1. The Mercury Battery Problem
A huge number of classic film cameras from the 1960s and 1970s — including the Olympus Trip 35, Canon Canonet QL17, and many early SLR light meters — were originally designed around 1.35V mercury oxide button cells (commonly the PX625 or PX400 types). Mercury cells held their voltage steady and predictably right up until they died, which made them ideal for precise light meter circuits.
Mercury oxide batteries have since been banned worldwide for environmental and health reasons, and haven't been manufactured for decades. This leaves owners of these cameras with a real, practical problem: the batteries the camera was designed for simply don't exist anymore.
Check Wein Cell Battery Listings on eBay UK2. Why the Voltage Difference Matters
The most common modern replacement, a 1.5V alkaline button cell, physically fits many of these old battery compartments — but 1.5V is meaningfully higher than the 1.35V the meter circuit was calibrated for. That difference causes the light meter to under-read the correct exposure, which usually results in the camera indicating less light is needed than there actually is, leading to slightly overexposed photos.
Alkaline cells also don't hold voltage as steadily through their discharge curve as mercury cells did, so the degree of error can drift as the battery ages, making consistent exposure even harder to predict.
3. Your Replacement Options
| Option | Voltage | Notes |
|---|---|---|
| Zinc-air hearing aid cell (e.g. Wein Cell MRB625) | 1.35V | Closest match to original mercury cells; the most commonly recommended fix |
| Standard 1.5V alkaline (e.g. LR44/SR44 equivalents) | 1.5V | Fits many compartments but causes a meter reading error unless compensated for |
| External handheld or smartphone light meter | N/A | Bypasses the internal meter entirely; most reliable long-term option |
| Adapter with built-in voltage regulator | Regulated to 1.35V | Lets you use a common, cheap silver-oxide cell inside a converter sleeve |
Always confirm the exact battery size and voltage your specific camera model requires before buying, as compartment sizes vary between models.
4. How to Check Which Battery You Need
- Open the battery compartment (usually on the base or front of the camera) and look for a stamped code near the battery slot, such as PX625, PX400, or similar.
- Check your camera's original manual if you have it, or search the model name plus "battery" to confirm the original mercury cell type.
- Match that code against zinc-air or regulated-adapter replacements sold specifically for that code, rather than assuming any small button battery will work.
- If in doubt, an external light meter sidesteps the whole compatibility question entirely and is often the most dependable long-term fix.
5. Common Cameras Affected
This issue shows up across a wide range of classic cameras, including:
- Olympus Trip 35 — uses a mercury cell for its light meter, though the shutter itself is fully mechanical and works without a battery.
- Canon Canonet QL17 — relies on its meter battery for accurate aperture-priority automatic exposure.
- Early Pentax Spotmatic models — many original Spotmatic bodies used mercury cells for their internal meter before later revisions changed the circuit.
- Various rangefinders and compacts from the 1960s-70s — check your specific model's battery compartment for a PX-prefixed code.
Cameras like the Pentax K1000 and Nikon FM use more common, still-available battery types for their meters, so this issue is less of a concern with those models.
6. Frequently Asked Questions
Why do vintage camera light meters read wrong with modern batteries?
Many cameras from the 1960s and 1970s were designed around 1.35V mercury oxide cells. Mercury batteries are now banned, and the modern 1.5V alkaline replacements output a different voltage, which throws off the meter's exposure reading.
What battery replaces a mercury PX625 cell?
A zinc-air hearing aid battery such as the Wein Cell MRB625 outputs the correct 1.35V and is the most commonly recommended direct replacement, sized to fit the same PX625 compartment.
Can I just use a 1.5V alkaline battery instead?
You can usually fit one physically, but the higher voltage causes the meter to under-read the correct exposure, typically leading to slight overexposure unless you manually compensate via the film speed dial.