Hi Al, Your diagnosis sounds correct-the panel will take any battery from a 1.2-6.0 Ah-proberably best to take the lid off and directly replace whats in there at the moment-obviously when you take the lid off it's going to go into tamper-just put your code in to shut it up-when you've put lid back on just reset the panel.
Regards Richard-Conwy Intruder Detection