Sent: Tuesday, April 22, 2008 6:54 AM Subject: RE: voltage question for the smart man
Nominal Current draw = Power / Voltage. Typical steady state current draw for a running vehicle would be 80 watts /28 VDC = 2.85 Amps. Typical steady state current draw for a vehicle with the engine not running would be 80 watts / 24VDC = 3.33 Amps if the battery is new and in good condition, and 80 watts / 20VDC = 4.00 Amps for a weak older battery. If the Display Screen is turn down less power and less current is drawn. At the lower and upper environmental temperature extremes, thermal management routine s kick in to turn heaters or cooling devices on, and more power and current is drawn.
Current flows from the battery's positive terminal to the computer and and returns back to the battery through it's negative terminal. The battery is basically an energy storage device. Energy is removed from the battery when you place a load on the battery, in this case the computer system. The vehicle's alternator system generates electrical energy to replenish the energy drawn from the battery, to keep the battery fully charged. There really is no good way to measure how much energy is returned to the battery because the device that puts the load on the battery (the computer system in this case) only draws enough current to meet its needs.
During equipment bootup, different power draws will be experienced as the various systems and subsystems are brought online. Inrush currents will surge and vary until the computer system achieves steady state operation, usually within several minutes.