Silverbased

In an earlier post I discussed the problem of old photo gear designed for 1.35 volt mercury cells—a battery type banned in recent years because of the metal’s toxicity.

I concluded that the easiest solution for most people is to substitute an inexpensive #675 hearing-aid battery (with a spacer ring if needed to fit your battery compartment). Their zinc-air chemistry is a near match to the voltage of the original mercury cells.

PX13 mercury, #675 hearing-aid, and Wein cell batteries

I also noted that there is a more expensive commercial product, the “Wein cell,” which is also a zinc-air type. But these are specifically marketed as mercury-replacement batteries for photo equipment.

While Wein cells are indeed effective for this, photographers have often speculated whether there is anything special about them. Are they simply standard hearing-aid batteries repackaged at a higher price?

The Wein replacement for a PX13 or PX625 battery does have some physical differences from a 675. The Wein’s metal case only has two air holes, not four. This helps keep them from drying out as quickly.

The Wein battery has the same dimensions as the 675 hearing-aid size, but adds a metal ring crimped around it. This can be important in making electrical contact with some battery compartments, for example the metered Yashica Mat models.

But is the Wein cell chemistry any different?

I have no insider knowledge of how Wein produces these batteries; and my only test equipment consists of a simple Radio Shack multi-meter.

Yet I made some curious discoveries about the voltage of zinc-air batteries (of both types), which photographers using these cells ought to know about.

While researching my earlier post, I became curious exactly how much current a typical camera meter circuit was drawing. By making temporary connections to the battery compartment of my Olympus OM-1, I found that with its meter needle centered, it drew roughly 0.2 milliamps from its PX-13 mercury cell.

It was more convenient and accurate to use a simple resistor rather than a camera as my dummy load when comparing batteries. So I used a 6.8 kΩ value I had handy, which drew a current comparable to the OM’s meter.

Testing zinc-air battery voltage with a resistor dummy load—hmm!

At this point I discovered an interesting anomaly. The voltage of a mercury or silver-oxide battery scarcely changed when put under this load—a drop of maybe 0.005 volts. But with either type of zinc-air cell, the story is very different.

Under no load, the zinc-air readings begin at about 1.42 volts. But, when connected to the load, their voltage begins to drop significantly. After five minutes or so, the 675 hearing aid battery dropped to 1.37 volts. The Wein cell went down to 1.33 volts. All these numbers showed slight variations when I repeated the tests.

I wondered if the Wein cell’s fewer air holes were “starving” the cell for oxygen, causing the lower voltage. But covering two air holes on a 675 cell didn’t seem to change its voltage readings.

Now, my original PX13 mercury battery tested at about 1.34 volts under load. So there does seem to be some support for the claim that Wein has somehow “tweaked” the zinc-air chemistry, to give a voltage closer to that of mercury cells.

But is this difference important?

Hardly! Far more significant for accurate exposures is that you turn on the meter several minutes before taking a reading. With either zinc-air battery type, the voltage will be too high immediately after turning on the camera—possibly leading to underexposure. Once you become aware of this effect, you will definitely notice your needle drifting for the first few minutes, even when metering a scene with unchanging light.

Now, the “gold standard” for mercury battery replacements is to use a silver oxide battery (approximately 1.58 volts) with an adapter that adds a miniature diode to the circuit. By introducing this small voltage drop, a very close approximation to the mercury battery voltage can be obtained. The voltage doesn’t drift after being switched on.

But in fairness, we should note that even this solution can give different voltages depending on the specific diode chosen, and the current draw of the meter circuit. The voltage also changes somewhat with temperature. Frans de Gruijter discusses both these factors at some length in his definitive battery-adapter article downloadable here.

Given all these uncertainties, I continue to feel hearing-aid batteries are the most sensible real-world mercury replacement for many photographers. Not perfect perhaps—but close enough.

But remember, don’t try to “save the batteries” by constantly switching the meter on and off. When you go out shooting, switch on the camera, uncover the lens cap, and leave it that way. Zinc-air batteries have a high capacity; they’re less likely to die from electrical exhaustion than from the inevitable drying out of the chemical paste inside them.

After that you’ll need to splurge on another 75-cent hearing aid battery. Then grab that vintage camera, get out there, and keep shooting.

See this earlier post for more about the mercury battery issue.