I recently bought a 2008 mini cooper R56 base. It has a newish battery, and does not show any battery or alternator fault lights or error codes.
But it recently left me stranded with a dead battery. I now put it on a trickle charger when it sits in the garage, and I keep an eye on the battery voltage. I noticed that after it has been on the charger the battery is at around 13V, and jumps to 13.8V after I start the engine. So it looks like my alternator is working OK. However if the car has been off the charger for a couple of days the battery voltage drops to around 12.4V. The car still starts, but the battery voltage stays at around 12V and it looks like the alternator is not charging. I can still drive the car, but if I stop the battery usually does not have enough juice left to start again.

I measured battery voltage and alternator output several times, and the magic battery voltage appears to be12.7V. Above that the alternator kicks in after starting, but below that the alternator stays off.
Does anybody have any suggestions for further diagnosing or repairing this? I am afraid to bring it to a dealer, since they will most likely just start replacing batteries and alternators, which may not be the issue.

Also; I read all the threads about battery registering and coding on the forum, and determined that my mini does not have the IBS system (no connection box on the battery terminal).

Thanks!

 

Three easy diagnostics you can do:

1. Measure battery voltage at rest.
2. Measure voltage *during starter cranking*.
3. Measure voltage when engine is running.

I'm sure you've done those- but- if the cranking draws the battery down significantly (the battery fails a "load test") then the charging system is working hard to charge that battery, which will demonstrate as a low voltage reading- the battery can create a significant load on the system. If the battery drops below 9 or 8 volts while cranking, it's probably needs to be replaced.

Battery voltage is a slippery number- a bad battery will easily present 12v at rest- but that does not tell you anything about it's ability to handle a load, nor to be charged.

 

My 2010 LCI charges the battery at 14.9 Volts once the engine is running, irrespective of how low the battery voltage had dropped to prior to starting the car (in two instances as low as 11.5 Volts for example). The onboard voltage only drops/increases slightly for an instance if there is a substantial load change (e.g. if the head lights are switched on or off) and almost immediately returns back to 14.9 Volts.

The alternator keeps the onboard voltage somewhere between 13 and 14 Volts (sorry, don't remember the exact value) once the battery is almost full.

EDIT: My car has an AGM battery and IBS.

 

Fakename - I also suspected the battery, but the difference in voltage is so dramatic that I thought it had to be something else.
Giorgos - Thanks that is useful to compare what I am seeing to a well functioning car

It looks like my alternator is not getting exitation when the battery voltage is low. Maybe a bad connection, or maybe the ECU decides that the alternator does not need to charge at (the wrong) times. Any suggestions how to further trouble shoot these two possibilities?

 

First step should be to take the battery to get load tested.

 

I'd check the connection plugs & grounds first. But it does sound as though the alternator is on its way out.

 

Apologies, I forgot to mention that my car has an AGM battery (btw: still the one from factory) and IBS.

Charging voltage for non-AGM batteries is usually recommended to be in the range between 14.2 V and 14.4 V (the latter being the max limit for non-AGM batteries). Unfortunately, I don't know at what voltage an R56 charges a non-AGM battery.

 

Update: It was not the battery; I bought a replacement and the car did exactly the same. I was able to drive some long distances, but always had to "jumpstart" the alternator, by hooking the battery up to another car (with battery voltage above 12.7V) during starting.

This weekend I finally bit the bullet and changed the alternator. The alternator that was in the car when we bought it looked brand new (aftermarket), and still had a nice "triple checked" sticker on it. But the very old old and dirty (but OEM looking) alternator that I got for $55 from ebay fixed the issue. The shiny one on the picture is the bad alternator, the filthy one is the good one!

By the way, I tried really hard to to the alternator swap without putting the car in service mode, but impossible. Even after removing VANOS sensor, dipstick and exhaust heat shield, and some cutting on the plastic radiator support it was too tight. Getting the car into service mode turned out to be easy though..

 

From what you’re describing, your alternator seems to work fine when the battery is at higher voltage but doesn’t kick in when it’s lower. That could still point to a weak ground or a bad connection somewhere in the system. If you haven’t already, I’d start by checking all the connections, especially the grounds, for corrosion or looseness. Another thing—if the battery itself is struggling to hold a charge, it might not give the alternator what it needs to start charging. A load test could confirm if the battery is the issue. When I had to replace mine, I went with a group 26 battery, which fit perfectly and had enough power to handle everything without breaking a sweat.