Your battery charger will have a "read-out" that gives the number of amperes the charger is putting out. For understanding what has been said above, try this:
Turn off EVERYTHING you can so noting other than the batteries takes current from the charger. Say the read-out now sez "1.5". That means that the batteries are now consuming 1.5 amps of current for charging
purposes, i.e the batteries are storing that quantity of power away for future use.
Now turn on the dome light over your dinette. The read-out might jump to "2.8" or so. That means that the dome light is consuming (2.8 - 1.5)A = 1.3A. The current put out by the charger simply "splits" so the batteries continue to take what they need, and the rest is accounted for by the "house loads".
Now turn on, say, the bilge pump
. The read-out might jump to "7.9". The batteries are still consuming 1.5A and the light 1.3A for a total of 2.8A. The pump
is therefore consuming (7.9 - 2.8)A = 5.1A.
Now turn off the dome light and the read-out will drop to (7.9 - 1.3)A = 6.6A, being 1.5A for the batteries plus 5.1A for the pump
It's that simple. Just go play with it until the pattern becomes clear to you.
For these simplistic purposes you can envisage current flowing through a wire, measured in amperes, as analogous to the volume of water
flowing through a pipe measured in gallons per minute. Current splits where there is a "junction" in the wiring
, just like water splits where there is a "TEE" in the piping.