The charging system supplies the accessories first- - - -then whatever is left over goes to recharge the battery. The load of the starter lowers the terminal voltage, so the charging system only has to raise its voltage a little to force the charge back into the battery. It makes no difference what other accessories are drawing current. If there happens to be a situation where high beam headlights, full speed heater blower, and a few other high-draw accessories are operating, (Booming sound systems with high wattage amplifiers, for instance) it just takes longer to recharge the battery. As the resistance at the battery posts goes up, so does the voltage to maintain the charge rate, up to the point where the voltage regulator kicks in and keeps the charging unit (generator or alternator) from overcharging the battery.
A battery acts sort of like an accumulator in a hydraulic system or a storage tank in a water system. It stabilizes the pressure spikes the pump (alternator) produces when load changes, up or down. Never, EVER disconnect a battery cable with an engine running. I've seen spikes of over 100 volts in a 12V system on an oscilloscope when someone disconnects a battery cable.
Jerry
