ElectricFlight L
ast time I described how to meas- ure the internal resistance of a Li- Po cell using a voltmeter and am- meter with a load. This resistance is the voltage drop under load divided by the current. I was sure I could use a small micro- processor to do this automatically. The popu- lar Arduino microprocessor systems seemed to be just the ticket. The problem was meas- uring the low (400 mA) current accurately. Fortunately the Adafruit INA219 Current Sensor (
http://www.adafruit.com/) contains a precision amplifier and a 12-bit A to D con- verter that accurately determines current by measuring the minute voltage across a 0.1 ohm, 1% sense resistor. This indeed does the job. I put together a Li-Po battery tester based on an Arduino Uno and a serial LCD readout. A single flashlight bulb is used as the load.
When the subject battery is plugged in, the no-load voltage is shown. Pressing a but- ton starts the test with a MOSFET complet- ing the circuit to the bulb. The current is al- lowed to flow for ten seconds. The loaded voltage and current is then read and the re- sistance is calculated and displayed. The MOSFET then removes the load when the test is complete.
If the loaded voltage drops below 3.2 volts, the test is aborted. This indicates the cell is toast and should be tossed. This sure does beat using two VOMs, writing down the data, and doing the math with a calculator. Of course the resistance varies inversely with temperature quite strongly and com- parisons should be made at the same tem- perature. That being said, you can weed out a bunch of duds in a hurry.
This column is not the place for instruc- tions on how to build the Li-Po cell resist- ance tester, but if you drop me an e-mail, I will be happy to supply a parts list, schemat- ic, and Arduino “sketch”. I am currently re- ducing the size of the unit so I can put the battery that runs it inside rather than being an external dongle as shown in the photo.
by stew meyers You can reach Stew Meyers at 8304 Whitman Drive, Bethesda, Maryland 20817, or via e-mail at
stew.meyers@
verizon.net
PHOTOGRAPHY: STEW MEYERS
Arduino-INA219 used as a recording VOM to measure the capacitor. The motor is set up on the charge stand and the VOM has been inserted between it and the capacitor.
I had advised not to charge Li-Pos below 10 degrees C. This is a bit too conservative. The problem arises at zero degrees C or 32 degrees F. A 10 degrees F not 10 degrees C buffer would be more reasonable. So don’t charge below 42 degrees F.
One of our active indoor fliers, John Krouse, regularly shows up at our flying ses- sions with a variety of inventive non-rubber powered models. Some are CO2, some elec- tric. He quite often uses R/C as throttle only relying on freeflight trim to keep the model from hitting the walls. A few weeks ago he had a capacitor powered electric model. This used the “Power UP 2.0” from http://www.
easybuiltmodels.com/pow.htm. This is a kit that allows you to mount an electric motor to a standard paper airplane.
PowerUP consists of a lightweight super- capacitor attached to a 6mm motor with a 2- inch pusher propeller via a carbon fiber shaft. The all-up weight is 7 grams. “Simply clip the module onto the airplane nose with the propeller at the rear, charge it for 15–20 seconds using the separate battery pack and let her go for over 30 seconds of flight dura- tion.” The motor runs while the capacitor is being charged. John had his on a balsa and tissue canard and a twin boom balsa and foam pusher. He did not fully charge the ca-
Arduino-INA219 Li-Po cell tester. When the button (at left) is pressed the cell resistance is read out. This version is powered by the black dongle. Thrusters (above) Microflyer Power unit top, PowerUP unit bottom.
46 MARCH 2014
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