RTC (REAL TIME CLOCK ) or backup battery its being used for giving backup supply to digital clock crystal circuit .
its help to putting the time of watch its real position , in the old mobile device which are don’t have backup battery or rtc battery when we remove the main battery of mobile phone, the clock has been reset . After then aging we putting the main battery the device the time of device need to setting again.
The back battery is very small & compact its look s like as wrist watch battery, its 2 type. Back up battery are provide 1.5v to 3.0volt supply to digital circuit, normally in the mobile device we have found 3.0 volt back up battery its small size & it also provide long life service .
If you need to identify the backup battery its working or not , just remove the main battery from mobile device & put it aging , if the phone asking for setting after power on that means backup battery or rtc battery of the device faulty . Its need to be replaced & fix the issue, if after put the battery in handset time & date working fine that men’s backup battery working fine.& no need to be required to replace .