APM 2.5 and problems with 5V power supply

Hello to all,
first time that I’m in this forum.

I just started my copter career with the 3DR APM 2.5. Yesterday I did the first short and successfull indoor flight with it and my quadx. All looked fine for the first flight.
Today in connected the Lipo again but the receiver showed a voltage of only 4.69V. No arming possible. I connected to USB as well and it raised to 4.8V, still too low.
The only thing I did yesterday, was to attach a Lipo warner by solding it to the power distribution board. I detached it today, no change

A maybe helpful information… Powersupüply is throught an ESC, means JP1 is set.

Any help?

Thnaks
Torsten

Problem solved. I changed settings in the TX to get low voltage warnings. But I produced that problem by doing it.