Weincell MRB400 1.35V Zinc Air Battery Z400PX PX400 EPX400 RM400
General Uses: Cameras,( Vintage Spotmatic Pentax)
Also known as: PX400, EPX400, RM400, H-B, V400PX, E400N, PM400R, KX400, T400N
More specific uses: Cameras,( Vintage Spotmatic Pentax)
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 two 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.