I'm designing a portable device containing a Li-Po battery and a very accurate Real Time Clock chip with integrated crystal.
When in service, the date and time can not be adjusted. This is a fact for this project and it can not be changed.
So, the plan is to set the date and time once after production and before taking the device into service.
The device is going to be re-checked every two years which gives the oppurtunity to check and adjust the time of the RTC.
Because we can not guarantee that the Li-Po battery will never be deeply discharged, we decided to install a coin type battery of 3V 48mAh (BR-1225).
This coin type battery alone will not be enough for the whole service life (5 years) of the device. The RTC consumes 2.5uA.
Apart from changing the battery every two years, I was thinking about the following circuit to use the Li-Po battery as the main powersupply for the RTC
and the coin type battery as a backup.
Unfortunately, this requires a diode with a low forward voltage and an ultra low reverse leakage current of less than 68 nA:
Since Lithium primary batteries are not rechargeable, use of a reverse IC current blocking diode and a protective IC
resistor in series is required where there is the possibility of charging in the equipment circuit.
Use a diode with a low leak current as the reverse current blocking diode. To maintain the characteristics of a coin
type Lithium battery, the total charging amount of the battery during its total usage period must be kept within 3%
of the nominal capacity of the battery.
For example, assuming that a BR-1225 (48mAh) will be used in a memory back-up power supply for 5 years, charging
by the leak current of the reverse current blocking diode should be no greater than 3mAh, thus:
3mAh / usage period (5 years x 365 days x 24 hours) = 68nA.
other words, a leak current blocking diode whose reverse current is not greater must be selected.
Source:
https://eu.industrial.panasonic.com/sites/default/pidseu/files/downloads/files/panasonic_lithium-handbook-2015_interactive.pdf page 14
Any ideas for the best diode? I was thinking about a BAT54.