Hi guys!

In a plane you use the power module or attopilot to measure current and voltage of the main batteries.
In a plane the servo rails must be powered by it's own power source, and with the zenner diode, if the power module fails to provide power to pixhawk/apm, the source of the servos takes over.
But if that source isn't the same as the main batteries?
If it that source is really an alternate battery, how can we know the state of that battery?

And also, just to feed the servo rail, what kind of battery you guys consider good enough for a plane, I mean not too big, and not too small to be exausted before a long flight?

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Hi Roger,

Yes you can use a second battery to power the servos. 

Ardupilot has the option of monitoring an additional battery. All information is on the wiki, but unfortunately is a bit fragmented and not really intuitive (Dev's are doing their best to keep the wiki up to date, but is't an easy task) 

you should find in your parameter list and change to:

  • BATT2_MONITOR  --> set to 4
  • BATT2_VOLT_PIN  --> set to 13 
  • BATT2_CURR_PIN --> set to 14 
  • BATT2_VOLT_MULT -->set to 10.1 (assuming you are using 3DR power module)
  • BATT2_AMP_PERVOL --> set to 17 ( assuming you are using 3DR power module)
  • BATT2_CAPACITY --> set to the capacity of your secondary battery. values are in mAh with increments of 50 ( example: a 5.65Ah battery will be 5650) 

The image below shows you where to connect the second battery monitor. Remember pin 13 is voltage and pin 14 is current.

Thank you very much naish88.

But forgive me for my ignorance, and tell me that you are sugesting me to use a second power module for the second separated battery, just for me to confirm that I've understood well.

And if I've understood well, I'll have to connect the new power module to the ADV 3.3V plug, which has only 5 pins, and so I would have to use a 6to5 pin df13 cable, and perhaps reorder the cables.
And with the cable in place and parameters well configured, I'll just have to configure mission planner to also display the new data.
Would you mind please to share the link of where there is this information about monitoring a second battery that you've shown me?
Once again, I thank you so much for your help, it has been very...... helpful! :P

Yes that's correct, the easiest way to monitor a second battery is to use one additional power module. 

You will need to get a DF13 5 pin connector and rewire that to the power module.

You only going to need 3 wires from the power module.

POWER MODULE      --->                 PIXHAWK ADC3.3 connector 

Pin 1 ( +5v)                 --->                  N/C

Pin 2 (+5v)                  --->                  N/C

Pin 3 (current )            --->                  PIN 14 ( Pin 2 on the connector) 

Pin4 (voltage )            --->                   PIN 13 ( Pin 4 on the connector ) 

Pin5 (GND)                 --->                   N/C

Pin6 (GND)                 --->                   (pin 3 on the connector) 

Remember pin counting is left to right ( 1-2-3-4-5-6 ). 

On mission planner, you can setup to display the voltage on the quick tab. Simply double click on any of the quick info to swap it for something else, then select "Battery_voltage2"

Here you find the wiki info on dual battery: 

Also to reply to your previous question about power usage and battery sizing for the servos... really depends on your setup. The best will be to you to measure out the current consumption of your servos with no load, and the one at stall. After you done that you choose the stall current - 30% as your average power consumption, then multiply that for the N of servos you have and for the flight time you are planning on flying. 

Wow! That is extremely important information!
I wonder why they haven't gathered it yet in a single page on the wiki.
I hope they can copy & paste what you just wrote!
Thanks a lot!

Roger please just note that there are some issues I discovered today.

  1. On mission planner the voltage of the second battery is displayed in millivolts instead of being scaled down to volts. 
  2. Current measurement of the second battery has not been implemented yet. 

Alright, I'll take that into consideration!
Thank you!

Awesome work naish88!!  I gave this a go last night and it worked a treat, this will be really good for monitoring a payload like a camera's batteries.

Thanks naish, very good information. I did it and I can see battery 2 on MP. But it see milivolts. How can I change to volts. Thanks again.

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