Review on Exell MRB400 Zinc Air Battery: Long-Lasting Power for Z400PX, E400N, EPX400, H-B, KX400, PM400R, PX400, RM400, T400N and More by Vanessa Harrison

I got this battery for the same reason as almost everyone here, for a light meter in an old Pentax camera. Unfortunately, the battery was completely empty when we received it. I made sure to read the details on the back of the box, removed the little sticker covering the vents and let the battery sit for hours and the voltage never went above 0.2V. I kept checking with a multimeter and confirmed that the battery was low. Rather than risk those batteries on your old camera, I would recommend getting a light meter app for your phone and using the camera without a light meter.