The official BMW guide, as listed in earlier posts, is to "connect a battery charger to the jump start points under the bonnet.
This allows the IBS to monitor the charge state of the battery".
People should follow BMW guidelines.
However, I believe it is better to directly connect the charger to the battery terminals as long as it is suitable for AGM batteries.
(Assuming there is an AGM battery fitted).
The following explains why. Skip it if your not interested
BMW Efficient Dynamics.
The car braking system is used to slow a vehicle down and looses energy in the form of heat, dissipated by the brake discs & pads.
Some of this energy could be used to charge the battery, in the form of regenerative power.
The Intelligent Battery Monitor system is used to protect the battery from over charging.
The battery capacity must be registered with the IBS system, in order for it to monitor how much energy can be dumped into the battery, without over charging it.
The IBS system uses a 70% charge target for the battery. (70% charge of the battery capacity rating).
Regen braking can use the 30% available capacity to dump energy, reducing brake disc & pad wear.
The IBS also has a derating curve for the battery age and reduces its capacity rating accordingly, again to prevent overcharging.
The problem I see here is, there is a likely safety factor, used to keep the values on the low side to ensure no overcharging occurs.
So the battery capacity will be under-rated.
The aging capacity reduction will be over estimated
(There's probably a temperature derating function and maybe others).
All leading to the battery capacity being well under estimated.
The biggest killer of lead acid batteries is under charging.
This leads to sulphation.
So the principle of keeping the battery at a nominal charge of 70% is not doing it any favours.
In reality, it's problably kept at a level of charge significantly less than its actual capacity.
Using the under bonnet terminals for charging allows the IBS to monitor the energy the charger puts into the battery.
It'll adjust what the car alternator puts into it on subsequent journeys and likely well under-charge it again.
So, get on those battery terminals. Charge away!
I do.
