So does my C-Class and my 2015 SLK 200 with tuning box, both these cars have the sensor disconnected, both batteries are now being fully charged not just to 75% as before, on the C-Class stop/start stays orange, on the SLK the light goes out, both cars have recently passed MOT`s with the sensor disconnected. According to the Honest John Web site the fuel gain between the SLK R171 and the SLK R172 is 1.9%, the R172 has Direct fuel injection, stop/start, energy regeneration and turbo which the R171 does not, all that tech for just 1.9% fuel gain, you don`t need an expensive battery on the R171 either.CLA have 2 battery, 1 main for the car, and an other one for stop/start.
By disconnecting the sensor you will loose stop/start, energy regeneration and brake hold, The reason an expensive AGM battery is needed is to allow stop/start and regeneration to function, thats it, its also needed ifthe battery is inside the car.13.6 volts WILL NOT overcharge the AGM battery neither will 14.3, do the maths. Both my Mercs charge at 14.3 volts when cars are first started, on a long drive the charge rate drops to 13.6/8 volts. The MK1 3.2 Audi TT first appeared in 2003, the battery was situated within the car and had an AGM battery, the car had a standard alternator which gave a continuous charge of between 13.8 and 14.3 volts , some of these batteries have lasted 10 years or more in the UK, this should show that a continuous charging regime of 13.8 to 14.3 volt is fine for an AGM battery.As for your constant charging method with the sensing circuit disengaged, interesting idea, though have you refilled electrolyte levels due to burn off? One can literally hear the internal bubbling at >13.6V charging levels with constant current feed. Though it seems you have been lucky