With SOC in the 80% range, the controller doesn't deliver that much power to my AGMs, but picks a lower value it believes is safe and healthy given the SOC. Let's say it is using 171W, 13.5V and 13A (numbers not far from reality but not necessarily real).
It's all good!
Now, plugin a 135W laptop charger, and turn on the inverter.
Controller meter continues to say 171W, 13.5V, 13A.
Can any controller provide extra power because it knows there is a load on the system above and beyond the "load" of recharging the batteries?
I am not an expert, but my understanding of how these systems works may be slightly different than yours. My understanding is that current flows because of voltage (electrical pressure) differentials. A charger can't "force" current into a circuit (or a battery) without increasing the voltage (pressure); alternatively a voltage drop in a circuit will cause more current to flow if it is available from a current source. If you add a load into a circuit that is at a certain voltage level, the voltage will drop unless a current source with enough extra current to meet the new load is available. The extra current is either drawn into the circuit and the voltage doesn't drop, or insufficient current is available and the voltage drops.
If you have a charge controller (current source) with active electronics that is monitoring/maintaining a certain voltage level in a circuit that includes a battery (such as following a program to maintain a battery storage system) then the current flow from the charge controller is doing two things: it is satisfying any load in the system, and it is forcing energy into the battery (if and only if) the battery can accept more energy at the current voltage level. If you add a load to the circuit, the charge controller will have to increase the current flow (if it can) in order to maintain the voltage level in the circuit. Either the current flow increases, or the voltage drops. If neither of these occurs, something else must be supplying enough current to meet the load without dropping the voltage, such as the battery. But this would mean that the battery wasn't accepting any energy from the charge controller at the voltage the charge controller was maintaining in the circuit, or else it wouldn't be able to supply current at the circuit's voltage.
According to my understanding, if you add a load to your circuit (the inverter, powering your laptop through its AC adapter presumably) and neither the voltage drops or the current increases as seen by your charge controller, then the power to the inverter must be being supplied by the battery at a voltage equal to the voltage that the charge controller is already trying to maintain. Perhaps the amount of current required by the inverter/laptop is very small (for example, the laptop batteries might be near peak capacity and the laptop charger is not drawing any A/C current at the moment) and the additional current flow from the battery or the charge controller is thus very small and the voltage change is too low to be captured by the charge controller sensor? Alternatively, the battery wasn't really taking any energy from the charge controller at its current voltage?
I wonder what would happen if you tried the same experiment but with a larger known load on the circuit (something that you know draws a steady 5 amps at 12 VDC or so, which should translate to at least 60 watts), like a lightbulb or a resistive load like an electric heater in a coffee pot?