Replace the standby battery. They're only designed for about 4 years use (I suspect yours has been in longer), some fail before this others may be ok for another year or 2, but manufacturers of batteries only design them for about 4 years use. The battery in the external/internal siren does tend to last longer as it is built differently and because of the power cut and the fact that the panel battery isn't holding a charge the siren is letting rip with it's battery. The siren should stop itself within 20 minuites to comply with noise nuisance and your neighbours. (If it dosen't then get it replaced as well before you have enviromental health breathing down your neck.) Hope this helps.
