Can a bad alternator drain the battery when the car is shut off?

Might not be charged. Might be getting an intermittent charge when driving which is just enough to start it once or twice but not after overnight. At least sometimes.

I've had this with a failing alternator. But also had it with a cracked battery connector clamp. In both case an intermittent fault was hard to diagnose as often was all tested fine on a static car.

But you'll know if they test the battery in the morning. If the dealers not giving you enough information get a 2nd opinion by getting it checked with an auto electrician. Dealers are very quick to throw parts at a problem at your expense.

Even if charging perfectly when driving, a fault in the alternator can absolutely discharge the battery when turned off.

A faulty rectifier in the alternator can cause non start after a very short period sitting.

Leaving ignition switched on will drain battery quickly. Alternator fault can produce the same result.

You appear to refuse to acknowledge that it can be the alternator at fault as diagnosed by main dealer.

Doesn't diagnose your problem, but a lot of people with older tractors and plant fit dead switches which are turned off each night, to prevent any stray drain, which can often crop up in older machinery.

Im surprised no one has mentioned that the battery itself might be at fault. If the alternator is ok, and the battery is bad, then the charge going in during a run will not be stored properly (damaged cells?). But back to prev posters .... you need to get it (battery & alt) checked. On a 2011 car, a new battery is more likely than a new alternator imho.

They'd be a light on the dash if the alternator was not charging Shirley ? I'd say it's a battery you need, if there is no light. Normal to replace a battery, try that first, if the problem is still there then take it from there, but battery is first step, anyone selling a battery will fit for free.

What's the voltage across battery terminals when the car is just switched off after running, and what's it in the morning before you try start it (This will tell if there is a leak or the battery is potentially faulty) ? And also as you start it what does the battery voltage drop to? I had a problem before where sometimes my car would start and other times nothing, was random too like it might start after 12 hours but mightn't start after a few or vica versa. Turned out to be a cell in the battery (The battery was 8-10 years old in my case) when I checked the voltages.

Don't assume just because it's a new battery that it's 100% OK. We had a new battery fitted in my wife's car at a main dealer a few years ago, and it was completely dead within a week. They replaced it with a new one, and the problem was solved.

Lads, don't forget a mechanic has actually inspected this car and said it's the alternator and there is absolutely no reason given to think it isn't.

Apart from your 'gut feeling', is there any particular reason you think the mechanic is lying to you? An alternator has a finite lifespan before a rebuild is required and you've done well to get 12 years out of the original. An alternator might not go bang and die enough to trigger the dash warning light, they can die slowly over time and eventually reduce their supply voltage to a level that can no longer sustain all the cars requirements.

The mechanics OBD scan would have told him exactly what the issues are.

Jesus, this is usually a fairly open and shut case. Straightforward job for any mechanic to troubleshoot this with a multimeter one would think.

Also to add to other posters comments if a battery dropped to 7V the damage has been done to the battery. It may be savable by de-sulphanation cycle on a charger but they are never at full capacity afterwards and it is best to replace as it will let you down on a cold morning.

In the last ten - fifteen years car have tended more towards smart charge system so the alternator can produce between 12v and 17v depending on load, state of charge of battery etc so it's a little more complicated than that

Ye, as I said above, a dealer would not be the most economical way to get it sorted and the €750 quote shows that.

As mentioned by JP, a good battery should be sitting around 12.5V so when you measured it at 9.6V it was still way down. I'd guess the dealers charged it fully and that may be why it didn't die while with them.

Hopefully you get sorted with your usual guy.

Had a similar problem on my car, turned out to be a short on starter motor that was draining battery overnight, new starter motor no problems since, took a while to find problem.

This really points towards the battery itself, not a drain on the battery.

It sure does.

Have you brought the car to wherever you bought the battery and told them the battery is not holding charge. If you got a new one this year then that battery is under warranty. Get it replaced, may need to get firm with them. If the new battery is still giving trouble then it's either the alternator or a phantom drain.

And would you go back and apologise and pay for the replacement battery if you then found it wasn't the battery.

If they sell a decent quantity of batteries they will have some form of tester or know there is a certain failure rate of the battery they supplied you with.

Had a similar issue on my E36 recently. When the engine got turned off you could see the battery voltage drop approx 1 volt every 5 mins. I disconnected the positive battery cable and found 3 ohms resistance between it and the chassis. I pulled each fuse one at a time and rechecked the cable to chassis voltage each time until I found the fuse that when pulled out, gave open circuit. The fuse was for the interior lights and cigarette socket. It was a faulty USB charger plugged into the cigarette socket that was draining the battery but strangely not blowing a fuse.

The only way to properly test a battery is with a load tester.

Your problem really sounds like something is draining it rather than a bad battery though. You should get a multimeter and test the current draw yourself. It's easy enough to do.

Don't think the voltage difference means anything. As I said, when the battery is charged put the car in the same state as it would be overnight, i.e everything off and doors locked, then test for current flow between the negative cable and the battery terminal.