Solar Charge Controller for medium power applications -> Note: some of the part numbers have changed from when the article was -> published to when the circuit board was manufactured. (C) G. Forrest Cook 1997 email: cook@solorb.com INTRO This article describes the companion for the low voltage disconnect circuit described in HP #60. This circuit regulates the charging of the battery in a solar system by monitoring battery voltage and switching the solar or other power source off when the battery reaches a preset voltage. A charge controller circuit can increase battery life by preventing battery over-charging which can cause evaporation of electrolyte. The absence of a relay and its associated coil current makes this circuit efficient for systems using small solar panels and batteries as well as for systems using larger current components. The charge controller was designed with the following goals: high efficiency, use of common parts, and operation with common ground circuitry. Some of the ideas used in this circuit were inspired by an article in QST magazine(1), this is a much simplified circuit. A circuit board is being made available with both the charge controller and low voltage disconnect circuits on one board, see the access section below. The charge controller circuit has been used with solar power input, it also functions well as a battery charger when used with any current limited DC power supply such as small "wall wart" transformers or a high current supply with a series resistor. SPECIFICATIONS Night time current drain: 0.6 ma Operational current drain: 19 ma (less without the LEDs) Maximum solar panel current: 3-10 A (see text) Voltage drop during charging: 0.5 V at 1 A THEORY During charging, current flows from the solar panel through diode D1, MOSFET transistor Q1, fuse F1 and into the battery. Power MOSFET transistor Q1 is the main switching device in the charge controller circuit, it connects the solar panel to the battery when the battery is in need of charging and power is available from the solar panel. As with the LVD circuit, Q1 is set up in a "high side" switch arrangement which allows for a common ground circuit, this is helpful in automotive and other applications. Switching efficiency is very high due to the low on resistance of modern power MOSFETS, usually under 0.1 ohm. Diode D1 is a schottky device, it prevents back currents from flowing from the battery to the solar panel. A regular silicon diode may be used but a schottky device will have a lower forward voltage drop and resulting higher efficiency. Fuse F1 provides a safety limit on the current available from the battery in the event of a short. Comparator U2 is used to control power to the rest of the charge controller circuit. When the solar panel voltage is lower than the battery voltage, the rest of the circuitry is disabled, this reduces night time idle current to the few milliamps consumed by U2 and its associated input circuitry. When the solar panel voltage rises above the battery voltage, the output of U2 goes negative, switching on transistor Q2 which provides power to the rest of the circuit. Resistor networks R1/R13 and R2/R4 scale the battery and solar panel voltages to a range that is useful to U2. Capacitor C23 prevents oscillation in the comparator at start up. Voltage regulator U4 is used as a reference for the battery set points, the reference points are adjusted via resistor network R11, R12, and R3. Comparators U1A and U1B monitor the battery voltage and switch states when the battery is fully charged (U1B) or has dropped to a voltage where charging should resume (U1A). The comparators drive a set-reset flip-flop circuit consisting of U3A and U3D. The comparator outputs are inverted logic, on is low and off is high. The output of the flip-flop is used to turn the oscillator consisting of U3B and U3C on and off. The flip-flop also drives the two LEDs which are used to indicate charging and battery full states. The oscillator generates a 10 Khz square wave that is stepped up to around 25 VDC by the voltage doubler circuit of D5,D6,D7, and C7,C8,C21. The gate voltage is higher than the battery's 13V, and is used to turn Q1 on fully. A ferrite bead, L2, is used to prevent oscillation in Q1. Resistor R9 discharges the voltage doubler when the oscillator is shut off. The technically picky may note that all of the ICs comparators are really common op-amps, not special purpose comparators, the op-amps are wired in a comparator configuration. The circuit is fairly dependent on the use of 741 and 1458 op-amp parts, use of other op-amps may require changing the values of R1 and R2. An equalize switch is included to allow for occasional over-charging of the battery, this switch works by raising the threshold of the high voltage sensing comparator, this has the effect of forcing the charge current on. Equalizing helps bring lower voltage cells in the battery up to a full charge. ALIGNMENT Alignment is straightforward, the equipment required consists of a multi-meter, a charged 12 Volt lead acid battery, and a 0-16V DC variable voltage power supply with a 10 ohm 25 watt resistor in series with the positive lead to limit the current. A word of caution is in order when dealing with circuits involving potentially high battery currents, the circuit should be placed on an insulating surface for testing and all wiring should be insulated, this lessens the chance of creating a short circuit and burning things up. Be sure not to reverse the polarity of the battery wires, doing so may damage the circuit. The voltages in this circuit present no shock hazard but the currents present a potential burn hazard. The first step of the alignment is to set the charge controller turn-on voltage with R13. Start out by turning R12 fully clockwise (toward positive), turn R11 and R13 fully counter-clockwise (towards ground). Connect the charged 12V battery to the battery terminals and connect the current limited variable power supply to the solar panel input on the charge controller. Connect the volt meter across the Schottky Diode, D1 with the negative volt meter lead on the cathode (bar end) of the diode. Adjust the variable supply from zero up to around 13V until the meter reads about 0.3V across the diode. Slowly turn R13 clockwise until the red LED just turns off, now turn R13 counter-clockwise again until the red LED just turns on. The second and third alignment steps involve setting the low and high points that the battery will alternate between when it is fully charged. Connect the volt meter to across the battery for this step. Turn the variable voltage supply to 15V. Adjust R12 counter-clockwise until the green LED turns on. Adjust R11 clockwise until the red LED turns on. At this point, the charge controller should be functioning and the LEDs should alternate. Observe the battery voltage and adjust R12 until the battery voltage peaks at the desired high charge point. Richard Perez recommends setting the high charge point to 13.8 volts for sealed gell-cells and to 14.5 volts for flooded cell (wet) lead-acid batteries. Richard also notes that these values are for solar applications where the sun only shines for part of the day, the values should be lower for applications with continuous power sources. The battery low set point should be set to 0.5 to 1 volt lower than the high set point, adjust R11 until the battery drops to the desired voltage before the charging cycle begins again. In a properly adjusted circuit, the two LEDs should alternate several times per minute, this varies with battery and solar panel capacities. If the battery voltage drops too slowly during the test, it may be helpful to connect a small 12V lamp across the battery, this will cause the battery to discharge faster. It may also help to adjust the voltage of the variable supply, this will vary the charging current and duty cycle of the flip-flop. CURRENT CAPACITY The current handling capacity of this circuit is determined by the MOSFET transistor Q1, diode D1, Fuse F1, and the current carrying wires in the path between the solar panel and the battery. An IRFZ34 MOSFET is rated at 30 Amps max and should easily handle 10 amp charging currents. A heat sink should be used on the MOSFET and diode D1 if you are running currents higher than 2 or 3 amps through the circuit. The peak current may be determined from the solar panel specs. Diode D1 can be an IR 80SQ045 when the max current is less than 8 Amps. higher current diodes such as the GI MBR1045GI rated at 10 Amps may also be used with a heat sink. For efficiency, it is important to use a schottky barrier diode here since it has a voltage drop of around 0.4V under load while a regular silicon diode has a voltage drop of around 0.8V under load. At 5 Amps, the silicon diode wastes 4 watts while the schottky diode wastes only 2 watts. The circuit board version of this circuit can handle about 8 amps maximum if the proper semiconductors are used. The fuse should be rated the same as the maximum current of the FET and/or diode D1, whichever is lower. CONSTRUCTION I built the prototype circuit on perforated circuit board using point to point wiring, see photo 1. Teflon insulation over tinned bare wire works well and does not melt under a soldering iron. Be careful not to overheat any of the semiconductors, especially the LEDs. IC sockets may save a lot of time and grief in circuit debugging. Wires between the solar panel, D1, Q1, F1, and the battery should be heavy gauge to handle the charging current. Be sure to use thick wires for the current carrying part of the circuit. In the prototype I built the circuit into a small plastic box and used banana plugs as connectors for the input and output Terminals. The circuit board version of the combination charge controller and LVD circuit is shown in photo 2. USE Connect the solar panel to the solar panel terminals and the battery to the battery terminals and watch the battery charge up. When the LEDs alternately blink, the battery is charged. A load may be connected between ground and the fused C5-Q1 source junction if the load current is lower than the fuse rating. The circuit board has the companion LVD circuit connected in at this point. Be sure to use battery cables that can handle the load current. If the circuit is to be connected to a high current source such as an automobile cigarette plug or a high current capable power supply instead of a solar panel, it will be necessary to use a high wattage series resistor between the positive power source and the charge controller solar panel input. A 10 ohm, 25 watt resistor would be a good value to start with. (1) The FET Charge Controller, Michael Bryce WB8VGE, QST, January 1992