Cinemax Macro C-802 Super-8 - Mercury battery

[Mario Hatzopoulos]

Hello everyone,

 

this is my first time on one of these Forums. But I recently bought a Cinemax Macro C-802 Super-8 and I've noticed I need a special battery for the light meter. Its a mercury battery and fortunately I still have the original piece that was in the camera when I bought it. Here's what it says on the battery:

• Brand: National Mallory
• Branding: H-3 D 3.9V (This is not a Button cell. This battery is about 2cm in height and about 1.5 cm in diameter)
• Country: Japan

My question is. Do these sort of batteries still exist? Or at least a format of them so I can use my light meter again?

Thank you for your answers in advance! :D

[Duncan Brown]

There are a fair number of replacements out there for the ubiquitous PX-13/PX-625 1.35V mercury battery used in about every SLR from that era, and I've seen them as meter batteries in some home movie cameras too.  It's weird that your bigger battery isn't an exact multiple of 1.35V because that was the "natural" output voltage of that cell type.

The big question is whether the meter depends on that exact voltage, or whether there's a voltage regulator built in, so it could actually tolerate a slightly higher voltage and still be accurate.  (That's the issue with putting same-size alkaline replacements in old SLRs - then your meter is miscalibrated.)  If you can answer that, it will guide your path a bit on what to do next.  ("Use a handheld meter" is one possible answer, sadly!)

A little googling doesn't tell me anything useful about that battery spec.

Duncan

[Mark Dunn]

Can you put up a photograph, with a ruler for scale? Might be easier to offer ideas if we can see it.

 

[Joerg Polzfusz]

This battery is most likely a stack of three coin-cells à 1.3V wrapped in plastic. This was common for some torches, some toys, …

[Steve Switaj]

Mercury batteries were popular in the 60's and 70's for photographic gear because they had a flat discharge curve and a long shelf life.

So, basically, they provided a consistent voltage for a long time in intermittent use, and that's a good match for the demands of photo equipment.

Though they were good at their job of being batteries, the downside was that oxides of mercury are very neurotoxic, and dumping all those batteries into landfills was... problematic.

So... where do we go now?

Well, first, there's no such thing as a 3.9v mercury battery. The basic chemistry of a mercury oxide battery provides a cell at about 1.35v, so as Joerg notes your battery is almost certainly 3 cells stacked in series in a common wrapper. It probably provided about 4.05v when new and unused, and settled in at about 3.9 v under it's rated load (little coin cells tend to have a pretty high internal resistance)

Probably the best match for the voltage and discharge curve of mercury today is a zinc-air battery. These are the types of batteries you see sold for hearing aids. They have a little plastic tab on the top that you peel away to allow air into the battery. Oxygen in the air starts reacting with a zinc compound inside the battery and... well... it batts.

They have an almost identical voltage, good power density, and a reasonably flat discharge curve, but they don't have anything like the lifespan of mercury.

If you can get a good measurement of the battery size, you could probably improvise a holder for three cells. The thickness is the important part, you can always pad the diameter. 

Look here for common sizes...

    https://en.wikipedia.org/wiki/List_of_battery_sizes#Zinc_air_cells_(hearing_aid)

Depending on how finicky your meter is, you might also be able to use three silver oxide cells.

That would be a lot more convenient because silver oxide cells come in more sizes than zinc-air cells.

Basic silver-oxide chemistry is about 1.5 volts/cell, though, so a stack of three is 4.5v.

The extra voltage is unlikely to damage your equipment, any good engineer would build in a lot more margin than that, but depending on the circuit it may affect accuracy.

Common sizes of silver oxide cells can be found here...

    https://en.wikipedia.org/wiki/List_of_battery_sizes#Silver_oxide_and_alkaline_cells

 

 

 

 

 

 

 

[Joerg Polzfusz]

BTW: This webpage claims that the camera takes „2 x PX“ for the lightmeter:

http://www.unau.de/museum/index.html?d_401_00_00_0006_Cinemax_C_8021366.htm

 

Do you have the manual for this camera? It should list alternative names for this battery. (Back in the 70s, every manufacturer used different modelnumbers, e.g. basically all Varta-batteries started with a V, all Kodak-batteries started with a K, …)

 

Edited February 19, 2023 by Joerg Polzfusz