[DieselJan]

The diode right at the top is indicated as 0,3V(marked HER508). The HER508 actually has a voltage drop closer to 0,4V. Germanium diodes normally have a voltage drop of 0,3V. Other diodes like 1N4007 has about 0,5V to 0,6V drop.

So depending on what high charging voltage you want, you can change that diode to a different one or put 2 or more in series. Lets say your alternator stock charges at 14V, and you want it to fast charge at 15V, you simply add 2 of 1N4007 diodes in series in the place of the HER508. Or add the 2 diodes and put a bypass switch inside cabin to bypass one so that you can decide what you want the voltage to be. 14,5V or 15V.

Remember that there has to be at least 1 diode in the position of the HER508 or it will work as stock, and the circuit will have no effect except using some power.

You have to measure your charging voltages before putting in the circuit so you know how to set it up. Mine was 14,42V stock. I also measured the current drawn by the sense input of the alternator. It was 1.5mA on my alternator, and designed the circuit around that. The circuit total current consumption is about 5mA when running.

If your alternator sense input draws more current, simply replace the 470 ohm resistor value with something like 390, 330, 270 or lower resistance. The lower the resistance the more current the total circuit will draw. You will know that resistance is too big if the battery voltage doesn't go low enough below the regulator voltage of your alternator.

Not all alternators has a sense input so you will have to do some research according to your car's alternator to see if this circuit can work for you.