Thinking About Batteries

As we work on assembling the airframe, we are also thinking about our battery strategy.  In our robotics courses we have developed a number of different drones and up until just recently we have powered them using the traditional RC-type lithium pouch cell (also referred to as lithium polymer) battery packs.  These batteries offer high discharge rates which work well for the high power demands required to fly multi-rotor aircraft.  These high discharge rates however come at the cost of cycle life, specific energy (Wh/kg), and safety. 

As a result of these shortcomings, we have begun to experiment with the 18650 cylindrical lithium cell in our drone projects.  These cells have been around for a long time and are responsible for powering everything from laptop computers to cordless power tools to electric vehicles.  When compared to the RC lithium polymer type cells the 18650 cylindrical cells offer greater specific energy, energy density (Wh/L),  and greater cycle life.  Our aircraft will not be taking off vertically (at least not the first iteration) so our power demand should fall within the range of the lower discharge rates offered by these cells.

These cells also lend themselves well for custom configuring and packaging to achieve the desired voltage, amp-hour rating, and space constraints.  We have already  fabricated a couple of quadcopter battery packs using 18650’s.  We design battery holders using CAD software and then print the parts on our 3D printers.  After the cells have been packaged to achieve the desired series/parallel configuration, we use our custom-built spot welder to fuse the nickel strip electrical connections.  Early tests have led to a 45 minute hover on one of our medium size quadcopters.

With electric vehicles gaining more and more traction we can anticipate battery technology to improve significantly. The increase in EV’s will lead to additional battery options in the form of re-purposing existing electric vehicle batteries for our mission.  Obtaining a battery from an electric motorcycle or part of a battery from an electric car and modifying it to meet our requirements may hold promise.

Regardless of what route we take in battery development we will have to make sure that we incorporate the necessary battery management system that will help us monitor the health of the system as well as spot any anomalies that could potentially lead to catastrophic failures.  We will also have to ensure that our cell interconnection strategy and packaging will prevent single point failures from propagating.

Expect much more in the way of battery discussion as we move forward…

CAD rendering of an 18650 lithium battery pack used to power on of our drones.
Cell holders being printed using ABS plastic.
This is our custom built spot welder. We use a 12V deep-cycle lead acid battery to provide the short 7 millisecond pulses of high current to fuse the nickel strip to the battery terminals.
The spot welding process in action. Each terminal receives 2 to 3 welds.

Time lapse video of the spot welding process.

A partially welded pack. The series connections are made after the parallel connections. The nickel strip is 8mm wide by 0.2mm thick.
A larger 18650 pack comprised of 160 cells. The cells are arranged in a 20S8P configuration for 72V at 27Ah (~2kWh of energy).
Comparison between pouch cells and cylindrical cells.
This drone was used to test our first 18650 (LG MJ1 cells) battery pack. It hovered for over 45 minutes on this 4S3P 10Ah battery with about 20% reserve energy.
This 650mm sized Hexacopter is scheduled to fly with a custom built 24 cell 18650 (Panasonic GA cells) pack in a 6S4P configuration.

Click here to see some of the drones our robotics students have designed, fabricated, and flown.

2 thoughts on “Thinking About Batteries

  1. I loved this post! You are always providing some great stuff.
    I shared with my Facebook, keep up the good work!

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