12 volt mains battery charger idea

I've recently tried connecting a battery on the DC side of a bridge rectifier with the AC side connected in series with the lighting ring (240v@12mA to 3A depending on how many lights are on). Is there any serious problems with doing this apart from the mains voltage being present on the battery terminals? As i understand, this configuration drops about 12v on the lighting ring, the lights are all cfl and i believe they are wide voltage range ones (85v to 264v).

My dad has been doing this for a while to charge a lead acid battery for the door bell, and believes this is a 'free' 12v battery charger, but i think there must be some disadvantages to it.

It's not free power, as the lights are likely to be dimmer or take slightly more current.

Or did you mean smaller purchase price? It would probably be cheaper than a transformer, but it is certainly more dangerous.

The other problem is lack of a cut-off. Lead-acid batteries should be charged to a fixed voltage, about 14 V for a 12V battery. You circuit will feed the same current even if the voltage goes far above that. You need to monitor the voltage and disconnect if it is too high. If you left it for hours, all the electrolyte would eventually electrolyse into hydrogen and oxygen, and with 240 V available you could get a spark..........