MrNoodly said:
If your charge controller is like mine, it disengages itself when it senses an another charging source.
Can't speak for every product, but usually charging sources will work together until the battery voltage, or the voltage seen at the charging source output terminals, on one of the charging sources, reaches or exceeds its programmed setpoint.
For example, if my solar controller is set for 14.4v absorption and battery is depleted and I then start engine, the solar and alternator combine their current until that 14.4v threshold is reached/exceeded, and only then the solar controller stops providing current.
Now the alternator can provide huge amperage and can quickly bring the only minorly depleted battery voltage upto and past 14.4v, so it would appear the solar controller instantly shuts off, but if the battery were well depleted, and the vehicles voltage regulator decided to only go to 14.2v, while the solar was still seeking 14.4v, then both should combine their current for a period of time until the high amperage of the alternator and the Solar,were able to bring the well depleted battery to or above the 14.4v absorption voltage setpoint of the controller.
The voltage regulation of the alternator and solar controller and any other possible charging sources can all play a weird game when all have slightly different setpoints. usually it is of no consequence to recharge time as the battery is up in the 80% range when they all start trying to confuse each other, and one then does not require multiple charging sources as the acceptance rate of the battery is limited at that point.
How much amperage is required to instantly, or nearly instantly bring a 50% depleted battery to absorption voltage varies greatly, with battery capacity, and battery health, and the cabling between alternator and battery all playing a huge part in the variability.
When new my group27 AGM maxed out my alternator at ~106 amps before voltage
Now with ~400 deep cycles on it 3 years later it only takes about 85 amps to instantly bring it to 14.5v
My vehicle takes 8.2 amps to run the engine at idle, and 12.2 amps at 2000 rpm. I can lower my vehicles voltage regulator to the mid 13's, and if the Sun is high, my solar panel can power a portion of my Van engine electrical needs.
It runs amazingly smoothly and is quicker revving when the alternator is not having to make any/little juice, compared to when it is cranking out 64 amps to power the engine and recharge a depleted battery.
each ~25 amps the alternator makes requires ~ 1 engine HP.