How to Modify PX4IO and 3DR Power Brick for a 6S Battery Pack

A 6 cell LiPo Battery will have 25.2V when fully charged which exceeds the 18V max of the existing DC/DC converter.

PX4IO

For the PX4IO mod some code modifications will be required.

  1. The DC/DC should be replaced with one supporting the 6S voltage range. I unsoldered the existing and soldered in a UBEC as shown:
    ubec_6s_mod.jpg
  2. Also, the 47K:10K resistor divider must be adjusted. As is, the 5.7:1 ratio will result in over 4V on the 3.3V max ADC input when a 6S cell is fully charged. You can replace either resistor, but I chose to just put a 12K resistor on top of the 10K making an effective 5.45K. Now the ratio is 9.624:1 so 26V will only reach 2.7V - easily within the adc range. Orange arrow below indicates the location of the 10K resistor on top of which I piggy-backed the 12K.
    resistor_6s_mod.jpg
  3. Now you have to adjust the parameters. Bring up QGroundControl and set the BAT_N_CELL parameter to 6 to indicate the number of cells in your pack. Finally, you have to adjust the ADC scalar parameter BAT_V_SCALE_IO. In my case, 17015 was the right value.

Once all this is complete, double check your circuit mods and connect the battery. Use a meter to test the voltage on the middle pin of one of the eight servo connectors. Should be 5.0V. Reconnect to the computer. Battery voltage should be properly reflected in the upper middle section of QGroundControl and no low battery or critical battery alarms should show in the communications monitor.

3D Robotics Power Brick

The power brick is commonly used with the Pixhawk. The mod for 3DR Power Brick is very similar to the PX4IO mod, with the one exception that no code changes are required. The brick is able to handle the extra voltage load.

Strip the plastic shrink tubing from the brick and cut off the IT DC/DC converter so it looks like in the photo below. I used wire cutters to cut the pins. Seeing I didn't want to keep the DC/DC converter I cut it up to make removing it easier.

Connect the BEC voltage input (red) to pin 1. Connect ground (black) to pin 2. Connect the BEC 5V output (red) to pin 3. I have hot glued the wires to keep them firmly in place. Make sure you do not place the BECs unused ground and signal wires below the BEC, ie: sandwiched in between it and the brick otherwise really weird things will happen even if the wires are not exposed.

I decided to hook the other end of the brick directly to the ESC to reduce the number of connectors. You can connect the DF13 6-Pos cable directly to the Pixhawk power connector without any code or parameter (need to test this still) changes necessary.

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