I am a relative newcomer to motorhoming (1st van bought July 2012). I have recently come across a problem that has me fooled. My CO detector is repeatedly activated when the van is connected via EHU to the domestic mains supply. This started when I hooked up at the start of winter and started to occasionally run a oil heater during old spells. At first I assumed it was the heater so removed it. The problem persisted so by a process of elimination (including shutting off all gas appliances at both the bottles and isolation valves and replacing the detector) I have now discovered it only occurs when on EHU and the leisure (not vehicle) battery is charging. I have 2 x 100ah batteries in tandem fitted. My control unit display is showing the leisure batteries at "poor" level and less than 11v despite charging, so I assume I have at least one knackered battery. I am resigned to buying 2 new batteries as I read in the forum that once one goes it is better to replace both. What does puzzle me is why is the CO alarm activated when the issue appears to be unrelated to non efficient combustion (unless the battery is cooking). Has anyone else come across this problem or have any ideas?