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My 1969 Olympus 35 SP like many other old cameras from the 50s through the 70s used obscure PX625/PX13 1.35 volt mercury-based batteries. These were popular because they could deliver very stable power supply for a very long time, ideal for the purpose of light metering. Because these where mercury-based and mercury is highly toxic for both humans and environment, countries ended up banning them.

A mercury-based Varta V 625 PX battery.

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), akaline batteries will output 1.5 volts with a less than ideal 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 photos with wrong exposure. And even knowing that you have to compensate for exposure, the alkaline discharge curve makes it pretty much impossible to guess by how much you should compensate as the battery ages and voltage drops.

There are a few (non-invasive) solutions to the problem, none is the perfect solution but they let us use our old cameras:

Wein Cell

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 (a couple of months?).

* In Europe Wein cells can be found here.

MR-9 Adapter

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).

My Solution

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.

A improvised adapter (o-ring and a washer) for hearing-aid cells. Note the 5 little intake holes for air in the cell.

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 (guessing it's pretty old), but the hearing-aid cell was fresh out of the box. Maybe the zinc-air reaction needs to build up until it reaches 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 stops off, maybe less, compared to the mercury-based cell.

Also, I wonder if starving a zinc-air battery from air will increase their life after being activated and used (by taping the air intakes). Update: It does!