Electronic – How to use CR2032 in parallel

You should never place two voltage sources in parallel. Their voltages are never exactly the same and that combined with a low output impedance gives a very high current from the higher to the lower voltage. The supplies will get very hot and most likely get damaged.

The way to solve this is to put a diode in series with each of them. Then the supply with the higher voltage will supply the power, the diode on the other supply prevents current from the first one from flowing in. Suppose PS1 is 12.5V and PS2 is 12V. Then the connected cathodes of the diodes are at 12.5V - 0.7V = 11.8V. So there's only 200mV across the diode of PS2, too little to let it conduct. If PS1 fails, the voltage drops to 11.3V at which PS2's diode begins conducting and PS2 begins supplying power.

There are few factors:

1) As it was mentioned - battery self-discharge. I've checked few datasheets and it's 1-2%/year at 20-23C. If your device will stay in hot place (70C under direct sunlight for example) - it will self-discharge much much faster. Also, if device will be frozen accidentally - battery might be damaged and loose capacity.

2) PCB leakage - can reach hundreds of nA in case of severe surface contamination in wet conditions. If you manufacture PCB in good place, clean & completely dry it, and then cover it all (including chips) with some protective layer (polyurhetane or something) you can virtually guarantee no leakage through device life.

So, if leakage won't be an issue, no extreme thermal conditions - device will likely work for about 20-30 years.