T
Theo Markettos
Guest
I have various bits of 20-year-old equipment with memory backup batteries.
These are all NiCd, and are quite capable of keeping the memory contents for
years. But they've all started crystal growth, and need to be replaced.
For example, one takes the obsolete Varta V280H, 280mAh 1.2V. The suggested
replacement for this is a NiMH, the CP300H. But the self-discharge is poor:
90% of capacity retained after a month. That means it takes 7 months for
charge to decay to 50%. Given the old batteries could retain memory for 10
years (memory supply current typ 2uA), that's a bit poor. Apparently also
the memory chip (PCF8583) has a habit of not reviving if it's been unpowered
for a long time (not sure if that's charge buildup on unterminated pins or
something else).
So:
Is that the best I'm going to get with a NiMH? Are there any better types?
If I use an alkaline button cell plus Schottky diode instead (to disable the
charging circuit), are there any pitfalls? Electrolyte leakage? Will the
Schottky behave sensibly at such low series current? The RAM chip is
specced up to 6V so a higher voltage won't harm it, but when I look up the
datasheet of a possible Schottky (ST's BAT41) all my currents are off the
scale. Reverse leakage doesn't look like it's a problem, but I'm a bit
worried about the Schottky not conducting at all in the forward direction
with such low currents.
For battery I'm thinking something like an good quality SR44 (one of the
locations is a very confined space, and originally used an 11mAh NiCd - I
might need something even smaller than an LR44/SR44). Renata's SR44 has a
self-discharge of 5% per year: http://www.renata.com/pdf/watch/DB303.pdf
Any other ideas?
Thanks!
Theo
These are all NiCd, and are quite capable of keeping the memory contents for
years. But they've all started crystal growth, and need to be replaced.
For example, one takes the obsolete Varta V280H, 280mAh 1.2V. The suggested
replacement for this is a NiMH, the CP300H. But the self-discharge is poor:
90% of capacity retained after a month. That means it takes 7 months for
charge to decay to 50%. Given the old batteries could retain memory for 10
years (memory supply current typ 2uA), that's a bit poor. Apparently also
the memory chip (PCF8583) has a habit of not reviving if it's been unpowered
for a long time (not sure if that's charge buildup on unterminated pins or
something else).
So:
Is that the best I'm going to get with a NiMH? Are there any better types?
If I use an alkaline button cell plus Schottky diode instead (to disable the
charging circuit), are there any pitfalls? Electrolyte leakage? Will the
Schottky behave sensibly at such low series current? The RAM chip is
specced up to 6V so a higher voltage won't harm it, but when I look up the
datasheet of a possible Schottky (ST's BAT41) all my currents are off the
scale. Reverse leakage doesn't look like it's a problem, but I'm a bit
worried about the Schottky not conducting at all in the forward direction
with such low currents.
For battery I'm thinking something like an good quality SR44 (one of the
locations is a very confined space, and originally used an 11mAh NiCd - I
might need something even smaller than an LR44/SR44). Renata's SR44 has a
self-discharge of 5% per year: http://www.renata.com/pdf/watch/DB303.pdf
Any other ideas?
Thanks!
Theo