Board power issue

Latest 3.5.7 stable release, nuttx OS, 90 amp power module with a bec connected directly to battery powering the servo rail. It measures 5.15 volts with a meter. I am logging as low as 3.9 volts according to the log and mavlink power_status messages. This is with the hexa disarmed and only the receiver is powered from the servo rail. All ESC power leads have been cut. I think the BEC is 3 amps. Do I need to go bigger? Power draw is under 1 amp measured between the hexa and the battery. I don’t know what else I can do to fix this.

Why are you powering stuff from the servo rail? The +5 volt BEC on the power module has more than enough capacity to power the FC, receiver, GPS/Compass, telemetry radio, and the receiver.

If you need power for other things like a gimbal, LEDs, retracts, or FPV gear then by all means use external BECs. I have 3 aircraft set up this way…

And BTW, if you are powering the receiver from the servo rail that power is also going to the Pixhawk via the RCIN port (which just happens to be were the receiver gets power when you use a power module).

The receiver only has one hot connection, and that is RCIN. It can’t power the rail because it only gets power from the rail. You are thinking of using an RC channel for RSSI, which would provide voltage back into the servo rail. There is a warning in the docs about that. GPS gets power from it’s serial port connection,and I was getting brown outs until I added the BEC. If the Pixhawk detects low voltage its supposed to take power from the rail as a backup according to the documentation. I’m not sure that is working correctly. I just realized I can force that by cutting the two voltage wires on the power port, leaving only rail voltage. The other option I thought of is providing +5V to the usb port. Or the documentation is not correct. I’ve researched this extensively short of digging into the source code, which probably means the boot loader too.

The addition of a SikRadio powered via the serial port was too much for powering only from the power module. The large LED would not even blink.

edited to add info.

I tested one of my rigs last night. I connected a BEC to a spare channel on the receiver and connected a battery to the BEC. The Pixhawk and everything else powered right up because +5 volts and ground are +5 volts and ground on the receiver the inputs as well as the outputs.

You must have something seriously wrong. I’d recommend pulling any BECs out and starting over with just the power module, the GPS/Compass, the receiver, and the telemetry radio. In this configuration you should be able to power every thing using just a USB connection. This configuration should also work when powered by the BEC in the power module. Any other devices like an OSD, retractable landing gear, LEDs, navigation strobes, a gimbal, FPV video transmitter and camera, MUST be powered from a separate power source.

If you cannot power the bare minimum (receiver, gps, flight controller, telemetry radio) from the power module, chances are the BEC is faulty.

I can power from USB just fine. I’m going to swap power modules.

Power is much better using a different power module. I cant run ChibiOs though. I get the dreaded logging failed message. networks fine. Oops, That is on the latest beta. I’m reluctant to report an error because of the voltage problems I’ve had.

I was getting the logging error on one of my Pixhawks. Replacing the SD card fixed it.

Another thing you might try is using the .apj file. I was using Mission Planner to flash the firmware and answering “Yes” to download ChibiOS. This failed on two different Pixhawks.

I was getting logging failed, bad compass health, and no IO thread heartbeat. On a hunch I downloaded the .apj file and used the “Flash Custom Firmware” option in Mission Planner. My 2.4.8 came up just fine, but a 2.4.6 didn’t. This time the only errors were logging and heart beat. Replacing the SD card did the trick.

I even went so far as to put 3.6-rc10 on my third Pixhawk and it runs fine too…

I need to pick up a couple of new SD cards. Any recommendations? I have been using SanDisk 64gigs. I suspect they are very sensitive to voltage. I can’t find another high speed card to test tonight. I’m running my own build right now to deal with autotune. I live on the beach in florida and there is always wind. Autotune keeps missing the level check but 50% of the time it’s only a few percent off. I changed the acceptable level values by 10% so I can finish autotune. I’ve been setting it to tune only roll and flying until battery failsafe forces a landing. That is when I started looking at voltages, trying to get every second of flight time