[JimCash]

This battery issue has to be thought of in a totally different light than previous experience.

It is probably true that the cold weather is the major factor here - but only because we are using much more power in cold weather and we also tend to have short trips.

BUT - contrary to some comments this is not because BMW are putting in cheap batteries or alternators. In fact they are now installing higher capacity alternators and AGM batteries.

The real issue that is causing confusion is due to the need to meet better fuel economy - and the process of braking regeneration.

The energy management system is designed so that when you are coasting or braking all fuel is cut to the engine and it is the forward motion of the car that keeps the engine (and alternator) rotation. It is at that point that the alternator is activated to provide full charge voltage to the batter. So it charges with no fuel usage.
BUT - so that this can function the batter must be able to take that charge (if even for a few seconds). To take a charge the batter must not already be fully charged - therefore the system never fully charges the battery in normal driving conditions.
- Basically normal driving charges the battery between 60 & 80 %.
- The system also tries to prevent charge from getting below 40% so that there is always enough power left in the battery to allow it to start the engine.
- To keep this balance it performs many tricks - reducing the power to various accessories according to a very complex set of rules, and increasing the output of the alternator when required (including raising the idle speed if required). But as mentioned above it does not do this beyond the 80% charge state.
- Since batteries do need the occasional full charge there is a timed program that does give them a full charge. I do not know the details of when it does that.

BUT all that does leave us very vulnerable in cold weather - especially when we treat our cars like we did years ago when they used much less power.

Bottom line - it is almost necessary to hook up a charger to at a frequency determined by your use of the car. Some people do it once a month - my dealer said some people are putting a trickle charger on every day.
Personally I am not a fan of always having a charger on the battery. I use a 3 stage intelligent charger and hook it up every 1-2 weeks - depending on how the car has been used. If I am ever sitting in the car adjusting settings, etc then I connect the charger.

This is an issue that BMW is struggling with. The additional charging system is one way they are trying to address this. I think they should be providing this as standard in any country that has winter conditions.
BUT - I do think BMW are being very narrow sited when they advise owners to take the car for a long drive. That does waste fuel and is directly against the concept of environment protection. It also does not work well due to the fact that the battery is still not fully charged in those conditions - but I suspect the people that are putting out that advise have not even bothered to learn how that system works.

Hope that helps.
Cheers
Jim Cash