Solar Piston V2

It’s the Year of the Rabbit. Someone needs to make a Rabbit Robot!

The updated design for the Solar Piston LiIon Battery Charger is here!

What’s Happening?

This circuit will safely charge a LiIon battery from a PV solar panel, or any other micro-energy power source. There are 3 states that the circuit can be in. The first is when the PV Source is not able to produce enough current and/or voltage to bias D2 and charge the capacitor C6. In this case, the IN on U1 does not go above it’s trigger, and it’s output remains low (internally connected to GND). In this case, the base of the NPN transistor, U3, feels GND and remains off. That makes the gate of the P-MOSFET, U2, feel +V through the resistor R2, and it remains off, disconnecting the circuit from the LiIon battery. In this state, the voltage at IN of U2 is well below its internal trigger

The second state is when the PV source is providing enough current and voltage to bias D2

What Is It?

U1 and U2 are both S-808 series Voltage Detectors from Seiko Instruments. The output of these devices will go low (internally connected to GND) if the voltage on the IN pin is below an internally set threshold. If the V on IN is above the threshold, the output is in high Z (high impedance state, essentially connected to nothing). These devices are the brains of the operation. U1 threshold is 3.5V, U2 threshold is 3.8V.

U3 is a 2N3904 NPN transistor rigged up as an inverter.

U4 and U5 are Power MOSFETs (P channel). U4 is the low resistance ‘switch’ that closes to provide charge for the battery. U5 is the low resistance ‘switch’ that opens to prevent over charging the battery.

D1 is a low Vf Shottkey diode. It’s job is to block current from flowing ‘back’ through the PV source.

R1,2,3 are pull-up resistors. 470K or 1M can be used to reduce leakage in the circuit.

For critical information about LiIon battery management and behavior, please go to Battery University.

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