A couple of things. First, taking only one sample is going to result in a noisy reading, try averaging a bunch of samples, like dozens of them. They don't take long at all. Secondly, on the odd chance that the onboard regulator is current limiting, take one or two batteries out of your pack and see what happens. Just jumper across the missing battery location. This will reduce the power dissipation of the voltage regulator (heat output) but still allowing plenty of head room on the input voltage to output 5V. If you have 6 brand new "copper tops" in your pack, the supply voltage is above 10V; 7.5V is likely more than enough.