What voltage should be supplied to the battery when charging?

I have an iPhone 5 that won't turn on, and I'm trying to figure out why. It won't turn on with or without the battery connected.

When I run a multimeter over the connections to the battery on the logic board whilst charging, it varies between 1.2v and 1.3v. That's pretty low, and even after charging overnight (approx. 8 hrs) the battery did not hold a charge so I'm planning on replacing the dock connector and battery. Before I do that I just wanted to check, what should be the expected voltage being supplied to the battery?