SIM800C losses time after power off!

Hi all,
I have a strange problem with the SIM800C about the timing. More specifically it seems that the SIM808 RTC module losses many minutes when the module is power off.
I had tested with 24-hour power off and after the power on the SIM800 gave me the time with 40min elapsed from the real time!
The battery for RTC was 12-20 type.
In the past I had worked with father of SIM800 the SIM900 and I had no problem with the RTC time.
Do you have faced any problem with RTC timing of SIM800C?
Any suggestions,

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