When you look to purchase a new battery, always remember to check to make sure that it is an FM55H ONLY battery. There are a number of sellers out there who are advertising FM55H/50/51 batteries. All of the ones that are marked this way that i have tried have failed in the Sony A100. You will almost certainly get the message "Incompatible Battery" if you try to use one.
If you check page 142 of your manual it states - Note that the NP-FM50 and NP FM30 cannot be used.
Why they dont work, i dont know, but i'd be interested to find out!