You're measuring the voltage with a voltmeter, right? That shows you the voltage- averaged over about 1/3 of a second. If you hook the meter up to a battery, it'll say 13V. If you hook it up to something that generates power in repeating pulses - say, 100V for 13 milliseconds followed by 0 V for 87 milliseconds the meter will still say 13V*. Hook a 13V headlight up to this 100V pulsing power source and it will be perfectly happy. If you have a GPS designed for 13V, it will be ruined by the same source. If this same power source were adjusted so that it put out 100V for 13 seconds, then 0V for 87 seconds, the voltmeter would alternate between 100V and 0V, and the headlight would burn out after a second or so of 100V. *Yes, I'm ignoring the difference between mean and RMS.