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Simple cutoff circuit for using a CC/CV supply as a charger


10 kW
Mar 10, 2010
Seattle, WA
I have a couple CC/CV PSUs that I use to charge my bike battery, and it's always been a bit annoying to have to remember to unplug the charger to keep the batteries from trickle charging once they’re full. I recently came up with this little analog parts-bin circuit that detects when the charging current drops below a minimum threshold and cuts off the charge current. There's only 2 active components, the FET that switches the charge current and a dirt cheap LM324 quad op-amp. It's intended to stay connected to the battery and it resets when the charger is disconnected or powered off.


The operation is pretty simple. It uses U1A to detect when the FET drain is above ground, which means that a charger is plugged in. U1A charges C1 when this occurs, causing the minimum current threshold to rise from 0. U1B senses and amplifies the voltage across the shunt resistor R7. U1C compares the amplified shunt voltage from U1B to the threshold voltage from U1A and turns off the FET when the shunt current drops below the threshold.

The minimum current threshold is set with R9, the 10k potentiometer. Vcc is regulated by D2 which is a 10V Zener diode. My battery is 60V so I used a 100V FET, STP40NF10L. The circuit draws ~2mA, but probably could pull less with a better op-amp.

I put it together on a row-connected 0.1" spaced protoboard and managed to fit the whole thing into a 1.3" square.

Check out my project page if you're interested in more details: Analog Battery Charge Stopper
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Not having analyzed the circuit ;) will a 1milliohm shunt (pilfered off a dead controller) work in place of the 20milliohm shunt, just adjusting the resistor values on U1B to compensate?
Yeah, that should work! The only potential issue issue is that the input sensitivity of the LM324 could be too low to detect low currents across such a low resistance shunt. The typical input offset voltage of the LM324 is 2mV, which would correspond to 2A across your shunt. It's worth a shot though, and maybe you could grind down the shunt to increase the resistance if you need to.