gardtec 800 alarm system reset?

I have a gardtec 800 series alarm system. This was installed by the previouse house owner. Last night the internal alarm went off even though the alarm wasn't set. On investigation it was indicating battery/fuse fault. The alarm was silenced by inputting my security code. As we didn't want the alarm going off again in the middle of the night I switched of the mains supply to the alarm. At 6 oclock the alarm went of again ( I don't know what the alarm said but on further checking in daylight I think it was saying the battery was down). To silence the loud internal alarm I disconnected the alarm box resulting in a small alarm noise from the code box. On investigation this morning I found the main internal fuse blown which I replaced. The code control panel remains dead and I can't reset the small alarm or reconnect the main internal alarm. Am I right in asumming the battery has flatened and if so how long will it take to recharge itself