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:
Any other ideas?
Thanks! Theo