Hearing aid batteries as a replacement for mercury cells

FishyFish

FishyFish

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It's a well known fact that zinc-air hearing aid batteries can be used as a replacement for 3.5v mercury cells in older cameras, so I bought a pack of 674 batteries the other day to go in the OM-1n I picked up last week. However, I didn't read the description properly and it turns out that they're 4.5v cells - not that far of a 1.5v silver oxide cell - so I presume the meter will be noticeably out?

Have I missed something, or are there 3.5v hearing aid batteries available that I've overlooked?

TIA
 
Is it a 674 battery? The usual size for the older cameras is the 675 cell which gives 1.45v output (after the air breather hole is exposed). The old mercury cells gave 1.35v which gives a possible metering difference, depending on the camera's circuitry.
 
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In case you aren't aware, once these batteries are activated by removing the small paper tab they are effectively being 'used' and their life will be determined by how much power is drawn by the camera. Activated but unused they can last up to about three weeks before they die, my smaller size 13 in my hearing aids last about 6 days in normal use, 14 days if I don't switch my aids on - yup, sometimes I've left my spare pair of aids activated but not switched on so I waste the batteries - doh!
 
RustySnapper said:
In case you aren't aware, once these batteries are activated by removing the small paper tab they are effectively being 'used' and their life will be determined by how much power is drawn by the camera. Activated but unused they can last up to about three weeks before they die, my smaller size 13 in my hearing aids last about 6 days in normal use, 14 days if I don't switch my aids on - yup, sometimes I've left my spare pair of aids activated but not switched on so I waste the batteries - doh!
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Thanks. Yes, I know. I've used Wein cells before which are also zinc-air batteries, just the correct voltage and considerably more expensive. The low cost of 675 batteries is what is making me consider them as an alternative, as long as the camera metering is not affected too adversely.
 
Wein cells are the same voltage and chemistry as the hearing aid batteries just have reduced air contact so last longer as moisture and CO2 reduce life. The full electrochemical potential of zinc air reaction is 1.65V however as there are losses at the anode, cathode and in the body the voltage on an cell with no load that has had a few minutes to start the reaction is at most 1.45V. However with even a slight load that drops to at least 1.4V and and then gradually reduces until about 1.2V when they go dead. The other option is a holder that takes a 1.5V small alkaline cell and has a Schottky diode that clamps the voltage to 1.35V, they are not cheap but last well. https://www.smallbattery.company.org.uk/sbc_mr9_adapter.htm
 
I have an OM1 and 'converted' it to use 1.5v batteries by purchasing an MR-9 battery adapter, which works well of course, but I regretted it only when I realised how easy it is to add the diode myself, there being lots of room under the bottom cover. The adapter is quite expensive ~£30 and I have a background in electronics, so it would have been easy for me to follow this guide:

http://learncamerarepair.com/downloads/pdf/OM1-Diode-Ver2-1C.pdf

If you feel confident with your soldering skills, you might like to give it a try.
 
I've looked at the Schottky diode modification and it looks relatively straightforward. I've not done much soldering before though (and can't even remember where my soldering iron might be!).

I do also have a battery voltage adapter that is currently sat in my 35RC (which I bought from the Small Battery Company a few years ago). I could just swap that around and use SR44s as an alternative.

Given what @dmb has said with regard to the hearing aid betteries, I might just try one of those first and see how it fares - I have some to hand - and somthing like HP5+ probably won't be too upset if the metering is a bit out anyway.
 
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FishyFish said:
I've looked at the Shottky diode modification and it looks relatively straightforward. I've not done much soldering before though (and can't even remember where my soldering iron might be!).
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Just make sure that you buy some 60/40 solder on ebay, not the lead free, which melts at a higher temperature and makes it more likely that you will cook semiconductors.
 
MR-9 adapters work well.

I have one for my lunasix meter which originally used 625 batteries ( I think)

AFAIK the adapter is compatible with cameras etc that require the 675 battery.

An initial outlay , but perhaps a better option long term
 
I bought a couple of Wein Cell batteries last week. They seem to be readily available.
 
I also took the MR9 route for my OM1 and Lunasixes. I much prefer the longer battery life from the silver cells.
 
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