M3 E46 help needed, is this an alternator issue?

[just1e]

My BMW E46 has been off the road for several years, battery was dead for ages so decided about a year and half ago to buy a new battery.
After fitting the new battery I started the car a couple of times but a few weeks later I found the battery dead again and couldn't be bothered to start the car for 6 months or so, by that time the battery was completely dead so I bought another battery.
This time I made sure to start the car every couple of weeks, but after several weeks the battery was completely dead again so charged it up and ran the car and left it running for 10 mins but then the car stalled and wouldn't start as the battery was once again, completely dead. Also found that the alternator was burning hot and smelling of burning.

Does this sound like a problem with the alternator or is this a problem elsewhere?

Now scared to connect the battery.
I am thinking I should remove the alternator and get it tested, unless anyone can suggest a better option?
Thanks in advance for any advice you can offer!

[Cactussed]

That'd be my first port of call.

[e21Keith]

Does the alternator get hot with the battery connected?
If this is the case it's discharging through the alternator.

[kev b]

My alternator repair guy says it is pretty common for the diodes to fail when a car is started after a long lay-up.
Happened to my FIAT Coupe even though it was inside a dry building for two years.

[E-bmw]

Replace the alternator & battery (which will now be stuffed) and you will be fine, but you need to learn the lesson that batteries aren't "fit & forget" or the same could well happen again, it has been an expensive mistake.

[e21Keith]

If the battery is indeed goosed they usually come with a warranty of minimum 2 years, so just take it back for a refund. Unlikely they will actually make any checks on the battery and you have nothing to lose.

[Mr_Dave]

Worth remembering that an alternator is incapable of charging a completely flat battery, it will just burn itself out instead (as you've found!). You might get away with it once or twice, but the rectifier will toast itself.

So, if you need to jump start a car that has an entirely flat battery, leave the jump leads connected and the other car that you're jumping it from running for a few mins to get some juice into the battery.

[robbiekhan]

Also invest in a trickle charger if the car is sitting idle even just for several weeks. My M3's battery died after 6 weeks sat on the drive during Lockdown v1. Replaced battery and bought a trickle charger and no issues since.

Also once all sorted, replace your final stage resister, this can lead to battery drain as the FSR keeps the fans/HVAC active when it starts to fail. The average life of the FSR is 8-10 years, especially if it's still the old design FSR which BMW changed in later years. My replacement is by Miele. I have changed the FSR on 3 E46s now and they were all around the 8yr age mark.

Telltale signs the FSR is on its way is that the vents may blow different temps between left and right side, of the fans will ramp up and down seemingly on their own or the climate control may do funky things. Essentially anything funky going on with the vents will generally be FSR related.

Edited by robbiekhan on Friday 12th August 15:48