Why is Battery draining fast and doesn't charge via laptop usb?

[buzz15uk]

Hi, I am running Nougat 7.1.2 Beta, although the problem seems to have started with Nougat 7.1.1, whereby my phone is discharging quickly, 20% in 1 1/2 hours. It also connected to a laptop and says it is charging via usb but the power is still falling, the screen showing battery use by app does not add up to 100% (79% battery life and 5% used by apps) and the phone rebooted yesterday at around 30% battery. I understand this is becoming a regular issue but is there any resolution?

Many Thanks

Rob