Choose a new rtc backup battery
We need to run on 1.8V
#1 Updated by Kevin Woo about 8 years ago
$0.75 / battery, 3V we need to step it down but its the best we can do. Uses existing battery holder
#2 Updated by Kevin Woo about 8 years ago
Wikipedia says that the typical capacity is 25mAh with our current draw of 5uA. This leads to about 208 days of running only on the battery. There is also a self discharge of .1mA. This may dramatically lessen the lifetime of the battery.
Alternatively we can use: http://www.allspectrum.com/store/product_info.php?cPath=76_156&products_id=1589&osCsid=f6ab02f15e80a8597fbd2cce9064f1fe&sdesc=CR1220+3.0V+Lithium+Button+Cell+Battery%2C+38mAh%2C+%281pcs%2Fbl%29+%3D%3DSHIPS+GROUND+ONLY%3D%3D+Model+%23+CR1220-BP1
This is a 1220 but has a capacity of 36mAh which gives us 316 days. It costs $0.99 each.
#3 Updated by Kevin Woo about 8 years ago
Using 2 diode drops. Schottky and standard diode -> .3 + .7 in the worst case leads to 2V which is a little dangerous. Common case will be .8 + .4 which leads to exactly 1.8V.