fizzit
10 kW
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
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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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?
