Replacement Battery Doesn't Fit!!!

Hi,

Wonder if anyone can help?

Advised by the fiat garage to replace battery after a recent service - which I am in the processes of doing. However the replacement battery is too big to fit in space left by old battery. Have checked battery specs on side of case it does say suitable for a Multipla on the side!?! Checked with battery supplier and they confirm its the right one for a petrol multipla 1.6 coming up for 3 years old.

I have noticed that there is a small tray which the old battery sat on. This is being held in place by a rubber plug. I'm sure if I pulled hard enough it would come away. I'm not sure I want to do this as the same sized battery would then slip left and right. Even so the new battery is about 4 inches wider and would be a very tight squeeze.

How hard can it be to replace a battery?!?

Any ideas?