Classic film cameras and analog light meters were designed in an era when power supply was simple, stable—and now obsolete.
The PX625 / MRB625A mercury battery, once the gold standard for exposure circuits, is no longer available. Yet thousands of perfectly functional cameras still rely on it.
This article is not about nostalgia. It is about keeping your equipment accurate, usable, and safe—with a realistic evaluation of modern replacement options, especially the V65HT battery.
1. Why PX625 Is Not “Just Another Battery”
The original PX625 mercury cell delivered 1.35V with exceptional voltage stability.
That stability was critical because:
- exposure circuits were fully analog
- no voltage regulation existed
- meter accuracy depended directly on supply voltage
Any modern PX625 replacement must be evaluated not by size alone, but by voltage behavior over time.
2. Devices That Depend on PX625 Accuracy
????️ Film Cameras (Meter-Only Power)
Typical examples include:
- Canonet QL17 / QL19
- Olympus OM-1 (early versions)
- Minolta Hi-Matic and SRT series
- Yashica, Konica, Ricoh rangefinders
In most cases, the battery powers only the light meter, not the shutter.
That means: wrong voltage = consistently wrong exposure.
????️ Handheld Light Meters
- Early Sekonic and Gossen models
- Scientific and industrial analog meters
These devices assume a stable reference voltage—something many modern cells fail to provide.
3. Evaluating the V65HT Battery as a PX625 Replacement
The V65HT battery is a rechargeable Ni-MH cell with a nominal voltage of approximately 1.2V. While lower than the original PX625, this difference is predictable and consistent—which matters more than absolute voltage.
Key strengths of the V65HT battery
- stable discharge curve
- rechargeable (long-term cost advantage)
- environmentally responsible
- physically compatible with PX625 battery compartments
As a PX625 replacement, the V65HT avoids the biggest risk of alkaline cells: voltage drift.
4. Real-World Metering Behavior
In practical use, cameras powered by a V65HT battery typically show:
- a constant under-reading of ~½ to 1 stop
- no progressive drift over time
- repeatable results across lighting conditions
This consistency allows easy compensation via:
- ISO adjustment
- mental exposure offset
- calibrated testing roll
That reliability is precisely what defines a usable PX625 replacement.
5. Installation and Use: Best Practices
Before installing a V65HT battery:
- Fully charge the battery
- Clean battery contacts
- Verify exposure against a reference
Recommended references:
- smartphone light meter apps
- a digital camera set to manual
- a controlled test roll of film
This step ensures confidence before critical shooting.
6. Common Mistakes to Avoid
❌ Using LR44 alkaline cells
→ unstable voltage, drifting exposure
❌ Using SR44 without voltage reduction
→ systematic overexposure
❌ Long-term storage of zinc-air cells
→ self-discharge even when unused
A properly chosen PX625 replacement like the V65HT battery avoids all three issues.
7. Who Should Choose the V65HT?
The V65HT battery is especially well suited for:
- active film photographers
- users of multiple PX625-based devices
- photographers who value repeatability
- anyone seeking a sustainable long-term solution
Collectors seeking perfect historical voltage may prefer zinc-air, but for regular use, V65HT is the more practical choice.
8. Bottom Line
The PX625 is not obsolete—it is unsupported.
Choosing the right PX625 replacement determines whether a classic camera remains a tool or becomes a display piece.
The V65HT battery delivers predictable behavior, long service life, and responsible reuse—making it one of the most sensible power solutions for vintage photographic equipment today.