I have both the ACP-12U and ACP-7U. Although the marked output voltage on the ACP-12U is 5.7V, it is measued by a multimeter to be 6.02V. The ACP-7U is marked as 3.7V, but measued to be 9.9V. I also has a UBS cable charge for Nokia, it is measured to be 8.2V.
All of them works when I plug them to my Nokia 6230i phone, I am not sure why there are such hugh difference between then marked voltage and the real measured voltage. Anyone ther knows more about chargere can explain the reason?
I honestly have no idea, though I'm sure if you've already been charging phones for quite a while, with no ill side-effects, then I don't think you have to worry about this too much.