Why's the battery in Tim's VW dying such a sudden death?

Dear Car Talk

Dear Car Talk | Aug 01, 2010

Dear Tom and Ray:

I purchased a new 2010 VW CC, and the car has 5,000 miles on it. During a long weekend, I left the car unused for four days, and the battery died. There were no lights on or external power drains. On Tuesday I went to the dealer where I purchased the car and was told: "There is nothing wrong with the car -- they just do that because there is a lot of power drain due to the computer." I called another VW dealer and was given a similar answer. My question is, How can a car company design a car with a battery that lasts only four days? Shouldn't a car be able to sit for more than four days without the battery dying? Thanks.

-- Tim

TOM: Yes, it should. In fact, VW itself says the car should be good for 30 days without a problem. So I think you have every right to go back to your dealer and ask him to investigate further.

RAY: In our experience, most cars can sit for about two weeks these days before the battery is too weak to start the car.

TOM: Why is that? Well, there are certain electronic components that continue to run even when your car is turned off. There's usually an alarm system, and there's the evaporative emissions system, which needs to cycle whether the car is running or not. Normally, there's enough juice in the battery to keep that up for a couple of weeks, unless it's extraordinarily cold outside.

RAY: But there are a couple of things that could shorten that time frame. One is a weak battery. That seems unlikely on a brand-new car, but maybe your battery is defective.

TOM: Or perhaps you were making lots of very short trips leading up to the long weekend, and never really charged up the battery.

RAY: The other possibility is that something is draining the battery too quickly. The dealer should be able to put an ammeter on the battery with the engine off to see how much current is being drawn off. If it's an excessive amount, then he needs to figure out what's causing it.

TOM: Maybe there's a fault in the alarm system or evaporative emissions system. Maybe there's a courtesy light that's staying on. Maybe a trunk light.

RAY: Or maybe there's a factory defect. I remember that VW did have a problem with new cars being delivered to dealers with batteries that had drained during shipping. That problem supposedly got fixed, but it's something the dealer could look into for you.

TOM: If you can't figure it out, and this continues to be a problem, your last-ditch solution would be a trickle charger that you can leave on the battery when you go away. That'll ensure that the battery stays charged, that you're able to start the car and that you don't lose all the presets on your radio. But that's kind of a Mickey Mouse solution for an expensive new car. So push the dealer a little harder, and tell him what VW said. Good luck, Tim.

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