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 !!!!
Ford Mondeo - 2008 Mondeo - 'Low Battery' AGAIN...! -
"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'.
I have a flat battery and a new super smart alternator unfortuneatly it will only charge the battery to its last known position which is flat so I still have a flat battery.An alternator works on the resistence and output of the battery if its for example a 70amp unit the alternator will deliver charge until the battery can deliver 70 amps while running the car at the same time (once a car is running you can actually remove the battery and it will still run)and then the alternator output is minimal the alternator output will increase as the battery requires charging.
There are a few more complications but thats the basic what the garage have told you is rubbish worthy of a mills and boone.
Well,reporting back. Dropped car off am. and said I had seen the same problem reported on motoring forum. I was advised not to go on them. Only complaining people subscribe!!! Anyway,car collected.No fault found. They charged the battery,checked the alternator rate and found no draining. Apparently battery must have lost a charge over time. I again explained I had just returned from 2 recent trips,each about 200-300mls and then at least the battery should be well up. Ah, I thought.I am getting somewhere when the next reason arrived. Maybe the battery was not fully charged when I bought it. (Nearly 2yrs ago! It was a Ford Direct supplied by the same garage. Nothing else I can do now I suppose. Sadly, for the 2nd time following warranty work (I had a faulty auto headlight sensor last winter) I had to ask for written proof of what had gone on today. They must be aware that I'll show it after the warranty period ends, if the "fault" shows up again. If it does I'll say it should have been fixed the 1st time. What else can I do ?
Volkswagen has confirmed the pricing for the eight-generation Passat saloon and estate. Deliveries will begin in January 2015, with prices starting from £22,215 for the saloon and £23,745 for the estate.