Technical Information About 357 Batteries
I have been looking at various batteries to power some
new products I am developing at work. While I had Energizer/EverReady
technical staff helping me with information, I decided to find out more about
the 357 (SR44) style batteries we use in most of the OM cameras.
Some new information:
- The silver oxide batteries from all manufacturers contain something
like 0.3 to 1% mercury! This helps preserve the anode. Internationally up to
25mg of mercury per cell is considered acceptable.
Although various battery books list that AgO cells lose capacity at up to
5% per annum, Energizer claims their cells lose capacity at 2% pa or less at
room temperature. Sealing is now so good that tested shelf life exceeds
10years. In accelerated (high temp aging tests) the cells have even longer
projected shelf life. This would means it is hardly worth refrigerating
cells. (Normally ~doubles shelf life for every 10C reduction).
High current pulse rated cells like the 357 all use KOH electrolyte. Low
current rated cells like the 303 use NaOH electrolyte. Vendors like Duracell
(who don't actually make their AgO cells themselves) mark their cells
357/303 and ship only the high current version. It used to be that the KOH
cells were much harder to seal as the electrolyte is more active and attacks
the seals and cell separators. The capacity was also a little lower as the
separator needed to be thicker. This is no longer true. Why then, does anyone
make NaOH cells anymore? The answer is watch vendors often demand a NaOH cell
as they still believe the sealing is a problem and are worried that the KOH
cells are more likely to leak and cause damage. The sealing on some vendors
cells may still be problematic, so avoid generic brand cells.
The 357 cell is capable of short circuit currents up to a couple of hundred
mA when new and for short durations (<100mS).
The Energizer 357 cell capacity under low rate conditions is higher than
the nominal 175mAh rated , approaching 200mAh. (The very old data book values
used to be 200mAh but this was at 35C (wrist temperature, which helps a
little.) not 21C. This helps with OM4's and 2'S's which have higher "off"
standby current. The 357 cell has been significantly reformulated over the
years since first introduced for use on LED display watches.
It would be interesting to compare Energizer's cells to those from other
vendors. In particular an accelerated life test would be interesting and a
high rate capacity test to see if there really are differences.
A battery vendor who makes batteries aimed mostly at the watch market told me
his batteries don't last more than 4 years and quoted me a loss of capacity
of as much as 10%pa. It would definely pay to refrigerate these cells!
(Although not in the deep freeze).
A very significant issue for OM use, is the change of battery internal
resistance as the cell ages, and of course near the end of discharge life.
This causes increased battery voltage drop at peak camera operating currents
at shutter opening.
One vendor's batteries show an almost doubling of resistance after one to two
years of storage. In general the resistance increases sharply in the last 20%
of discharge life and is what ultimately causes sudden camera lockup in OM's.
The resistance change in storage appears to depend on the vendor's process
and may be the reason why certain "new unused cells" cause lockup in the OM
occasionally. The resistance in the case of the watch vendor's cells,
increases after a year or two, and then drops off again to closer to new
values after about 2 years.
Based on Everready's longevity claims (not tested myself) their 357's do not
suffer from the large increase in resistance or fail after a some years of
storage. Hence my recommendation: stick to Everready.
The battery life in OM use, is probably as much related to battery resistance
near end of discharge life, as it is to rated capacity in mAh. It would be
interesting to run some discharge-resistance curves to compare different
When I asked Everready about the '76 type (SR44 size), which are shown as
having a sloping discharge voltage near end of discharge, the tech support
person said he thought they no longer had that characteristic, although he
was not completely sure! Cells with a shelf like end of discharge curve,
were engineered at one time to allow easy end of battery life detection.
contributed by Tim Hughes