Excited to get to work with my new Edge 830 I paired all of my devices one by one (Garmin HR Monitor, Garmin Speed Sensor, Powertap P1 Pedals) and went for a ride. The Powertap P1 pedals consistently recorded very low wattage values, about nearly half of what I would have expected; I have been using power for 4 years so I know what say 200W feels like and this was wrong.
1. I un-paired, and re-paired the P1 pedals. Same result.
2. I un-paired the pedals, changed the batteries, and re-paired them. Same result.
3. Searching online I discovered that I still needed to set crank length: it defaults to 172.5, whereas I use 175mm. After altering it to 175mm I retried. Same/similar result.
I began to suspect the Edge 830 was faulty as the pedals had been fine with my old Edge 800.
More searching around on the internet and discovered that the whilst the Edge 830 supports two interfaces (ANT+ and BLE) the BLE interface is somewhat limited in comparison to the ANT+ interface - which supports more concurrent devices and more bandwidth. By accessing the Device information on the Edge 830 for the power device I could see that it was listed as using BLE, and I could also see that the device name was P1.R (presumably meaning P1 Right pedal) - there was nothing listed for the Left pedal. I then suspected this was the problem - the Edge cannot communicate with two pedals via BLE, only one. You have to use ANT+.
The problem was there was no obvious way to get the pedals to connect via ANT+ rather than BLE, but this is how I did it...
Un-paired the pedals again.
Selected... Add Device | Power, and this time ignored any power devices listed as (for example E3 A8 A2), and selected the power device listed as '6535'. This worked. Basically, I think the P1 pedals publish the BLE interface using a hexadecimal name like 'E3 A8 A2', whereas the ANT+ interface is published using a decimal name '6535'. When I connected to the decimally-name power device everything started working perfectly.
These are older pedals now, and there isn't much information on them so I'm adding this here in case it helps anyone.