under-exposure and because the under-exposure is compensated for during printing. (so you would have greater problems if you were using slides). If you do find you need to compensate, 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.
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 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.
A fourth possible replacement is the Accucell 1 used only for watches to replace the 387.
There is some further information available in our Frequently Asked Questions page. Look for the question 'Do you know of a replacement for the PX13 / PX625 / MR9 Mercury battery?' The same principles apply to replacing the PX400.
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