Solar Piston V2

It’s the Year of the Rabbit. Will there be a Rabbit Robot?

The updated design for the Solar Piston LiIon Battery Charger is here. This circuit will safely charge a LiIon battery from a PV solar panel, or any other micro-energy power source.

What’s Happening?

There are 4 states that the circuit can be in. The first is ‘Not Charging’. This happens when the PV Source is not outputting enough power to charge the capacitor C6 above 3.7 volts. When this happens, 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 the internal internal trigger of 3.8V, so its OUT pin is connected internally to GND. The gate of U5 (P-MOSFET) feels GND through U2’s OUT pin, and remains on. If there is any rise in voltage on the PV Source above the voltage on C1 + 0.3V (the forward voltage of D1) current can flow and charge C1.

The second state is when the PV Source is providing enough current and voltage to charge C1 up to 3.7V, the trigger voltage on U1. When IN on U1 feels 3.7V, it turns off its output, and disconnects the base of U3 from GND. U3’s base feels +V through R1 and turns on, pulling the gate of U4 to GND.  U4 turns on, and connects the capacitor to the battery and charging happens as C1 drains into the battery. If the source (PV, Piezo, Wind Turbine, Bike generator…) is not powerful enough to continuously charge the battery, the voltage on C1 will fall until it drops to 3.5V, at which point the Voltage Detector, U1 will turn on it’s OUT pin, connecting it to GND. This, in turn, will pull the base of the NPN low, turning it off, and the gate of th P-MOSFET U4 will get pulled high, and it will turn off, therby disconnecting the charging capacitor from the battery. As the PV Source is continuing to provide power it will send the charging circuitry into oscillation, pulsing current into the battery.

If the PV Source can provide enough voltage and current to C1 so that the Voltage on U1’s IN pin is above 3.7V, The PV will charging circuitry will remain on, and charging will happen continuously. This condition would constitute state three.

The fourth state is another oscillating situation. In this case, the PV Source is providing more than enough voltage and current to charge the battery through the charging circuit, and the voltage on C1 rises above 3.9V. At this point, the IN pin of U2 feels the voltage trip it’s trigger setting, and it turns off the GND connection of its OUT pin. This disconnects the gate of U5 from GND, allowing it to be pulled up through R3, and the MOSFET turns off, disconnecting the PV source from the circuit. As the charge on C1 continues to drain into the battery, it’s voltage falls. When it drops to 3.8V, U2 turns on again, pulling U5’s gate low, and allowing the PV Source back into the circuit. This oscillation will continue as long as the Source is providing excessive current to C1.

What’s It Made Of?

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 P-Channel Power MOSFETs (BS250). U4 is the low resistance ’switch’ that closes to provide charge for the battery. U5 is the low resistance ’switch’ that opens to prevent overcharging the battery.

D1 is a low Vf (~0.3V) Shottkey diode. It’s job is to block current from flowing ‘back’ through the PV source.

R1,2,3 are pull-up resistors. 1 or even 10M 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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