Drone dock charging with BMS

[art-mx]

Hi All,

I was thinking about an autonomous drone dock charging station, and I found that BMS is the right way to do this. The trick is that the motors on a relatively large drone can consume up to 100-120 A and a BMS that can deal with that load is massive and unfeasible.

Then I thought, it's actually not necessary to balance the 100A discharge during flight, only the 10A charge. So it might be feasible to have the following setup.

The question is: Will it work?
I would appreciate any feedback!

 

[Grizzl-E]

Your drawing is off but yes the concept is often used here when running high powered motors. On a side note, as far as I know the positioning accuracy doesn't exist yet to do what you want to do. Unless you can program in a target for an optical flow sensor to recognize after the gps gets you close. Otherwise you'll never drop your "drone" onto a set of contacts.

 

[circuit]

Hi art-mx, perhaps this BMS would fit your needs? Only 95*65*8mm and handles up to 150A.
https://endless-sphere.com/forums/viewtopic.php?f=14&t=76011

 

[art-mx]

Thank you for the responses!

Grizzl-E , Would you mind pointing out what is wrong? https://docs.google.com/drawings/d/1UKxaBk7dThxhOcjnPv_2_yGrdFPH7nDaBgf1_udnjXc/edit?usp=sharing
As to the positioning accuracy, there are many ways of achieving this. It all is up to the application. To name a few:

Motion Capture: http://flyingmachinearena.org/videos/ - millimeter accuracy
RTK GNSS: https://en.wikipedia.org/wiki/Real_Time_Kinematic - centimeter accuracy
Computer Vision: http://www.asctec.de/en/uav-uas-drone-applications/computer-vision/ - varies, but can work quite well in a pair with a Flow Sensor.

circuit , That seems to be a good piece of hardware, even too functional for my current application. It would fit nicely though on our high payload hexacopter project: http://diydrones.com/profiles/blogs/huge-high-payload-hexacopter

There are a couple of things that I would appreciate in a drone BMS:

1) Reliability. By using a BMS, you are essentially "entrusting it with the life of the UAV". If it fails, the machine goes down. In my opinion, it should be the most reliable part of the electrical system. For example, there could be a similar "soft back-off strategy" in case of overheating, as in Vedder's VESC. (don't know if it's possible with a BMS, but you get my idea)

2) Support for two battery systems, with one being the main "high voltage drive battery" and the other, "low voltage backup battery", just for keeping the flight controllers and onboard computers on during the main battery swap process. In this scenario, the backup would be receiving a balanced charge when a drive battery is connected.

full switch-mode DAC up to 500mA and full battery voltage range (for example, to charge 12V battery or supply any range of voltages for variable fans or analogue SOC gauges)

This sounds close, could you explain this a bit?

 

[circuit]

It really would be a shame to see such UAV crash, especially due to failed BMS.

At the moment it has no "early warning" output to inform about low battery condition, but it is one of highest priority features planned.
The idea would be to support at least one of these methods:
1. Logic output for "battery low"
2. PWM output to represent battery voltage or maximum available current (it would estimate maximum available current based on current voltage, set current voltage and measured battery impedace).
3. Analogue output, same as 2.

However it does have all the parameters available via USART line. If you are using Arduino or any other custom control board, you could read the data periodically (it also has a periodic broadcast in CycleAnalys v3 format).

As a backup battery, I guess you could have the BMS to only inform the onboard computer about battery low condition, but never cut the power. It would be up to operator to land safely. Anyway damaging your battery is worse than crashing your UAV and the battery inside.
Or you could use a simple secondary low capacity battery with diode...

The DAC is used to generate various voltages, for example as 12V contactor drive. It could probably charge external battery, if current is somehow limited.

 

[art-mx]

The current way the battery life is handled in MAVs is quite crude, although works.

It is done by feeding a 0-3.3v analog voltage and current sensing into the autopilothttp://copter.ardupilot.com/wiki/common-pixhawk-overview/. 3DR Pixhawk https://store.3dr.com/products/3dr-pixhawk is the most common nowadays on both consumer drone products and research purpose vehicles.

The battery capacity is estimated by autopilot firmware by knowing the total capacity of the battery used onboard and the current measurements from a sensor. As well as looking at the voltage measurements, but they can sag and fluctuate a lot during flight. The autopilot then decides on the actions that need to be performed, either land in place or return to home position.

It would be nice if your BMS could seamlessly work with Pixhawk (3.3v analog output) and provide more precise analog battery capacity measurements without any preliminary configuration.

As an addition, the full parameter list(USART that you mentioned) could be fed into either Pixhawk serial or Onboard Computer serial, but that will require software integration.

 

[circuit]

Thanks for your input, I have included a feature request in our project management system. Since all the hardware is in place, this will be an easy software implementation.
Meanwhile I will be starting a sell thread for preorders.