You have to be about 3 tenths a volt over of terminal battery voltage, from a charge source for current to flow to the LA battery.
This is based on a new healthy battery. You have to overcome the internal resistance of the bat as well as v-drop between the bat and the charging
source. Bad cables
, and poor lugs will result in a higher voltage potential needed for amps to flow to the battery. The same goes for a battery as it ages and devlops a higher internal resistance due to age, and sulphating.
An 12 volt alternator
spinning at it's design speed will have a no load voltage of about 22-30 volts depending on design, and weather
or wye connected. It's 3 -phase so you will have to do some math to correlated it to a single
phase charge source.
The charge potential of any source is the terminal voltage of the battery, as a negative added to that of the voltage potential of the source.
So a LA at 12.4 volts and a source potential of 18 volts, produces a amp potential of 18 v- 12.4 volts = 5.6 volts by its watt potential. As the battery moves higher in voltage the, a voltage limited device will produce less potential current to the battery.
With an alternator
this is probably ideal, because the overhead of the alt is a such a great potential. and the battery soon fills and requires less amps.
, we are limited to 18 volts, with pmw. This is where MPPT
comes into real effect. With the much higher potential from higher voltage panels. We can see better charge into the bats. An MPPT controller, on an 18 volt panel is going to provide littel reward after losses to the controller.
You just can't compare an alternator to a solar