Is there a good way to monitor a CR2032 coin cell's voltage, that does not load it down when other power is off?
We are using a CR2032 coin cell for a battery-backed real-time clock. When main power is on, I want the (ARM) processor to measure the coin cell voltage to within 100mV or better to detect a 'coin cell failure' condition. ADC channels are available.
When main power is off, the average measurement circuit loading must not exceed 1uA to avoid discharging the coin cell, and the measurement circuit must not charge the cell.
Desired coin cell lifetime is 10 years. Typical draw is currently 1.2uA, which uses 105mAh of the 240mAh capacity. Aging eats another 10%. Component variations, higher temperature and humidity, any board contamination may cause earlier failure.
Hoping for something fairly compact and inexpensive.
Edit: there is not enough room for a relay.