The battery on my A2000 is at the front of the machine
about the middle, almost under the middle floppy drive
slot. And yes, its a barrel shaped thing about 3/4th of
an inch long and about 1/2 inch in diamiter. It most likely
needs to be replaced. I replaced mine with a 3 1/2 volt
phone battery. If you are unsure how to go about replacing
it, do a search of the older threads here on A.org.
There should be something on it, if you can't find anything
then ask here on A.org. Someone will explain how to do it.
Back to your boot problem, try pulling all the cards from
the card slots. Just a bare MotherBoard with the floppy
drive. Boot from your WorkBench disk. Unless you have
a bad WB disk or more serious problems on the motherboard
it will boot.
If you can get that far (boot on floppy) then its trial
and error to find which add-on card is causing the problem.
If it boots on floppy then add something (Harddrive maybe)
and try booting again.