3DR Solo iMX6 as video and telemetry link

3DR Solo, have a great onboard computer, the IMX6 runs Yocto Linux : link to the Linux source: https://github.com/OpenSolo/imx6-linux

Among it’s best features is the HDMI-in, USB, and mini-PCIe port which can easily use rather powerful MicroTik router/Wi-Fi modules.

I reverse-engineered the remaining pins, as only the 3 and 6 pin headers were known to the community.|

Battery 7-24v, is the main power source.
+5v input does not seem to be essential for normal operation.
First USB host (D+,D-) is a normal USB2.0 host, compatible.
SEL(select) and OE(output enable) are two GPIO pins used by Solo to select where this USB host is connected: During normal operation the OE control if it is connected to any device (or not) and SEL controls if it is connected to the Cube (for firmware upgrade via bootloader) or the gimbal bay connector.

Unless you need more than two USB devices, you can ignore SEL and OE, and just use D+/D- for the USB host as usual.

Then there is the USB-OTG port. If you pull down the “Sink to GND…” pin, then the USB-OTG port becomes another USB host port. Otherwise, the USB-OTG port is in OTG mode, and will act as a slave/device – so that you can connect it directly to another host(usually a computer) This pin is not routed to anything on the Solo main board I tested.

The default onboard Linux is compiled only with FTDI and CDC-ACM (Communication Device Class – Abstract Control Model) profile, so it will instantly work with all kinds of Arduino/STM32’s that normally show up as a ttyUSB or ttyACM device.

DSM outputs a Spektrum RC control signal from the Solo Controller, it is optional to use it.

Finally, there is the usual RTS, CTS, RX, TX that is used for MAVLink telemetry, the hardware flow control is optional.

There is one more pin “not used by Solo” that is not routed anywhere, but on the iMX6 it is certainly connected to something, maybe another GPIO

As-Is, you can take out the iMX6 from a Solo, and use it and a Solo controller with any MAVLink source.
If no MAVLink stream is presented, everything else will still work, but the controller will auto-sleep after 10minutes or so. This could probably be easily fixed/changed in the controller’s software.

Simple desoldering of the Solo Battery Connector

Users of BMSOne can find it hard to remove the special Solo battery connector from a dead battery, this is a guide on how to do it, with a focus on simplicity.

No desoldering vacuum station, preheater, or reflow oven required. 🙂
Or: you can just buy it here (somebody else is selling, I am just providing the link for your convenience I am not paid to do so)

Safety First :

People are afraid of the dreaded thermal runaway, where the Lithium-Polymer/ion battery spew gasses & fire when shorted/damaged.

Basic physics tells us that the energy to do that, must be available, or else, there will not be smoke/fire hence:

Drain the battery – method 1:

Connecting a small 12v ,5-10W bulb to a bad battery (no reason to open it like I did in the photo below)

DO NOT think a 50W bulb will drain it better, it will make the voltage drop faster(and battery will switch off) – in the end , slower discharge is better.

In this case, I stuck the wires into the +/- connector. Switched on the battery, and let it sit on a concrete floor until it dies.

Drain the battery – method 2:

You may as well hover the Solo till it can’t fly no more, then let it be on until the battery dies completely

The BMS will fail to keep the FET’s on at somewhere around 5.7v or so, When the pack voltage falls that low, it will switch off, .. feel free to let it recover for a while then switch it on again..

When the battery is empty:

Solo Battery tool tells me that this battery is drained, no cell is even near 3.0v even an hour after it switched off
I have seen the voltage drop far enough to it to switch off more than once.
A properly drained battery, where all cells are low, do not have the energy to cause any fireworks, let me prove it to you.:

Opening the battery shell:

(will describe the process later – many have already done it.. if YOU did a video on it, feel free to let me embed it here.)

Extracting the connector:

I used an electronic hotplate, set to 300°C
You can use a cooking plate just fine.
The plate should not be red hot, let the PCB sit there for a few minutes, then start pushing the PCB down (so that the pins get a better thermal connection, and get pushed thru in the end.)

BMSOne – the universal BMS

This BMS is very flexible and universal, you can fly any 4-cell battery chemistry/capacity that the Solo can handle in terms of voltage & weight, anything that produces 10-20v(I did not actually research/test Solo’s upper voltage limit)

The PCB is less than 50x40x9mm (balancing connector is the highest component) The weight is only 7.6g

There are three mounting holes that should enable nice mounting inside the GPS bay (I did not test it, just measured it.)

A user made a great and very detailed guide for windows-users. It is very user-friendly, and requires very little prior knowledge about computers/serial communication to follow. bit.ly/37QRJHm

20 January 2020: Firmware 1.4
Verified support for Solo 1.3.1…1.5.4 – this firmware corrects a minor typo in 1.3 that prevented it from working on Solo 1.x.x as advertised thanks to the people behind a much more PnP version: https://jonbrotherton.fws.store/JBBMSOne_Liion_Edition/p5973142_20662335.aspx

23 September 2020: Firmware 1.3-test.
This is not a release, because it does not do anything new for OpenSolo users, it does, however, respond to messages Solo 1.3.1…1.5.4 request – so if anyone flying those versions can confirm that it works – it will become a release.
There are no other changes in features/adverse side effects, only this additional code.
Another Bonus of that is that the Solo Battery tool can read the main voltage properly. 🙂

13 June 2020: Firmware 1.2 released. Thermistor added for monitoring of battery temperature. All new devices are delivered with a thermistor.
All boards ver1.2 (with “Temp” input) can use a 100k NTC thermistor.
All boards ver1.1 devices can be modified with an NTC between GND and pin22 of the microcontroller, and a pullup resistor of 100k with a pullup to +5v.
I have also added some LED blinking.

16 Mai 2020: The first FIRMWARE UPDATE is out! (1.1) And I have published a configuration for Tattu 5200mAh battery (see below) – from now on, this will be the default configuration, as I have the impression that more people fly with Li-Po than Li-Ion.

Important information: (for devices shipped before 16 Mai 2020):The BMSOne comes configured for Li-Ion cells, (down to 2.5v/cell – check datasheet.) – watch the cell/voltage pack reading unless you are sure you did configure it properly for Li-Po

Operation modes:

The device can provide remaining capacity information based on one of the methods:
S0 : remaining capacity is estimated based on voltage.
S1 : actual capacity reported based on mAh consumed (assumes you start with a fully charged battery of defined capacity on boot)

Firmware upgrades:

It can be fully configured using an FTDI cable (or any USB<>Serial cable with RX,TX and DTR signal- DTR is only used to enter the bootloader, and only useful for flashing, although it is possible to flash it without DTR too)

All firmware versions apply to all BMSOne hardware versions, unless stated otherwise.

To flash the new firmware you will need the avrdude application.
On Linux, it’s installed by: “sudo apt install avrdude”
The firmware upload command is:
avrdude -patmega328p -carduino -P /dev/ttyUSB0 -b115200 -D -Uflash:w:BMSOne.1.1.hex :i

Windows users need drivers for the serial<>USB cable, avrdude application: http://download.savannah.gnu.org/releases/avrdude/avrdude-6.3-mingw32.zip and the firmware (download link below). Extract both firmware and avrdude to the same directory.
Then replace the “/dev/ttyUSB0” in the example command with the “COMx” name of your serial port, and execute the command.

Please note that the programming protocol is Arduino compatible just enough to make it work with avrdude, but it’s not fully Arduino compatible as a bootloader, (and much smaller).

Firmware 1.1: Perfectly nice interpolation and smooting.

With firmware 1.0, this would be a graphical representation of the spent percentage (SoC-by Voltage) with Li-Po , due to Li-Po’s low voltage range, the remaining % would jump a bit in certain areas.
With firmware 1.1 , the estimated SoC is very nice, less jumpy, maybe except for some flying I did, but very nice to watch in the display.

Firmware 1.1 also has some cosmetic changes in the terminal and comes with the Tattu 5200mAh 35C linearization curve as standard.
It will NOT overwrite your existing configuration. To apply Tattu /Li-Po defaults, look up Tattu further down on this article.

CAUTION: The bootloader is well tested, it handles interruptions in upgrade and can upgrade/downgrade firmware regardless existing of version. You should NOT attempt to use “avrdude”, “winavr” or “avrdudess”, or any other tool/commands except those described above. The only known case of messed up bootloader reported to me for BMSOne or SoloBatt tool, was caused by experimenting with such tools.

Firmware 1.2 : Battery pack temperature sensor:

Firmware 1.3-test – should work with Old Solo v1.3.1…1.5.4

Firmware 1.4 – Works with the old Solo v1.x.x

This image has an empty alt attribute; its file name is BMSOne11top-2.jpg
BMSOne $35

BMSOne w/XT60 $40

20cm balancing cable JST-HX $1

Super-Simple solution:

If you dislike configuration, finding suitable battery etc.. please see the super-user-friendly version made by some nice people at: https://jonbrotherton.fws.store/JBBMSOne_Liion_Edition/p5973142_20662335.aspx
Basically: It gives you a simple plug-and-play solution, none of the setup/configure/install/mount stuff.

This is the BMSOne:

On top, balancing connector, FTDI(configuration) on the left, Solo connector on the right, with the option to solder battery wires directly.
The backside. B+/− is where you connect the battery, if you do not use a Solo power connector, (or not a Solo) – then output is +/- and SDA/SCL is clearly labeled.

BMSOne+XT60 connector

The Silicone wires are 5cm long (measured from PCB to where they enter the XT60)

You can extract the gray power connector from a dead battery: Instructions here: https://madhacker.org/simple-desoldering-of-the-solo-battery-connector/

Direct connection to Solo motherboard. Instead of moving a Battery-Connector the BMSOne, you can remove the Solo’s male battery connector, and just solder in the BMSOne permanently.


A few people asked whatever BMSOne is “plug’n’play”.
Yes, it is kind of that, the simplest job is done right out of the box, once you connect a battery thru it, you will see the battery, and cell voltage, which is the most important information.
To have a nice, linear, SoC based on voltage or current used, some configuration may be required, there is no “one setup fits all” configuration. (but the default is pretty much food for most Li-Po’s) This is the price of having one BMS, many batteries, vs having a dedicated BMS for each pack.

As you no longer have the shutdown signal from the battery, the GoPro will remain on after pulling the battery, so you need to switch it off manually.

Power for programming/configuration:

Hardware version 1.3 uses a diode, and the device can be connected to FTDI regardless of battery being connected or not. For calibration, +5V from FTDI should be disconnected.

Hardware version <1.3 have a solder-jumper “JP1” on the solder-side of the PCB.

If JP1 is open, just connect FTDI , and provide power from a battery (FTDI’s +5v is disconnected)
If you wish to do programming/configuration without a battery connected, solder a blob over JP1 then connect FTDI cable with +5v – it will power the device. – do not apply battery powered from FTDI.


Configuration, like firmware updates, is done over FTDI-adapter.
For configuration, any USB-Serial adapter will do.
You could use the typical 6-pin FTDI adapter, but you can also just connect a 3-wire adapter, using GND,RX,TX, while powering the device from a battery.

When you connect to it at 115200kbps, (8N1) and power it up, you will see:
BMSOne v1.1
Set the capacity of the battery in mAh using the command:
it will respond:
Set capacity: 10000
Now, you have defined a 10Ah full capacity.

You can type:
S0 //Select voltage-based SoC (State Of Charge)
S1 //Select capacity-based SoC
S! // No thermistor, fake 22.5°C temperature
SH //100K Thermistor
SW //write configuration to flash – until you do this, all your changes/calibration are temporary and can be reverted by rebooting the device.
SR //read config from flash (it happens on every boot)
SS //Show config

Displaying Configuration:
Voltage table: 104, 110, 113, 116, 119, 122, 126, 129, 132, 135, 138, 142, 145, 148, 168,
Capacity table: 0, 4, 12, 20, 29, 37, 45, 53, 62, 70, 78, 86, 95, 99, 100,
SoC by Voltage
Cell voltages(mV):3727,3731,3739,3744
Pack voltage(mV):14941
Current (mA):-152
Full capacity: (mAh)10000
Remaining capacity by Voltage: (mAh)9900
Remaining capacity by Current: (mAh)10000

SD //Show internal data

You can also perform AD calibration.
When calibrating voltages/current sensor, it is important NOT to provide power from the FTDI connector. (otherwise, the precision will suffer.)
On the solder side of the PCB, there is a solder jumper “JP1” – to ensure a correct calibration, you can remove its solder blob – OR, simply disconnect the +5v pin from the FTDI connector.

With a battery hooked up, you can measure its cells, then provide a command like this: the numbers are voltages in millivolts.
let’s say you measured 3.256v, 3.231v , 3.255v and 3.263v then enter:
SC3256,3231,3255,3263 // calibrate cell voltage in millivolts for 4 cells.
If you can’t measure with 3 digits, – let’s say you can measure with lower precision “3.21v” then enter “3210”

SI10220 //Calibrate current sensor, value in mA “SI10220” = 10.220A
Again issue the command with a known current flowing through the device. It is better to calibrate while sensing 10A than 100mA.

Capacity by voltage linearization table:

For Li-ION:

SV10.4, 11.0, 11.3, 11.6, 11.9, 12.2, 12.6, 12.9, 13.2, 13.5, 13.8, 14.2, 14.5, 14.8, 16.8 //set 15-point percent linearization table
SP0, 4, 12, 20, 29, 37, 45, 53, 62, 70, 78, 86, 95, 99, 100 //sets 15-point percent table.

So by the example above 11.3v (third value) equals to 12% , (while 11.5v will be interpolated to a little less than 20%.

For Tattu 4S1P 5200mAh 35C

Thanks to Gary Pang’s generous donation of such battery, here is a configuration for this battery:
SV14.0, 14.1, 14.2, 14.3, 14.4, 14.5, 14.7, 14.8, 14.9, 15.0, 15.2, 15.3, 15.8, 16.0, 16.8
SP0, 8, 21, 27, 41, 49, 57, 64, 72, 80, 85, 88, 98, 99, 100

ArduCopter 4.x battery safety settings:

These settings are for ArduCopter 4.x , which you should be using with OpenSolo 4.0 or newer. The BMSOne does work with OpenSolo 3 and AC3.6.x too, but it does not work with the original, old , forked 1.5.x versions of the autopilot, (nor should you fly the old versions)

If you use S0 mode(default), set low and critical-voltage thresholds:
If you use S1 mode(capacity based), also set low and critical-capacity thresholds:
Finally, these parameters define what action is taken:

SoC Voltage mode explained:

S0 = State-of-Charge by Voltage
This mode is beautifully simple and reliable, as you can use any number of battery packs, with same type of cells, and have perfectly fine SoC estimation (see battery percentage drop)
Advantage of this mode is that it works by voltage under load, so you can have one BMSOne and any number of packs, and any state of charge on those, and still, it will “just work”

We all remember how fast the remaining capacity drops on a Solo battery at the end, 20% can fall to 3% within one minute.

The first graph is made using these packs’ first flight.
It estimates energy as consumed a bit fast in the beginning, then just a little faster after 19/20 minutes.

Then the voltage/capacity table is corrected.. and watch the second graph, beautiful linear estimation, no surprises there even as the battery gets to 2.5v/cell.

Another aspect of SoC by voltage, is that it is most accurate when at normal cruise/hover. During a fast climb, the voltage will drop, and while climbing, you will see a lower estimate, and during a fast descend, you will see a higher estimate.

21minute flight example:

(I flew in bad wind conditions, now I see that the flight had the 4.0.3 PID issue that surely affected the flight time in a bad way)
4S3P Samsung INR18650-30Q (12 cells) 620grams with BMSOne.
GoPro Gimbal with Gopro 4 Black. AUW 1950g
Cells bought from Banggood (please observe that this is not a referral link, I get no kickback, nor can I guarantee the authenticity of a later batch).

1297 sec (21min) long flight in light rain and 2-4m/s wind.
I landed at 2.6v/cell, yet the manufacturer’s datasheet use 2.5v as the cutoff.

How to make a linearization table:

Let’s say you have a battery with given chemistry that you want to charge to 16.8v max and fly down to 14.0v minimum voltage. None of the known tables work well for it.

On a day with calm or no wind, fly slowly or hover well out of ground effect.
Monitor voltage of the individual cells and fly the pack down to the desired 0% state, for example, 14v.

The most important part is not to affect the discharge curve by hard flying or rapid climbs/descends. You want to see the pack’s normal voltage drop under normal load.
Get the log from the Solo, and plot BAT.Volt using APMPlanner2 or similar tools.

Now imagine, or draw some number of data points you want to read out, they can be spaced out by time on x axis, or , better, you can increase density where the voltage is less linear like illustrated with red lines below:

Your first data-point would be full pack voltage 16.8v, 100%
The next one, the voltage that is measured right after takeoff, you can see voltage drop to 16.25v under a given load, but you know it is still full, so the next data point is 16.2v, 99%
The next data point may be read out at the second red line, 15.7v, and the red bar’s position, maybe for example 10% into the flight, – then the remaining capacity(flight time) is 90% – so the next data point is 15.7, 90%
several data points later you land at desired 14.0v … you end up making a final data point 14.0v , 0%.

Now let’s convert the data points to the table: x are points we did not bother to calculate in this example:

SV14.0, x, x, x, x, x, x, x, x, x, x, x, 15.7, 16.2, 16.8
SP0, x, x, x, x, x, x, x, x, x, x, x 90, 99, 100

Solo/Ardupilot SMBUS BMS

Latest progress:

See https://madhacker.org/bmsone/ – the resulting product.

13 January 2020:

It’s working as expected. (The balancing connector header had 2mm pitch on PCB (my bad), therefore, this test version got a wire.)

All Calibration/setup/programming and firmware updates are done over the FTDI header.

Please do not think much about the poor flight time I got during early tests, the cells had a bit too high internal resistance and produced heat/dropped voltage too much. Better cells could be used.

11 January 2020:

Preliminary documentation for configuration over Serial port (serial port is used for configuration and firmware updates):
Example commands in bold.

SC3256,3231,3255,3263 // calibrate cell voltage in millivolts for 4 cells.
Users can also omit any cell to if one-by-one calibration is desired for some reason, like SC,,3300, – this may be useful if calibrating by a single, precise voltage standard source.

SI10220 //Calibrate current sensor in mA “SI10220” = 10.220A

SV11.3, 11.7, 12.6, 13.3, 14.3, 14.7, 14.8, 15.7, 16.0, 16.2, 16.3, 16.4, 16.6, 16.7, 16.8 //set 15-point voltage linearization table

SP0, 5, 10, 15, 25, 45, 55, 79, 84, 88, 91, 94, 96, 98, 100 //set 15-point percent linearization table

SF9000 //set design capacity in mAh , here: 9Ah

GV //get firmware version
SW //write valibration/linearization tables to flash

5 January 2020: designed some initial PCB’s – now with individual cell measuring and current sensing.

(I hope to start selling them as a product before March 2020)

15 December 2019:

Currently handled I2C requests (all that Solo is requesting:)


This is an early version of a BMS for any 0-5v/cell chemistry 4-cell pack for ArduPilot.

Right now, it is more correct to call it an BMS emulator, than a BMS.
A final product would measure individual voltages and the actual current. I started this way because it does not require a custom PCB and a minimum of discrete components.

This early version does handle all the communications, and makes the autopilot set the actual capacity (9Ah), reports voltage, and calculated individual cell voltages.

It does also calculate and report remaining capacity by doing a 15-point linearization and then interpolating a “voltage_to_capacity_table”. This method is good enough once one knows the battery characteristics and fly at about the same current later.

Test Flight1:

I did not wait for the 4S3P Li-Ion pack to charge fully. (Samsung 3Ah 18650-30Q cells) The outdoor temperature was about -5°C, batteries. Flight time was 17 minutes. With GoPro4Black + the brushless 3DR Gopro gimbal. The battery pack was 677gram. Included in that weight is 22gram that is the BMS microcontroller PCB, Solo battery connector and wires/connectors to battery.

The predicted SoC percentage (blue) is clearly not perfect (yet) it was good at the end, not at the beginning, but that is not a problem, as it is easy to change by modifying the table.

Notice that it can still climb at as low as 10.5v , not exceeding 70% of throttle.

Post-flight thermal image: (somewhat affected by some kapton tape on the battery pack.)

TODO / Future:

Given enough interest, I plan to make a fully standalone BMS that can be user-customized to any chemistry, (min/max voltages) and so on.
With balancing, real current sensing, fuel gauge etc.

History of this site

madhacker.org existed since ~1995   Originally it was a site about my buildlogs, hacks, modifications, and technical that you may call a “blog” today.

With time – I’ll restore  some more of the old articles,  most of it will be very outdated, stored just for fun.