Acer Aspire 5101 – cmos battery?

Hello all,

My year old acer has started to lose track of the date and time and i need to reset this in bios every now and again. i am guessing it is a CMOS battery error and think i will replace it, but i was just wondering if any one can tell me what type of battery i will need? and where it will be located?
The amin board is an Acer Navarro.

thanks in advance

will