A rechargeable PX625 replacement?

[Doug Richardson <doug@meditor.demon.co.uk>]

For some time, Ive been trying to find a replacement for the PX 625
mercury battery used by classic cameras such as the M1 - M5, Cl and
Leicaflexes. As a newcomer to LUG, Im aware that this problem has already
been discussed, but I thought it might be useful to provide a summary of
what Ive learned so far, and of a possible solution Ive found. 

As result of restrictions which the US and EC are placing on mercury
batteries, manufacturers have responded by creating the PX 625A. This
generates 1.5 - 1.8 volts, rather than the 1.35 volts of the mercury cell.
For most applications, this higher voltage is no problem, but it is
incorrect for camera exposure meters. 

For the moment here in the UK, mercury cells are still available, but
its obvious that US supplies are rapidly dwindling.

There are only four realistic options:

1 the Wein air-zinc battery

2 an adaptor able to mount another type of battery into a PX626 holder and
reducing the voltage of the cell to the level created by the mercury cell

3 a rechargeable equivalent to the PX625.

4   an adaptor which allows a small rechargeable battery to be fitted into
a PX625 battery holder  

(Theoretically there is a fifth option - for someone to devise a
modification to the cameras and meters which would allow them to use a
more modern battery - but this seems unlikely. It would involve modifying
the Leicameter, M5, CL, Leicaflex, Leicaflex SL and Leicaflex SL2.)

What are Leica doing about this problem?   Nothing.  As Leica users, we
need to find our own solution.

To date, I havent even seen a Wein cell, but according to discussions
Ive seen on the internet and CompuServe, these are expensive and have a
limited lifetime. Option 2 - a combined adaptor and voltage reducer sounds
promising. One has been marketed since last fall by C.R.I.S. Camera
Services, but I havent seen one or talked to anyone who has used this
device, so dont know how well it works. If this had been discussed
already, could someone copy the relevant postings to me? 

Ive been focussing my attention on options 3 and 4 - rechargeable cells.
Around 1970, a NiCad cell with the same shape as a PX625 was available
from DEAC. This was known as the DK-50, but it is no longer manufactured.
A present-day equivalent of the DEAC DK-50 might be a good solution, if
one can be found. The number of people who use these classic cameras must
be many thousands - not enough to justify a manufacturer putting a DK-50
replacement into production, but Ive been trying to find a manufacturer
somewhere in the world who still makes a cell of this type.  

The overall dimensions of a PX625 are 15.5 - 16mm in diameter, and 5.5 -
6.2mm in height. The rim of the cell is of 15.5 - 16mm diameter, but the
lower body of the cell was only some 12.5mm in diameter for most of its
length.  

Several companies in Pacific Rim and southeast Asia offer NiCads
of 15.5mm diameter and 6.2mm height, but these are straight-sided, so will
not fit the battery housing on most cameras. They would probably fit the
Leicameter. Im still waiting to hear from one battery company in Hong
Kong, but I think the chances of finding a rechargeable with the shape of
the PX625 are remote. But Ill keep looking.

What I have located is a small straight-sided rechargeable with a
diameter of 11.7mm and a height of 5.3mm. It would be a relatively simple
task to make a small aluminum PX625-shaped adaptor which would allow this
cell to fit our cameras. 

The main problem I can see is that while the mecury cell produces 1.35
volts, and the newer PX625A delivers 1.5 volts, the NiCad starts at around
1.3 volts when freshly charged, but soon drops to around 1.22 volts.  At
around 1/3 capacity, its voltage then begins to decline sharply.  A
nickel-hydride rechargeable of the same size is also available, but I
havent any details of its discharge characteristics yet. The 1.5 - 1.8
volt cell causes meters to read incorrectly (by a stop or more, Ive
read), but what happens with a 1.2 volt cell? Does the meter read low by a
stop? 

According to my early-1970s edition of "The Leica Way", the DEAC
DK-50 NiCad cell could be used in the Leicameter and Leicaflex, but the
battery test deflection of the meter would be a little less than normal.
No mention was made of any metering inaccuracies.  Does anyone have
experience of using the DEAC NiCads in the Leicameter, Leicaflex, M5, or
CL? If so, were they aware of meter errors?  How long did the battery hold
its charge?

Regards,

Doug Richardson