I seem to have a drain that is causing my battery to go flat. It seems to have started around the time that i installed a new pioneer MVH head unit and edge subwoofer. These both turn off and on with the ignition and i used a wiring harness from Halfords to install the stereo. I have since removed the radio and sub and i still have the same problem. I did a 200 mile journey yesterday ( after jump starting it which took ages ) and had no issues whilst driving. This morning i come to start the car and it is flat again (doesn't even attempt to start and won't lock) with the battery reading only 9v . If anyone has any help it would be greatly appreciated as i don't have the time or money to get it looked at professionally. 

You need a new battery, 9V means one of the cells is totally dead.

My battery went flat overnight once, had various other random electrical faults.

 

The battery was knackered 

Josh- most of the fast fit places and breakdown services will test a battery FOC. Tip, if you do buy a new one, don't let the fast fit dispose of it for you. Find a scrap metal place and weigh it in. I was pleasantry surprised to get circa £10 for two 12AH mobility sealed batteries . Makes that replacement battery a bit cheaper.

Cheers everyone. Bought a new battery and seems to have done the job.

I and others have mentioned on this forum that colder weather sorts bad or failing car batteries  out. 

Basic electrical theory Andy. Cold oil means higher cranking current. Cold batteries don't work as efficiently as warm ones. Then there's age .On a float system, battery efficiency tends to fall of as batteries age and sulphation creeps in. But the tendency to do away with cell access means that keeping an eye on SG cannot give a clue as to battery condition.

Edited 8 October, 20196 yr by VWD