Cheap Easy Mercury Battery Replacement

Back in the Good Old Days, a number of cameras were designed to use the PX13 or PX625 mercury battery. Because the mercury battery's voltage output was so reliable and constant, SOME (not all) of these cameras were built without any form of voltage reguation in the meter circuits. Now that mercury batteries are no longer available and the only 625 cell we can get is a 1.5 volt alkaline, these models tend to give erratic readings (the error varies with light level, so it can't be easily compensated).

The following is a very economical way to power these cameras at 1.4 volts, close enough to the original spec to give good meter readings.

*** NOTE ***
Some cameras (like the Yashica Mat 124G) have a contact at the edge of the battery box for the + connection, rather than using the cap as a contact. If you have one of these, instead of the 18AWG wire indicated in the sketch, use a single-strand 12AWG wire and strip off the insulation. When you roll the wire into a circle, make it slightly undersize so it will fit snugly on the battery and slip it over the battery instead of into the battery box. This will provide the electrical connection to the + contact as well as making the battery fit the box.
- Many thanks to Peter Calafell for this tip

One or the other of these approaches should take care of almost any camera designed for PX625 mercury batteries. One exception that I know of is the Canonet QL17, which loads the battery edgewise... you'll need to stuff in a little aluminum foil or something to increase the thickness of the 675 battery to make it work in that one.

***~~~ Even Better! ~~~***

Don't want to mess with cutting wires and tinfoil and stuff? Jon Goodman, of camera seal kit fame, now has neat metal adapters to securely hold a 675 Z/A cell and make it fit the space of the Mercury 625! As always with Jon, the product is excellent and the price is very reasonable. Contact him at jon_goodman@yahoo.com for details.

For those who are curious, below is a set of discharge curves for all of the common camera battery chemistry systems. As you can see, mercury and zinc/air essentially share the same curve, while silver has a similar profile but at a higher voltage and alkaline just sort of wanders all over. Of the two lithium curves at the top, please note that the chemistry used in cameras is Li/MnO2, not Li/SO2. The lithium curve does not look as flat as the silver, zinc and mercury cells on this chart, though it's better than alkaline. This data is from a 1986 Duracell publication. [There is another Lithium chemistry, used (I think) only in AA cells, which produces a very flat 1.5V output curve... almost like a silver cell. This is Lithium-Iron Sulfide, or LiFeS.]

The basic idea for this adaptation came from Leon Schoenfeld, who has an excellent Topcon site.... he has done the math on the voltage-error problem, and rather than repeat it all here I'll let you go to his site to study up (see links below).

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