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.