BMW 320Ci (E46) flat battery, caused by light?

Question

I just bought a second hand manual BMW 320Ci yesterday, and after driving around in the backyard for 15 mins this morning, I found I have a flat battery this evening. The engine won't start and I can only hear clicking sound when I switch the key. The door glass is very hard to move when pressing the button which usually adjust it smoothly. Because I got this 2004 159K KiloMeter car not so expensive, I was very worried if the seller sent me a problematic one. I am/was a computer nerd with some DIY experience and I am really a new fish on cars.

When the car stopped ignition this evening, I found the light switch was in the side-lights instead of the "0" position (I think there is difference between Australian and other/European version, in this picture the OFF(12 clock position) is presented as "1"), maybe I made a switched the night before or in the morning, but I really couldn't remember.

So the question is: Can this cause a flat battery?

If I turned it on in the morning, will it cause a flat battery in 12 hours? Why doesn't the designer set the lights off automatically when I lock the car?

BTW the battery was replaced in 2011 and the warranty is 36 months. All time in my mention was in Sydney local time, if that causes confusion. If I miss anything please just ask. And please forgive me for being a new ignorant driver :)

Update:

My neighbour confirmed that the side lights were ON at least 10:00AM in the morning. So it got lit for at least 9 hours during the day. And my seller said he didn't drive it frequently in the past month. Hope I could fix it by purchasing a battery charger.

Answer 1

Can this [leaving lights on overnight] cause a flat battery?

It is quite likely that leaving the parking lights on overnight has drained the battery.

The first course of action would be to jump-start it and see if things hold out.

You may also want to make sure that it is getting properly charged by the alternator (13.5 - 14 V with the engine is running).

Why doesn't the designer set the lights off automatically when I lock the car?

Automatic lights were introduced as an option around 2003 on higher-spec BMW's. It seems that your vehicle does not have that option.

Answer 2

Yes, leaving the parking lights on overnight will drain the battery.

You may well find that, especially if it has been run flat a few times, that the battery will hold less charge than before. A 4 year old battery may well need replacing (q.v. My own car which recently needed a new battery despite the old one being less than 3 years old...)

The parking lights aren't switched off automatically as they are parking lights - intended for when you park at the roadside on an unlit road, to warn other drivers of your vehicle's presence. Some cars will have a separate switch for this purpose, or another method (for example some VWs will leave a sidelight on if you switch the car off with the direction indicator switched on)