I am afraid it is going to be a bit more involved! In desktop computers ( I mean towers or desktops) the MoBO battery is usually a CR203/5 etc and that costs only £2or3 and is easily replaced!
In case of a laptop it is possible to replace the batttert( It is called C-MOS battery) and due to the compact design it might be a more complicated task!I suggest you entrust it to a qualified repair person!