Not sure this is a good idea if you have any battery management system !
Maybe the system will continue to treat the battery as the old aging one !
The MK1 TT did not have a battery management system, Hoggys battery lasted 14 years mine is 8 years old and still going strong, it does not matter how many systems run off the battery the manufacturers will provide a big enough battery and an alternator capable of providing enough charge to maintain the system. The main reason for the Battery Management System (BMS) is stop/start, the Engine Management Unit (EMU) needs to know the state of charge (SOC) of the battery to ensure the car will start again after each short stop of the stop/start cycle, the BMS on the negative rail of battery provides this information. The second reason for the BMS is to enable the EMU to manage any regeneration, ie overrun, braking etc when the car is not being driven by the engine but by the inertia of the vehicle,
When the EMU senses this state it instructs the alternator to increase the charge into the battery, in some makes of car this charge will increase up to 17 volts, this energy is free, no petrol used. To accommodate this high charge burst the battery needs space otherwise boom, to make space the EMU will not allow any charging other than from regen if the SOC of battery is above 75-80% the EMU will always try to maintain the SOC at that level, that`s fine with a new battery, not so good with older batteries that probably still have some useful years left if stop/start was not functional, not so good if car is used on short journeys with little regen available.
The real on the road fuel gains appear to be small with this system in place, the system also adds another layer of electronics that will probably go wrong as the car ages. If the BMS is disconnected which I did 6 months ago on my 2012 Merc nothing alters, the car drives as normal with no EML, it charges normally at between 13.8 and 14.3 volts same as my MK1 TT, with the device disabled any type of battery can be used, the EMU doesn`t care as it doesn`t know the SOC of battery or the type of battery installed so no need for coding new battery, the battery will now charge same as older cars like MK1, I disconnected sensor to disable stop/start which on the Merc cannot be coded out. One other benefit is that the alternator will now always attempt to fully charge the battery which is better if car is used for short journeys.
If stop/start has already been coded out any fuel gains from regeneration are so marginal there is little point in this more complex system.
