Its early, I just linked to this post from my email and I'm trying to figure out what you are talking about voltage? Then it dawns on me the relationship you are viewing. And then it dawns on me that there is a mathmatical relationship here that could be of great value to those who don't have a duty-cycle reading mulit-meter.
This thread began talking about 90% duty cycle and now is talking in voltage. One needs to undersatnd a relationship here. The actual activity here if viewed with a scope is shown in my DIY article. The voltage over time is a square wave of battery voltage or zero voltage: up, down, up, down, etc. The measurement that matters is the amount of up versus down. This is called duty cycle. But if one is viewing this with a digital multimeter the voltage one reads is the average. Thus a reading of 6.5v when battery voltage is 13v means you are looking at a 50% duty cycle.
Allowing for circuit resistance, I propose that a ratio of measured voltage to battery voltage will give one side of the duty cycle. By this I mean that a signal that is up 60% is thus down 40% and its duty cyclle can be refered to as 60% or 40% depending on your point of view.
Bosch Master, ASE Master, L1
33 years MB technician