[TimV]

To lower the currentdraw from the alternator, i want to do a trick like some hypermilers are doing with there hybrid car.

Pushing some current in to the cars battery, but it is limited in this case. (dont have an electric motor pushing me around )
If this is succesful, i can go to an altdelete, or kill switch.

I have an 14,8V 26Ah battery. But i cant directly hook it to the cars battery. Maximum charged voltage is 16,8V and empty voltage (under load 10.8) 12V

Or i can hook it up to have a 29,6V 13Ah battery.
Maximum charged voltage is 33,6V and empty voltage (under load 21,6) 24V

Then i need some kind of step down module.

Otherwise, i can't charge it high enough, because charging it to only (lets say 15V) then it is only charged about 40 to 50%

The point is to grid charge this battery at home, and use it to lower the stress on the alternator. And thus maximizing fuel economy.