How to calculate RTC battery life time?

[Dat Tran]

Hi everybody,

STM32H7 RTC document says "The RTC consumes 225 nA at 1.2 V", does it mean 225nAh?

if so, it a battery has 140mAh / 225 = 622,222h hours, which is around 71 years?

Thank for help.

[TDK]

> STM32H7 RTC document says "The RTC consumes 225 nA at 1.2 V", does it mean 225nAh?

No, nA is a current, nAh is a unit of charge.

> if so, it a battery has 140mAh / 225 = 622,222h hours, which is around 71 years?

If the battery didn't self-discharge or degrade in any way, yes. Also note current consumption goes up with voltage, and your battery won't be at 1.2V all the time.

[Dat Tran]

Thank so much.

[TDK]

I can't find the 225 nA value. The current consumption in low power modes is in the datasheet. 225nA seems way too low, it's like that is just the RTC portion of the current being used.

[Dat Tran]

Hi, I found it here: STM32H7-WDG_TIMERS-Real-Time Clock (RTC), in second page. Is it correct?

Yes, we only care about RTC to calculate how long or battery, 1 year, 2 year..... Not consumption of all entire system.

[TDK]

I would rely on the datasheet rather than promotional material. The datasheet for the H742 says typical current consumption is 0.24 uA in vbat mode with LSE/RTC on. I have no reason to doubt that value.

[Dat Tran]

Thanks,

Just small wondering, isn't 0.24uA ~ 240nA, not much far away 225nA?

[TDK]

Yes, it is. I’m surprised it’s that low but I don’t have a reason to doubt it.

[waclawek.jan]

Note, that that's for rev.Y. Numbers for rev.V are roughly twice as much.

Also, to avoid any disappointment - we all know the value of "typical". There surely will be a temperature-dependent portion, too, as leakage will play non-negligible role here. Also, LSE's consumption (and stability, precision etc.) will depend on the minute details of the crystal and surrounding circuit.

But yes, a decently designed circuit will probably tick away on a single CR2032 for decade.

JW

[Dat Tran]

Thank a lot.