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, Thanks,