Replacement for PX625 battery

Started Sep 26, 2020 | Discussions thread
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Overrank Senior Member • Posts: 2,898
Replacement for PX625 battery

I'm starting this thread as a place to put information about replacements for the PX625 battery, as this comes up now and again, and having it in one place makes it easy to point to. It would be interesting to know what cameras this is really an issue for, since I've owned a couple of Canonet 28s and they've been fine with the alkaline batteries, whereas the Canonet QL17 iii is reported to need to the correct voltage. Maybe I'm just less fussy about exposure ......

The problem

Some old cameras and meters (e.g. Nikkormat FTN, Canon Cannonet QL17 iii, Gossen Lunasix 3, Pentax Spotmatic F) used the 625 mercuric oxide cell. Batteries based on this technology are now illegal in many countries and often a replacement is needed.

There are three main issues:

Voltage - the voltage of the mercuric oxide cell was 1.35v, the voltage of many of the popular alternatives is 1.5v or so

Discharge characteristics - the voltage of the mercuric oxide cell was fairly constant until it was fully discharged, whereas some cells (in particular alkaline) have a voltage that decreases steadily with time

Fitting - some of the options don't fit into the available space in the camera

The options

Use a PX625A or similar

PX625S - Silver Oxide

Alkaline and silver oxide versions of the PX625 are available. These are the same shape and size. However the alkaline cell has problems with both voltage and discharge, whilst the silver oxide version has problems with voltage (the discharge curve for silver oxide is pretty constant like mercuric oxide). Some cameras, e.g. the Pentax Spotmatic F, have a bridge circuit that compensates for the voltage, others don't. Alkaine 625s are around a pound each, silver oxide versions are around £6 each.

Use an alkaline or silver oxide 675 and an adapter

SR44 / 675S in brass ring

Small brass adapters are available on eBay and elsewhere. These convert the cheaper 675 batteries to fit in the same space as a 625, but do not change the voltage or discharge characteristics, so the problem can persist. Alkaline cells are a couple of pounds for 10, silver oxide batteries are around £4 for 10

Use a zinc air hearing aid battery with an adapter

Zinc air hearing aid batteries have a flat discharge curve and a lower voltage of 1.4v -1.45v. Therefore it is possible to use these with the adapter above as a replacement for the 625. The downside is that zinc air have a relatively short lifetime of around a month, and can discharge blue goo when then are used up.

Use a Wein cell

Wein Cell

A Wein cell is an engineered replacement for the PX625 which uses zinc air technology with a special electolyte to give a voltage of 1.35v. It comes in a package which is the same shape as the PX625 and has a longer lifetime than hearing aid betteries. Wein cells are more expensive than hearing aid batteries (~£5 each)

Use a MR-9 adapter

MR-9 with a 386 battery

A special adapter is available that takes a 386 battery and drops the 1.55v to 1.35v. The combined package is the same size as the PX625. This is an expensive solution (around £30 each). At one time a special dual size adapter was available for the two batteries needed for a Gossen Lunasix 3, but these are now out of production.

Have the meter recallibrated for the silver oxide 625

This is probably the best solution, but it requires work on the camera. It also requires rework if you replace the camera with another.

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