1. Introduction:
Pixhawk 3DR kit is delivered by default with a 4S maximum power module. For those wanting to use 5S or 6S or higher voltage batteries there is currently, to my best knowledge, no “how-to” guide for the Pixhawk board. I therefore decided to document it for others who might need it too.
For those who would like the same “how-to guide” for APM 2.x , here is a link I wrote a while ago:
Pixhawk comes standard with three (redundant) ways to powered it up:
1-USB : not used to fly obviously; just useful on the ground for connection on a ground station software.
2-The power module port accepting a maximum input voltage of 5.7volts (and will not get destroyed up to 20 Volts input)
3-The RC input pins. Will accept a maximum voltage of 5.7volts also (and is also protected up to 20 Volts)
This guide assumes a use of Pixhawk’s power module port which provides not only a way to power the board but also the pins to measure current and voltage values of the main battery.
This guide assumes a use of a 6S battery in combination with a Attopilot current & Voltage sensor board. This Attopilot “power module” replaces the 3DR 4S limited power module. The Attopilot board comes in three flavors: 45 amps, 90 amps or 180 amps.
The choice of the right Attopilot board (45A, 90A or 180 A) will depend on your motor/props combination: take the Attopilot version that has the smallest amps capacity above your max multicopter current consumption. However we will introduce in this guide a way to use the 90 amps Attopilot board to measure up to 150 amps, still using Pixhawk’s power module port.
2. Attopilot description:
An Attopilot board provides three wire soldering pads to solder : a current measurement wire, a voltage measurement wire and a ground wire. See picture below:
Attopilot 90A support up to 50Volts for a maximum of 90A. However the resistor specifications exceed the 90A limitation which makes it possible to use it for measuring 150 amps (we will take this as a assumed max current as our example for the rest of the explanation).
The datasheet of Attopilot specifies that the Voltage measurement wire outputs an analog voltage of 63,69 milliVolt per Volt. Similarly the current measurement wire outputs an analog voltage of 36,60 milliVolt per Volt.
So for a 6S battery the maximum analog voltage values will be:
-For voltage measurement: [min 0V - max 1.6V]
-For current measurement: [min 0V – max 3,3 V]
3. Pixhawk power port description (pinout):
Reusing the excellent pixhawk infographics published in the wiki, the image shows circled in yellow where the power port is on the pixhawk board.
The power port is a so-called DF13 connector with 6 pins.
The six pins of this connector are assigned in the following order, starting by the red wire on the leftmost pin:
Power Port Pinout Description:
- 1- Vcc (5V input)
- 2- Vcc (5V input)
- 3- I (Battery current measurement analog voltage input)
- 4- V (Battery voltage measurement analog voltage input)
- 5- Ground
- 6- Ground
4. Wiring Case 1 : to measure up to a maximum of 90 amps
The connections between Attopilot and Pixhawk are shown in the illustration below:
We have added an optional BEC in the illustration that would be connected to the Vcc and Ground wires of the power module. It is optional as Pixhawk could alternatively be powered via the RC inputs.
4. Wiring Case 2 : to measure up to a maximum of 150 amps
The connections between Attopilot and Pixhawk will integrate resistors to be able to measure up to 150 amps.
Indeed the ADC of this power port on Pixhawk has a range of 0-3.3V. This means that for the maximum true current of 150 amps, we want the current analog wire of Attopilot to output maximum 3.3Volts (as it is the case in case1 for 90 amps max without additional resistors).
Note : this part has been updated with a resistor scheme simplification (only one resistor to add in parallel rather than the originally classical R1, R2 resistors divider) thanks to a contribution of Bo, a diydrones member who analyzed in depth the Attopilot circuitry.
So we will build a small resistors divider on wire 3 (current measurement) & wire 5 or 6 (Ground) as follows:
The Attopilot current measurement output masks a circuit that contains an existing output resistor called Rl. According to the Attopilot datasheet, the following equation links to measured current I (called MeasuredCurrent in the equation), the output analog voltage for current measurement Vout, and an existing Rs resistor in Attopilot:
What we want is to get Vout = 3,3 when Current =150 amps. To do this we will add a new external resisto (Rx) in parallel with the existing Attopilot Rl resistor, so that the new resulting Rl resistor (called Rl’) must be (knowing that Rs = 0.5Mohm as per Attopilot specs):
Therefore we can calculate the Rx resistor value we need to add in parallel as follows:
So, Rx must be ~ 110kohm to have a Vout at 3.3V when the measured current is 150 amps.
You can choose another max amp output (but I would not advise higher than 150 amps with the 90 amps Attopilot, otherwise use the 180 amps version instead) and calculate the resulting Rx resistor
As a result, when the current is 150amps, Vout will have value of 3.3Volts.
5. Mission planner / parameters configuration in battery monitor screen:
In the battery monitor parameters screen, you can manually select which current and voltage sensor you are using. In the present case, you will select the power module and modify the following parameters to make the mission planner voltage and current display match the real values (measured using a wattmeter for example). The explanation below is an extract from the Arducopter parameters list.
Battery monitoring (BATT_MONITOR)
Controls enabling monitoring of the battery’s voltage and current
Value | Meaning |
0 | Disabled |
3 | Voltage Only |
4 | Voltage and Current |
Battery Voltage sensing pin (BATT_VOLT_PIN)
Setting this to 0 ~ 13 will enable battery current sensing on pins A0 ~ A13. For the 3DR power brick on APM2.5 it should be set to 13. On the PX4 it should be set to 100. On the Pixhawk powered from the PM connector it should be set to 2.
Value | Meaning |
-1 | Disabled |
0 | A0 |
1 | A1 |
2 | Pixhawk |
13 | A13 |
100 | PX4 |
Battery Current sensing pin (BATT_CURR_PIN)
Setting this to 0 ~ 13 will enable battery current sensing on pins A0 ~ A13. For the 3DR power brick on APM2.5 it should be set to 12. On the PX4 it should be set to 101. On the Pixhawk powered from the PM connector it should be set to 3.
Value | Meaning |
-1 | Disabled |
1 | A1 |
2 | A2 |
3 | Pixhawk |
12 | A12 |
101 | PX4 |
Voltage Multiplier (BATT_VOLT_MULT)
Used to convert the voltage of the voltage sensing pin (BATT_VOLT_PIN) to the actual battery’s voltage (pin_voltage * VOLT_MULT). For the 3DR Power brick on APM2 or Pixhawk, this should be set to 10.1. For the Pixhawk with the 3DR 4in1 ESC this should be 12.02. For the PX4 using the PX4IO power supply this should be set to 1.
This is a parameter to adjust to match the real Voltage value with the displayed mission planner value.
Amps per volt (BATT_AMP_PERVOLT)
Number of amps that a 1V reading on the current sensor corresponds to. On the APM2 or Pixhawk using the 3DR Power brick this should be set to 17. For the Pixhawk with the 3DR 4in1 ESC this should be 17. Units: A/V.
This is a parameter to adjust to match the real Voltage value with the displayed mission planner value.
There you go! I hope this will help you configure your pixhawk with higher than 4S batteries.
Cheers,
Hugues
Comments
Hi guys I am slowly discovering there is no support for the 6S group of people from 3DR. I have written a email asking when there is going to be a 6S power module. I do not like the way they make u use a dodgy wiring bodge. There should be a module by now. A lot of people online are asking the same questions as me. I have bought some cheap nasty thing off ebay http://www.ebay.co.uk/itm/291272159858?_trksid=p2059210.m2749.l2649.... If it works great but I wont be trusting it at all and don't think its the best idea but its all im left with.
Hope 3DR takes note and comes out with a solution very soon.
Yes Mayehem, the role of the protection from the Zener diode is to trim any voltage spikes above a threshold. Above a certain threshold, pixhawk will protect itself by shutting down. Spikes may be caused by servos or even other electronic devices or even wires capturing EMIs.
Hugues can you elaborate on not using servos with the Zener diode? From what I have read so far my understanding is that the diode regulates the power across the servo rails on the pixhawk board to prevent any spikes from connected servos. Appreciate your reply.
Comment
Thomas, you are running 6S batteries and you are worried about the weight of the attopilot? It is only a few grams and the piece of mind I get from the voltage and current monitoring, as well as the logs of that data is well worth the few seconds of lost flight time.
Thomas, if your BEC produces 5v (and no more than 5,6v) and you install the safety Zener diode, and you do not use servos, you should be OK (you could test first on the bench). However you will not be able to measure voltage or amps without a PM or an Atto or any other volt/amp sensor.
I think you guys need to find an electrical engineer. Electricity doesn't need to fly! Aeronautical engineers need not apply. ;)
see https://www.sparkfun.com/products/9028
The datasheet clearly explains that the 'shunt resistor' can handle up to 2W. The energy dissipated in the shunt resistor is based on the voltage drop across it and the current. Which clearly leads to either the 90A max rating or the 180A. Again read the datasheet, it actually explains the calculation
The 4KW figure mention is irrelevant as that is related to the complete system, and also pretty bogus, since unless you measured the current it's a figure created on components max current handling, not about what the system is pulling as it max current. (Find an Electrical Engineer, buy a multimeter and measure whats happening. A lot of this is empirical at the end of the day)
From a marketing perspective, larger number specs = more ca$h, and this seems true when marketing to persons who don't know better. And I bet that 30A ESC probably couldn't handle that amount current. It just makes people more likely to buy it over the 20A cheaper one. Not that it was more fit for purpose. (Note 3DR kits come with 20A ESCs, and they work as expected ;) )
With the numbers crunched by the engineer I guess you should be ok. Please keep us updated when you start testing the system.
I am not thinking about the spec of the components but the cross section of the power-rails on the card it self. Think of it; even at 180A and 6 cells you are generating close to 4KW, all going through the Attopilot card. Doubbe that for 360A!
I would be very cautious about pulling 360A through the little Attopilot board even if the components theoretically can handle it. Even 180A is high, at least for an extended time if you ask me.
Hi all, Great write-up. I'm currently running a 6s system which is built up from 2 3300 mAh 3S backs wired in series to power a single-motor fixed-wing.
I suppose one would want to use dissimilar connectors on the series wiring harness (i.e. special connector to mate with PM) to prevent accidental connection of the PM across the wrong (upper) battery pack (i.e prevent shorting 12v to ground through the pixhawk to the ESC-BEC ground)?
I guess a drawback would be that one would be monitoring the voltage across only one pack, and drawing slightly more current from that pack, but assuming the current draw of the Pixhawk is negligible compared to that of the ESC/motor, I would think the two packs would still end up at close to the same state of charge at the end of a ~20 minute flight.
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