There are three main reasons why a battery will go flat:

1. The battery is not serviceable.

2. A fault with the charging system.

3. Something on the car is not switching off when it's supposed to and causing the battery to drain. This can by any motor, solenoid or other resistance on the car, including the obvious things like the radio and boot lamp which I assume you've already checked.

Checking for a drain on a modern car is not such an easy thing to do because the control units do not go into sleep mode until after a few minutes after everything has been locked up and shut down. This won't happen if the bonnet is up and you're working on the car. Having said that the way to check is the same way it's always been. Make sure your keys are in your pocket and disconnect the battery negative terminal. Connect an ammeter between the terminal and the post. Take care not to switch on any loads at this point because your meter will probably only be fused at 10amps maximum. Any reading shown on the ammeter will be current drawn by whatever is switched on, and that is what's draining your battery. If possible leave the ammeter connected and close the bonnet with the ammeter outside so you can see it. Make sure the car is locked and observe the meter over a period of time, even an hour or so if you have to. About 0.2A is the maximum acceptable drain, and even that is a bit high. Check your battery's amp hour (A/h) rating. Let's say it's a 70A/h. That means that assuming it's fully charged and in top condition you can draw 70 amps from it for one hour. Similarly you can draw 1 amp for 70 hours, or 35 amps for two hours. Beware though these figures apply only to a battery that is in top condition and will completely discharge it down to nothing. The battery will be sufficiently flattened to be unable to start the engine long before this point so don't be confused. You can do the maths for your battery and the drain you have, if indeed there is one.

If you do have a drain finding what's causing it is another matter. The old fashioned way was to remove one fuse at a time until the ammeter showed no drain, but as I mentioned earlier it isn't quiet so straight-forward on a modern car.....

My 08 1.8 tdi threw up the message of low battery last night for the first time. I am so pleased I logged on tonight. Over the past 2 weekend I have driven about 250-300mls each time so I was surprised to see the message.

Last night I had turned on the radio without the engine running for about 15mins. thats all. I went back to Evans Halshaw today to query this. I was told that the modern "smart"alternator will only charge up to where the battery charge was, before it started draining, not continue charging to a full top up the old way.I think you know what I mean.

It was suggested I had maybe left the lights on overnight.(difficult to do these days with all the chimes and bells anyway!) or that I had left the radio on. This was the first time I had sat with it on ! I have the warranty until March 2011 so I have booked it in for a check tomorrow. Obviously something is not right. I thought it could just be a computer message blip as I had only driven home about 5mls after the message was seen, and nothing was displayed today.

Thanks for raising the subject, I will let you know what the result is. A bit worrying though isn't it. I hope it gets sorted before the warranty runs out. It was suggested, in view of the "smart alternator" comments above that when this happens a way of dealing with it is to recharge the battery myself and then the "smart alternator" will top it up to that level. Maybe thats why we spend thousands on these new cars !!!!

Edited by Avant on 19/10/2010 at 23:01

"I was told that the modern "smart" alternator will only charge up to where the battery charge was, before it started draining, not continue charging to a full top up the old way.I think you know what I mean."

Someone please tell us that isn't true. If that was all that the alternator did, the last thing it is is 'smart'.