To use my Leica M5, i use a Kanto MR-9 battery adapter, sold by The Small Battery Company, a London (UK) based company. The Kanto MR-9 is identical to the Cris Camera Service MR-9 adapter. All the MR-9 adapters employ a built in micro electronical device, that automatically reduce the silver oxide 1,55 voltage to the 1,35 voltage of the outlawed PX625 mercury oxide battery.
The best MR-9 adapter advantage is the use of very common and low priced 386 silver oxyde battery (used in the watch for example), with 1,55 voltage. The 386 battery don't discharge if the camera is not in use.
Instead, the Weincell MRB625 zync-air battery (with 1,35 voltage) is more expensive than 386 battery and discharge always, if use or not the camera. This drawback is due to the particular system of the Weincell zync-air batteries: to activate the MRB625 batteries is necessary take off the thin plastic foil that cover the holes of the battery, to allow the zync/air chemical reaction. The life of the MRB625 zync air battery go from 4 to 6 months or, if you replace the thin plastic foil over the holes of the battery after every use, go from 10 to 12 months (if you use the camera or not).
I think that the MR-9 adapter is a better choice than the Weincell MRB625 battery.
As alternative, you can use an SPX625 silver oxide battery, with 1,55 voltage. The silver oxide and the mercury oxide have the same constant output for the all useful life of the battery: therefore, to take the correct exposure reading, is sufficient to compensate the ASA setting. The dimension of the SPX625 battery are the same of the old outlawed PX625 battery.
Instead, the much worse choice is the alkaline 1,5 voltage APX625 battery.
Ciao.
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