Supercup selection and estimated lifetime calculation for RTC.

okeskin_mak · ‎2022-11-29

Hello.

I am using a supercap as rtc backup battery on my STM32G4 processor. There is a bit of high consumption as I drive the RTC crystal high. But the highest consumed current is 0.8uA.

The product I chose is SCMS22D255PRBB0. However, it discharges quite quickly. My charging voltage is 3.6V. My operating range is between 3.6V and 1.55V. after about two days i lose rtc.

Is my capacity selection wrong? I see them using supercap in many designs. Is there something I did wrong? Or does every project using supercup make a short-term backup?

Bubbles · ‎2022-11-29

Hi @okeskin_mak,

you can take advantage of the CubeMX integrated power consumption calculator to estimate the battery life. That should spare you surprises like this.

BR,

J

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okeskin_mak · ‎2022-11-29

Of course, first of all, I used cubemx during the design and it showed me 1.05uA consumption. Isn't it abnormal to run out of 2.5F supercapacity in 2 days even with this consumption value?

I have no problem with the current consumed by the MCU. Do you have an answer about supercap selection?

BR.