PX400 battery adapter for Pentax Spotmatic SP & Fujica ST

• Provides the correct voltage: The P312 zinc-air cells deliver a stable 1.35V under load—the exact voltage the exposure meter in older Spotmatics and Fujicas was designed for.
• Replacement for the mercury cell that is no longer available: Original mercury batteries are no longer sold, while modern 1.5V alkaline batteries produce inaccurate readings.
• Fits securely in the battery compartment: The included rubber ring centers the smaller cell and establishes contact, preventing it from rattling loose in the compartment.

Compatibility

Suitable for various cameras, including:

• Fujica: ST601, ST701 [...]
• Honeywell: Spotmatic 1000, Spotmatic 500, Spotmatic SP, Spotmatic SP-II, Spotmatic SP-IIA [...]
• Pentax: Spotmatic 1000, Spotmatic 500, Spotmatic SP, Spotmatic SP-II, Spotmatic SP-IIA [...]

Not sure whether the battery fits your camera? Here is a complete list of all cameras compatible with the PX400 battery type.

Package Contents

2x Adapters & Batteries

• 2x centering rings (rubber)
• 6x P312 zinc-air batteries

Refill pack: P312 batteries

6x P312 zinc-air batteries

Technical Details

• Material: Rubber (O-ring)
• Battery type: P312 zinc-air
• Voltage: 1.35V (under load)
• Replacement for: PX400 / EPX400 / V400PX

Description

Your Pentax Spotmatic needs a battery with a constant 1.35 V—originally, this was a mercury cell. These cells are no longer available today, so this set mechanically adapts P312 zinc-air hearing-aid batteries to the required format: A rubber centering ring holds the smaller cell in the battery compartment, while under load it delivers a stable 1.35 V—the exact voltage the exposure meter was designed for. Also fits the Fujica ST601 and ST701.

Electrically, the zinc-air cells are a direct replacement for the old PX400, EPX400, and V400PX types—the exposure meter works correctly without requiring you to adjust the camera. The rubber ring establishes the contact.

Note: Zinc-air batteries are activated by removing the seal. Wait approximately one minute before inserting them into the camera.

Frequently Asked Questions

Why not use a standard 1.5V alkaline battery? Alkaline cells start at 1.5V and gradually drop over their operating life. The Spotmatic exposure meter is designed for a constant 1.35V and displays inaccurate readings with alkaline batteries.

How do I activate the zinc-air cells? Remove the seal, wait approximately one minute, then insert them into the camera.

What is included in the set? The “2x Adapters & Batteries” option contains 2 rubber centering rings and 6 P312 zinc-air batteries. The refill pack contains 6 batteries without rings.

Which battery types does the set replace? PX400, EPX400, and V400PX.