Choosing your DC charging and split charging system.

Now we have added an external alternator controller to the same installation.

See all the extra complexity. That is purely and simply to get rid of the volt drop introduced by the diode splitter.

Now there is a sense wire to the alternator controller. This is what it uses to measure the battery voltage and thus control the alternator.

It is never made clear which battery bank this should be connected to. And with very good reason. Because whichever battery bank it is connected to it will still cause problems.

Remember that the volt drop across the diode splitter is dependant upon the current through it?

Assume the sense wire is connected to the engine start battery (which will usually be drawing less charge current). The volt drop through this half of the diode will be less than that across the half feeding the domestic bank. The result is that the domestic bank will be charged with too low a voltage.

If the sense wire is connected to the domestic bank, then the engine start battery will be charged at too high a voltage.

There is nothing that can be done about this.

Proponents of split charge diodes and alternator controllers are very careful never to mention this. But it is a fact. It cannot be denied. Either one battery will be undercharged, or the other one will be overcharged.