Mercury battery replacement.

[Tony Warren]

I have recently been given a Minox 110s which only functions with help of the dreaded mercury battery, two of them. Researching alternatives has suggested the Wein zinc-air cells which appears to be a 675 hearing aid battery mounted on a MN625-size lower plate. It is also an expensive option compared to the price of the 675 batteries. Down here in NZ a four pack of 675s costs 1/4 the price of a single Wein. Since they probably last about the same time I looked for a means of using the hearing aid version. The o-ring seemed to be the way to go and the attached shots show the result.
Maybe this is old news but a recent comment from Gary suggests it is not going away. Some cameras I found take the 675 size of battery, which is the same as the mercury version physically. Some others need more complex solutions. Is it worth sharing experiences here?

 

[Gary R. Smith]

I just purchased three Wein cells that are rated at 1.35 volts. According to the FTb manual, the battery voltage should be 1.3 volts however they only show 1 decimal place so in theory they could show 1.35 either as 1.3 or 1.4. The Energizer 675 looks to be rated at 1.45v. Oddly enough I noticed that the newly replaced Wein cell doesn't seem to respond to a Battery Check however the meter is working when it's turned on. Another thing I noticed is that while the Wein cells do appear to have an attached disk to fill in the receptacle pocket diameter, the one that I just inserted seems to have lost the disk and looks just like an unmodified hearing aid battery. Amazon sells six of the 675 for $11.50 which makes the cost per cell $1.90 v. $5.16 for each Wein cell (2.7x more). If there is no downside to a cell of a smaller diameter, I may try using the 675 (with or without the o-ring). The single cell that I need for the FTb sits in a cylindrical pocket.

 

[Brian Moore]

Is it worth sharing experiences here?

Absolutely. Although I currently have no need to replace mercury batteries, someone else may. But even so, I find it interesting to see the solutions people come up with for challenges such as the one you're facing, Tony.

 

[Tony Warren]

Absolutely. Although I currently have no need to replace mercury batteries, someone else may. But even so, I find it interesting to see the solutions people come up with for challenges such as the one you're facing, Tony.

It's an interesting subject, even only academically as you say Brian. Most problems involve the battery placed flat in a recess under the battery cover. Mine are on edge which makes it more difficult to retain them while I close the camera. There are some very ingenious solutions as you say. Worth a trawl if anyone has this problem iwth a camera that only works in auto.

 

[Tony Warren]

I just purchased three Wein cells that are rated at 1.35 volts. According to the FTb manual, the battery voltage should be 1.3 volts however they only show 1 decimal place so in theory they could show 1.35 either as 1.3 or 1.4. The Energizer 675 looks to be rated at 1.45v. Oddly enough I noticed that the newly replaced Wein cell doesn't seem to respond to a Battery Check however the meter is working when it's turned on. Another thing I noticed is that while the Wein cells do appear to have an attached disk to fill in the receptacle pocket diameter, the one that I just inserted seems to have lost the disk and looks just like an unmodified hearing aid battery. Amazon sells six of the 675 for $11.50 which makes the cost per cell $1.90 v. $5.16 for each Wein cell (2.7x more). If there is no downside to a cell of a smaller diameter, I may try using the 675 (with or without the o-ring). The single cell that I need for the FTb sits in a cylindrical pocket.

I think I would use Wein if they were the same price as yours down here in NZ. The lowest price here is NZ$19 with 675s NZ$8.50 for 4 = NZ$2.12 for one. The US Wein price is roughly half the NZ price with the Energizer much closer.

The Minox 110s looks to use the two batteries in series so ideally 1.7v so 0.2v extra shouldn't upset the meter unduly. Interesting that the latest Wein is full size as it were. Duracell produced a 625 zinc air at one time which was the same physical dimensions as the MN625 so Wein have maybe resurrected this casing size for camera use. It must save them something in manufacturing costs. The 675 are used in cochlear implants which is no doubt why they have survived alongside the smaller sizes.

The problem with the Minox is that the batteries are housed in the door and held in place with a sliding grip so the rudder helps stop them falling out. With a flat location it would be less of a problem I guess, even,as you say, just dropping one in or maybe with a little paper packing. The thickness may be a bit loose but easily fixed.

The batteries as shown have been in the camera for 6 days now and are still showing an OK test so even using a fresh one for each outing wouldn't break the bank and could be relied on. Less than the price of coffee.

 

[Gary R. Smith]

I debated spending for the Wein cells since I just purchased a shoe-mount meter that appears to work great, but you have to look at the top of the camera whereas with the FTb, you merely look through the viewfinder to match your needles.

 

[Bill Watts]

I just purchased three Wein cells that are rated at 1.35 volts. According to the FTb manual, the battery voltage should be 1.3 volts however they only show 1 decimal place so in theory they could show 1.35 either as 1.3 or 1.4. The Energizer 675 looks to be rated at 1.45v. Oddly enough I noticed that the newly replaced Wein cell doesn't seem to respond to a Battery Check however the meter is working when it's turned on. Another thing I noticed is that while the Wein cells do appear to have an attached disk to fill in the receptacle pocket diameter, the one that I just inserted seems to have lost the disk and looks just like an unmodified hearing aid battery. Amazon sells six of the 675 for $11.50 which makes the cost per cell $1.90 v. $5.16 for each Wein cell (2.7x more). If there is no downside to a cell of a smaller diameter, I may try using the 675 (with or without the o-ring). The single cell that I need for the FTb sits in a cylindrical pocket.

I would suggest the reason that the Wien cell does not respond to the battery check is it's internal resistance. This limits the current flow available from the battery and whilst it does allow enough current to flow for metering purposes, the battery check circuit has a lower resistance and draws more current than the metering circuit, hence not giving an indication when operating the battery check.

 

[Bill Watts]

As an engineer, my solution to the problem has been to fabricate brass SR 44 adaptors similar to the ones available online, and then modify them with a Schottky diode to drop the voltage to approximately 1.35v, thus providing a direct drop in replacement. They can be stacked if required to get the required voltages for cameras that required multiple 625 batteries.

Although I have found that with certain of my cameras the battery voltage has no effect on the exposure reading - presumably because that camera has a bridge balance type of meter which does not rely on a constant voltage instead of a current reading meter which does. The cameras concerned have meters that come to rest centred rather than dropping to the bottom of the viewfinder when switched off - whether or not this is reliably indicative of a bridge balance meter I don't know

 

[Tony Warren]

As an engineer, my solution to the problem has been to fabricate brass SR 44 adaptors similar to the ones available online, and then modify them with a Schottky diode to drop the voltage to approximately 1.35v, thus providing a direct drop in replacement. They can be stacked if required to get the required voltages for cameras that required multiple 625 batteries.

Although I have found that with certain of my cameras the battery voltage has no effect on the exposure reading - presumably because that camera has a bridge balance type of meter which does not rely on a constant voltage instead of a current reading meter which does. The cameras concerned have meters that come to rest centred rather than dropping to the bottom of the viewfinder when switched off - whether or not this is reliably indicative of a bridge balance meter I don't know

I don't have the skills or equipment to follow your solution Bill so I am more Heath Robinson. I understand the Pentax SLRs has the bridge circuit you mention. The centred needle theory is interesting but I don't have a camera to test it.