The supply voltage spec for their single action brake valve is 3.5 - 7.5 Volts. A 2S LiPo is 8.4 volts. To use 8.4 volts may be operating within the component's design margin. But, there is not much risk if a brake valve goes out either. Taking that risk on the Retracs may be another question. Seems to me manufacturers gives us the design specs so that the component does not fail prematurely. To ignore the specs increases risk. I am sure some folks have never even given it a thought to go to 8.4 volts so the risk is acceptable to them. I have mine regulated to within the spec.