April 11, 2023
My 1969 Olympus 35SP and many other old cameras from the 50s, 60s and 70s used obscure PX625/PX13 1.35 volt mercury-based batteries that could deliver very stable power supply for a very long time. Because these where mercury-based and mercury is highly toxic for both humans and environment, most countries ended up banning mercury-based batteries, and rightly so.
This creates a problem for old camera enthusiasts that were left without suitable replacements for their cameras.
Although there are alkaline cells with the same dimension specs of the mercury-based PX625 (15.4 x 6.2 mm), these will output 1.5 volts with the less than ideal alkaline discharge curve. Also, a 1.5 volts battery instead of 1.35 volts means the camera's light meter will be 1 to 2 stops off, resulting in underexposed photos. And even knowing by how much you have to compensate, because of the weird alkaline discharge curve, estimating the compensation gets harder to get right as the battery ages and voltage drops.
Apart from hardware modification like the diode mod or the meter recalibration, there are a few solutions to the problem, none is the perfect solution but they let us use our old cameras:
Wein makes zinc-air cells especially made for older cameras. These have the dimensions of PX625 and output stable 1.35 volts that these cameras require, so they are a direct replacement with no need of adapter or modification. Unfortunately they are rather expensive at around 8 € per cell.
Other than the cost, the biggest issue with zinc-air cell is their significantly shorter operating life (few months?).
* In Europe Wein cells can be found here.
The MR-9 Adapter is a adapter shaped with the dimensions of a PX625 battery that accepts a 386 silver-oxide battery (that outputs 1.55 volts) and reduces its voltage to the required 1.35 volts. Silver-oxide batteries last a bit longer than alkaline batteries and have a pretty stable voltage output through their life.
Because the output is exacly 1.35 volts, the light meter reading will be spot on, without having to compensate. The price is steep at around 30 €.
Hearing Aid Batteries
PR675/PR44 are zinc-air batteries for hearing aid devices that output stable 1.4 volts and while it's half a volt more than the ideal 1.35 volts, it's acceptable as the discrepancy is not that high (maybe +1/3 a stop off in the light meter). The issue is that they are smaller than PX625, both in diameter and height so there's the need for a physical adapter.
Being zinc-air batteries, these share the same life expectancy issue of the Wein cell, but they are dirt cheap (about 0.50 € per cell).
Although my camera came with one of those dreaded mercury-based cells that should last for a long time, I've been thinking about alternatives for when that'll happen. I'm also interested in how does the light meter reading compare between the mercury-based cell and the hearing-aid cell.
My solution was to to improvise an adapter with an o-ring of the correct size. I had some o-rings measuring 16 x 11 mm, it works, but it's a bit too snug, maybe 15 x 11 mm would have been better. Also added a washer because the original battery is thicker than PR675 cells.
I might try to ask a friend to print me a better adapter on his 3D printer.
The two batteries measured exactly the same 1.35 volts on the multimeter, I don't know how old this Varta battery is, I guess it's pretty old, but the hearing-aid cell was fresh out of the box (maybe the zinc-air reation needs to build up until 1.4 volts?). The light meter readings were of course exactly the same with either batteries.
Update: After a few hours I tested the zinc-air battery, and it's outputing 1.4 volts, so there's a buildup after activation. The light meter might be about +1/3 off, maybe less, compared to the mercury-based cell.