i m new in this forum and i buy an apm 2.8 with his power module.
All was ok until i disconnect usb and i connect the battery.
My RC receiver is on alarm because my RC receiver have only 4.1v and it 's not enough and when i connect mission planner with the telemetry i have this horrible message Prearm check: check board voltage and i check this voltage i found 4v … so alarm of mission planner the apm bip bip bip bip bip and my rc receiver bip bip bip because voltage is under 4.2v …
Unplug everything from the APM except your power module, plug in the battery, measure voltage.
If you get the same voltage then you have found the problem.
By the way, what battery are you using?
If the voltage looks good then proceed :
Plug one thing back in, say the receiver, and measure the voltage. If it looks good keep plugging components in until you find the one that drags your voltage down.