A customer asked us some question related to the battery life for the RTC and some good practices.
Basically, they’ve been using CR1220 (35mAh)、BR1225 (48mAh) batteries which they estimate they will get discharged after 2 or 3 years of normal use (highly dependant on the use but I believe is their estimate).
The following reasoning assumed that the RTC was always powered from the battery: However, just to be sure, I’ve done the maths from my side and given the best case scenario that the M41T0 is always in standby with a 0.9uA supply current, the 48mAh battery would last around 53333 hours or around 6 years (correct me if I’m wrong).
I don’t want to get too specific here as the objective of this question is to ask for any recommendation or hardware approach they can follow to increase the battery life up to a least 10 years, to avoid having to exchange the batteries that often as they expect the minimal maintenance and the final solution would not have the battery easily accessible.
I was checking the schematics with the customer and I could further analyze the case.
I would dare to say there are 3 different cases, so please feel free to comment on this to further confirm/correct me:
The board is powered and the RTC is in standby. In this case, the RTC standby current is 0.9uA. However, the RTC is powered by the 3.3V, not the battery (at opposed at what I thought at first, sorry), and almost all the current would be coming from this pin rather than the battery, is that correct? To further confirm this, we could measure the voltage difference in both resistors behind the diode (R37 and R36). Unless I’m missing something, the current from the battery should be very very close to zero.
The board is powered and the RTC is on since the time is being loaded. Similarly to (1), the current is obtained from the 3.3V, not the battery.
The board is powered off. This is where the battery come to play. In this case, we will have to take into account the leakage current from the diode, as D4A would be reverse polarized with around 2.7V-3V. Checking the datasheet, it should be around the following values:
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which would be around 0.05uA, because the major temperature source (CPU) would be powered off leaving the diode around 25ºC (or less). In this case, to the 0.9uA of standby current of the RTC, we would add this leaving the calculus to:
48mAh／0.95μA＝5052hours（5.7years).
Therefore, as long as the board is powered half of the time, a 48mAh battery should last 10 years.
You are right: when the carrier board is turned on, the RTC circuit is powered from the carrier board 3.3V. The battery only gets discharged when the carrier board power supply is not enabled.
Regarding the 10 years, I would say that it makes sense also to check if the battery can maintain the charge for such a long period or there would be some self-discharging situations which need to be taken in account.
Please check if the battery datasheet provides this information.
I did a quick online research and find some information:
Try google “world’s lowest power RTC” to find other options. Pay careful attention to what voltage the device cuts out at, as that can impact lifespan. In our measurements, we were able to get 6 months backup at room temperature with 1.5F supercap using the first link above.
