Constant current load board for plotting discharge curves

Constant current load board

I hacked together a constant current load board for the LiPo batteries so I can record and plot discharge curves. The hand-drawn schematic is shown below. I used parts that I had lying around, including an LMC660AIN quad op-amp, an NDP706BEL logic-level input N-channel MOSFET, and a 10-turn 5-kOhm potentiometer. I ran to the local Radio Shack to pick up the 1-Ohm, 10-Watt power resistor. I was pleasantly surprised they actually had that sort of thing anymore! Note that this circuit really only works up to about 2A, at which point the power resistor is smokin’ hot! So I’ll keep it down below 1.5A. For the small batteries I’m testing, I won’t need anywhere near that much current. The nice thing about using a 1-Ohm resistor is that you can put a multimeter across the resistor terminals on the DC Voltage setting and read the display as if it’s Amps. E.g.: Voltage = 0.15V on the Voltmeter display means current = 0.15A, or 150 mA. (BTW, “DUT IN” is where you connect the LiPo battery to discharge it; BAT+ to Q1 Drain, BAT- to GND.)

 

 

Constant current load schematic