The PX28 battery (same as 4SR44 or 4LR44) is a 6V battery that is used in some medium format cameras and light meters. I know it's used in the Pentax 6x7, some Bronicas, and the Mamiya RZ67. This battery is actually made up of four LR44 batteries (or SR44, G13 etc). The LR44 batteries are ubiquitous, and you can usually get several for a dollar. Unfortunately, the PX28 battery seems to be quite expensive, and not so easy to find. You can try to stack four LR44 batteries in the space for a PX28, but the overall length will be too short unless you add a spacer of some kind (like a folded up piece of tinfoil).
This DIY is to make your own PX28 battery using the bits from an old dead one.
1) Take an old dead PX28 battery and with a pair of small pliers pry the outer metal casing away from the body at the seam at one end.
2) Work you way down the seam to further pry the outer metal casing away from the body. This will expose the inner plastic or paper tube that holds the four LR44 batteries.
3) Once the outer metal casing is removed, the smaller LR44 batteries can be pushed out of the inner tube along with the metal end contacts. You'll want to keep the tube and the metal end contacts for the new battery.
4) There's some variation of the inner sleeve style between various manufacturers. The Varta ones I've tried have a plastic sleeve that's capped at one end with a smaller hole. This is nice, since it keeps the positive contact from falling out the other end. The Maxell has a thin cardboard tube, and a Duracell just has a piece of paper that's rolled up as a tube but isn't joined. If it's just an open tube, then you need some tape to hold the batteries from popping out the ends. Put the four LR44 batteries in the tube in series (each in the same orientation). The positive metal end contact will have a taller protrusion. and this should be placed in contact with the positive battery contact at one end. The metal contact for the negative end will have a shorter protrusion. I don't put that one at the outer end, since it's too easy for it to fall off. I put that one in the middle of the battery stack, with the protrusion in contact with a negative end of battery. This leaves the outer end battery with it's own negative end exposed.
5) Wrap some thin white freezer tape around the sleeve such that the tape extends a little over the end of the sleeve. This is so you can fold the tape over the end of the sleeve to prevent the LR44 batteries from falling out easily. The freezer tape is nice since you can write on it to identify what type of battery it is.
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