Hardware: type of the small cell close-by the USB-socket

[olke]

I saw a small cell close-by the USB-socket.
What is it for? (Bios, clock...... ???)
After how many years I have to replace it?
Which type is it (VARTA)?

Thanks
olke

P.S.: I had have too many trouble with PC-Bios cells.

[swifty]

you don't have to change this cell even if it is empty.
if your main battery is loaded an in the mobile you will not have any problems.
if your battery is completly empty or out for serval hours, your A780 will lose its time and date.
it lastes 2+ years before this happens.
if you dissasamle your mobile you can read the type of the cell for replacing.
if you have the type you can post it here

[gewe]

Why do you think Varta? The image shows Sanyo.

[jendral_hxr]

I've once replaced mine with similar (in size, also voltage) battery, yet it didn't seem to be working.