Hey all,

I've had my 2015 S for a few years now with zero issues. I recently had a battery drain issue that I traced back to an aftermarket line driver for a sub system i have installed. While chasing that down I checked on my battery to find it was the original battery from 2014, so I swapped in and registered a new battery just in case. Everything works great and fine, however I have a multi-port cigarette charger with a voltmeter built in that reads 12.4-12.8v anytime the car is on for the most part, I verified this with a separate voltmeter that reads identically to the adapter. The highest voltage I've seen is 13.5v and that's when engine braking at highway speeds, for whatever that's worth.

So my big question here is, is this a sign of a failing alternator? voltage regulator? It's about time for me to do the serp belt so ideally if the alternator is toast then I'd like to do them at the same time.

What are your thoughts? What does your cars voltage sit at when running? Im used to 14.4v being the norm, so the mid 12s is concerning for me, unless that's just where our cars sit...

Thanks in advance

 

I've been down this rabbit hole -- short version answer is that alternator output on a BMW, including your F-series MINI and mine, is dynamically controlled by the DME. You won't see consistent output because the DME is constantly turning it up/down/on/off based on all manner of inputs and calculations it's doing. You will sometimes sense patterns in when it goes up or down but you will occasionally think the pattern might be your imagination, because it looked different that one time/etc. It's not your imagination, and it was different, and you were right about there being patterns that aren't always consistent because... it's always, always dynamic.

So here's an example.

- After going down my rabbit hole, and replacing a battery that got killed by the actual problem of parasitic draw, and almost replacing my alternator before finding that parasitic draw (a draw that existed courtesy of BMW/MINI instructions no less, and that someone else had happen too after I figured mine out), I got all that patched up to find that the alt is 100% ok.

- At the same time, the charging patterns still feel weird, and they specifically strike me as being a bit meh if running in Green/ECO mode, or at least different if in Green/ECO mode, but I can't really prove it. I can just sort of intuit it by watching the ups/downs and the car's behavior.

- And finally, I think the behavior I see in Green/ECO mode, when combined with the kind of automatic stop/start feature use one will see with common city driving patterns (stop/go traffic), will gobble a battery pretty good in 3 years, in keeping with what I hear service shop-side about a lot of MINIs gobbling batteries in 3 years or so (more like 3-4 years I think, but you get the idea). In fact, when I had the parasitic draw in play, duplicating most serious "fallout problem" (EPS malfunction) was doable at least intermittently in Green/ECO mode, but nigh impossible to replicate at will in MID or Sport.

For another reference point on this, over in a BMW M5 forum discussion, post #32, I found this chunk of text. Note that "VR" = Voltage Regulator and "TIS" = BMW Technical Information System:

The takeaway here isn't the bit at the end about dimming headlights. Heck, if you have LEDs you definitely won't see that. The takeaway is the way the DME is directly fiddling with the alternator output in order to manage all sorts of scenarios that those of us who know alternators old-school aren't thinking about when we put the multimeter on the leads on that cigarette lighter/accessory adapter (I have one of those, woot!) Our insights are on point -- but with realtime computer-controlled alternator output, the rules have been changed on us.

Finally, most of my observations since I fixed the parasitic draw have been from using Bimmerlink in real time -- meaning I can see current values as well as voltage values, and I'm seeing what the DME thinks/feels/says they are. Pretty informative. Intuitive? No. But informative? Yes. Totally recommend trying that approach, even though it will probably make you wonder even more about how it decides what it decides and when.

I did actually replace the brushes on my alternator -- with the alternator still in the car, no less (that was quite the exercise) -- replacing that voltage regulator is not going to happen, it's soldered to a stator contact and I'm not getting into dealing with all that. I'm sure my change helped the alternator just a little for longevity, but in material terms as pertains to the output, eh, not so much.

Hope this is helpful.

 

Holy crap what a well written and informative reply. I've been on forums for years and have never been blessed by a reply like that. Thanks for all the info and examples.

So tl;Dr is it's probably just fine the way it is because it's constantly changing based on the dme doing what it thinks is best for the car.

I'm good with that, thank you again 😀