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By JOHN CLARKE 12/24V MPPT Solar Charge Controller Rev.1 The 3-stage MPPT Solar Charge Controller has been a popular project and one year later we have been able to make some useful improvements to its efficiency and include some extra charging options. The design retains the features of maximum power point tracking (MPPT) for optimum power delivery from solar panels and 3-stage charging for lead-acid batteries. A S PRESENTED in February 2011, the MPPT Solar Charge Controller was designed for use with 12V solar panels rated up to 120W or 24V solar panels up to 240W. A number of readers have requested modifications to allow it to be used with larger panels. Unfortunately, that’s not practical because the necessary component changes required for more power cannot be accommodated within the existing box size or on the PCB. We may publish a higher power MPPT Charge Controller at a later date but for now, we are simply presenting some enhancements to the original design, to make it run cooler and more 90  Silicon Chip flexible to use. The increased efficiency will be a significant improvement where the Charge Controller is being used inside a vehicle and is subjected to high ambient temperatures. The software in this revised design adds some options to the way the charger functions. As originally presented, the 3-stage charging feature includes bulk charging, an absorption phase and float charging. Plus there is the option to periodically run equalisation to make sure that all cells in the battery or battery bank have been equally charged. Fig.1 & Fig.2 show graphical representations of these charge modes. During bulk charge, maximum pow­ er is extracted from the solar panel, using the MPPT system, to charge the battery. Bulk charging ceases when the battery voltage reaches a cut-out voltage of 14.4V. This applies to a temperature of 20°C. It is compensated for other temperatures and is inversely proportional to increasing battery temperature; if the battery is hot, the cut-out voltage is reduced. The absorption charging stage then maintains the cut-out voltage (ie, 14.4V) across the battery for an hour to ensure the battery is fully charged. After this period, the charger switches siliconchip.com.au BATTERY VOLTAGE BATTERY VOLTAGE CUTOFF VOLTAGE FLOAT VOLTAGE BULK ABSORPTION CUTOFF VOLTAGE FLOAT FLOAT VOLTAGE BULK EQUALISATION FLOAT TIME CHARGE CURRENT TIME CHARGE CURRENT TIME STANDARD THREE-STAGE CHARGING Fig.1: the three standard battery charging stages. First is the initial bulk charge. Once the battery reaches the cut-off voltage, the absorption stage takes over to fully charge it. Finally, the float stage maintains its charge. to float charge whereby the battery voltage is maintained indefinitely at 13.5V at 20°C. Again, temperature compensation reduces the float voltage with increasing temperature. Normally, the MPPT Solar Charge Controller is left permanently connected to the battery or battery bank and for perhaps most of the time during daylight hours, the battery will be on float charge. Then, when a load is placed on the battery, the Controller reverts to the bulk charge mode when the battery voltage falls below 12.45V. The figure of 12.45V is somewhat arbitrary but is a level at which it is certain there is still substantial charge remaining in the battery. During absorption and float charging and whenever a load is placed on the battery, the charger’s current increases up to the maximum that can be derived from the solar panel. Only if the battery voltage drops below 12.45V will the charger revert to bulk charge. Some constructors found that the charger did not reliably start bulk charging when the battery dropped to 12.45V. The battery monitoring settings are critical in this regard. Some constructors of the MPPT Charger would also have preferred the battery to be bulk charged each day at the onset of daylight, as soon as the solar panels start to produce siliconchip.com.au TIME CHARGING WITH EQUALISATION Fig.2: the charging cycle with equalisation enabled. Instead of the absorption stage, the battery voltage is allowed to rise by 10% over the cut-off voltage to cause gassing within the cells. This charges the cells equally. power. Additionally, they wanted the Controller to revert to bulk charge from float charge whenever power is drawn from the battery. This would then occur before the battery voltage falls to 12.45V. Both options have been incorporated into this revised design and they can either be selected independently or together. With neither option selected, the MPPT Controller will operate under have the original arrangement, with bulk charge cutting in whenever there is less than 12.45V across the battery. Other software refinements With the first option selected, bulk charge is initiated each morning as soon as the solar panel starts generating power. The second option switches the Controller from float to bulk charge whenever power to maintain float charge is greater than that required just to maintain the battery in float. However, if bulk charging begins WARNING! When charging with the equalisation cycle, the battery will produce hydrogen gas which is explosive. For this reason, make sure that the battery is located in a well-ventilated area during charging. Additionally, if equalisation is used, the battery voltage will rise above 15V and this could damage any equipment connected to it. If there is any risk of damage to such equipment, it should be disconnected during equalisation. A test point (TP>15V & <11.5V) is provided on the PCB and this point goes to +5V when the battery goes above 15V during equalisation. This output could thus be used to automatically disconnect equipment when the voltage goes above 15V. A suitable circuit for doing this is the DC Relay Switch published in SILICON CHIP, November 2006. However, a latching relay switch would be more effective for this application since the relay only draws power when switching. A suitable latching relay circuit was published in June 2011. Note that the TP>15V & <11.5V output also goes to +5V if the battery voltage drops below 11.5V and only returns to 0V when the battery subsequently rises above 12V. As a result, this output can also be used to disconnect equipment when the battery voltage is low, to prevent over-discharge. March 2012  91 100nF 0.01  3W 1k LED3  A K LED2  A LED1  A K LED5  A K LED4 K  A K 2.2k 7 1k 12 11 13 15 16 10 2 1 9 E C A B E LK1* D +5V D3 AN4 RB2 AN1 AN0 MCLR LK2* A K K A2 100nF L1 5 H (12V) 10 H (24V) VR1 20k 4.7k 10nF E B (100k <at> 20°C) TH1 THERMISTOR 100k VR2 COMPENSATION mV/°C 100k 100 F 16V TP 5V D2 MBR20100-CT (24V) DSSK 38-0025B (12V) >15V & <11.5V 2.2k TP4 TP2 100nF 1k D4 A1 * SEE TEXT 3 8 18 17 4 33k 100nF K A 10 G RB0 RB1 6 7 IC1 PIC16F88 –I/P Vdd 14 Q2 TIP31C C Q3 BC337 Vss 5 RB6 RB5 RB7 RA6 RA7 RB4 AN3 AN2 PWM 100 B 470  1W K ZD2 18V 1W S 12/24V 3-STAGE MPPT SOLAR CHARGE CONTROLLER, REV.1 4 IC2b 470pF 2.2k 100 2 x 2200 F 25V (12V) 2 x 470 F 63V (24V) 10 F 35V A2 K Q1 SUP53P06-20 4 REG1 TL499A C BC337 2 8 TP1 100nF A K K K A2 MBR20100-CT DSSK 38-0025B A K D3,D4: 1N4148 TP GND A1 5 1 3 x 4700 F 16V (12V) 3 x 1000 F 35V (24V) B C E 10nF ZD1 30V 1W C G A D F1 10A S SUP53P06-20 K ZD1– ZD3 K A LEDS 22k (12V) 51k (24V) VR3 20k TIP31C 330 LEAD-ACID 12V (24V) BATTERY Fig.3: the circuit for the 12/24V Solar Charge Controller is based on PIC16F88-I/P microcontroller IC1. This monitors the solar panel voltage and current, the battery voltage, temperature (via the NTC thermistor), the compensation trimpot position and the equalisation switch S1. The resulting PWM (pulse width modulation) output on pin 9 of IC1 then drives Mosfet Q1 via transistors Q2 & Q3, while several other outputs drive the charge indication LEDs. SC 2012 EQUALISE THERMISTOR FLOAT 1 A K IC2: LM358 IC2a 8 ZD3 30V 1W 100  (12V) 1k (24V) 68k BULK 5 6 2 3 ABSORPTION 8.2k 100nF 1.5k 12V (24V) SOLAR PANEL EQUALISE S1  + 4.7k 22k (12V) 47k (24V) A1 D1 MBR20100-CT (24V) DSSK 38-0025B (12V)  92  Silicon Chip siliconchip.com.au D ZD2 4700 F/16V (1000 F/35V) 2.2k 2.2k 470  1W D3 REGRACHARGER HC RALOS TPPM SOLAR 100nF 10 100 Q3 LED4 100nF 100 4148 10nF LED3 1k ZD1 TP>15V & <11.5V 68k 470pF VR1 LED2 TP2 10nF 2.2k 8.2k 100nF TP5V TP1 TPG ALED1 1k TO CON3 THERMISTOR VR2 1k 1.5k 100nF 10 F VR3 100 F REG1 TL499A 100nF 4.7k (47k) ZD3 22k 4.7k IC1 PIC16F88-I/P 100nF (1k) 33k CON2 100 4700 F/16V (1000 F/35V) 100k F1 L1 4700 F/16V (1000 F/35V) 0.01  3W TO BATTERY + – D2 Q1 2200 F/25V (470 F/63V) 2200 F/25V (470 F/63V) CON1 (51k) 22k TO SOLAR PANEL + – 330 D1 IC2 LM358 at the start of each day, it is highly likely that the battery may already be fully charged and so does not need bulk charging. If it was bulk-charged, it would would quickly reach the cut off voltage. In this case, it is not necessary or good practice to then have a further hour in the absorption charge phase at 14.4V. Hence the Controller now monitors the time the Controller stays in bulk-charge mode. Should the battery reach the cut off voltage during bulk charge in less than one minute then the absorption phase is bypassed and the Controller will switch directly from bulk to float. The second software change involves detecting the start of a new day. The software needs to differentiate between the start of a new day and lifting of heavy cloud-cover or after a total solar eclipse; admittedly this last event is rare, the next one in Australia being in November 2012. Without correct detection of the start of a new day, the Controller could incorrectly initiate bulk charging throughout the day. So to detect the start of new day, the software monitors how long the solar panel fails to produce power. If it is four hours or more, it is deemed to be a new day, to allow bulk charge to begin. LED5 4148 K D4 TO S1 (EQUALISE) 21120141 Q2 Fig.4: the parts layout on the PCB is exactly the same as in the February 2011 article with the exception of the LK1 & LK2 linking options (see Table 1 below). The links are installed using solder bridges, as described in the text. Efficiency improvements More important than the software refinements are the changes to gain improved efficiency. While the original circuit is still valid, we have now specified a new Mosfet with much lower on-resistance, (RDSon). In addition, if you’re using the Charge Controller with 12V solar panels, you can use new double-Schottky diodes which have much lower forward voltage. With these points in mind, let’s have a look at the revised circuit of Fig.3. It is virtually the same as that published in February 2011 except that we are now employing the RB0 and RB1 inputs at pins 6 & 7 of the PIC16F88 microcontroller (IC1). These are used to select the charger options. Leaving these two inputs unconnected selects the standard option whereby bulk charging begins when the battery voltage drops below 12.45V. Table 1 shows the selections available with either or both LK1 and LK2 connecting the relevant pins to ground. Note that the LK1 connection is made using a solder bridge between siliconchip.com.au This is the view inside the completed unit. Note the extra cable gland at bottom left. This allows a figure-8 cable to be run to the external thermistor which must be mounted next to the battery to obtain accurate readings – see Fig.5. Table 1: Charging Options LK1 LK2 Operation Out Out Bulk charge initiated if battery <12.45V In Out Bulk charge initiated each morning (and when battery falls below 12.45V) Out In Switches from float to bulk charge when power is drawn from the battery or when battery falls below 12.45V) In In Bulk charging each morning and switching from float to bulk charge when current is drawn from the battery or when <12.45V March 2012  93 Parts List For MPPT Solar Charge Controller 1 PCB, code 14102112, 111 x 85mm (original 14102111 can be used – see text); available from SILICON CHIP for $25 + $10 p&p 1 diecast aluminium case, 119 x 94 x 57mm 3 IP65 cable glands for 4-8mm diameter cable 3 2-way PC-mount screw terminal blocks, 5.08mm pin spacing 1 SPST mini rocker switch (S1) 1 waterproof switch cap (optional) 1 2-way PC-mount polarised locking pin header (2.54mm pitch) 1 2-way polarised header socket with 2.54mm pin spacing 2 M205 PC-mount fuse clips 1 M205 10A fuse (F1) 1 NTC thermistor, 100kΩ at 25°C (TH1) 1 DIP18 IC socket 1 iron-powdered toroidal core, 28 x 14 x 11mm 4 TO-220 mounting kits (insulating bushes and silicone washers) 4 M3 x 15mm tapped Nylon spacers 4 M3 x 12mm countersink Nylon screws 4 M3 x 10mm machine screws 4 M3 x 6mm machine screws 4 M3 nuts 1 400mm-length of 1.25mm enamelled copper wire 1 50mm-length of medium-duty hookup wire 1 1m length of light-duty Fig.8 wire 1 25mm length of 6mm-dia. heatshrink tubing 1 50mm length of 2.5mm-dia. heatshrink tubing 5 PC stakes 1 100mm cable tie 1 20kΩ horizontal-mount trimpot (VR1) 1 100kΩ horizontal-mount trimpot (VR2) 1 20kΩ multi-turn top adjust trimpot (VR3) Semiconductors 1 1 PIC16F88-I/P microcontroller programmed with 1410211B.hex (IC1) 1 LM358 dual op amp (IC2) 1 TL499A regulator (REG1) pin 6 and the ground track from the underside of the PCB. Similarly, LK2 is made using a solder bridge between 94  Silicon Chip 1 SUP53PO6-20 60V 53A P-channel Mosfet (Q1) (Element14 Cat. 1684102) 1 TIP31C NPN transistor (Q2) 1 BC337 NPN transistor (Q3) 2 MBR20100CT 10A 100V double Schottky diodes (D1, D2) (24V version) 2 IXYS DSSK 38-0025B dual 25V, 20A Schottky diodes (D1,D2) (12V version only) (Element14 Cat. 1080066) 2 1N4148 switching diode (D3, D4) 2 30V 1W zener diodes (ZD1, ZD3) 1 18V 1W zener diode (ZD2) 3 3mm green LEDs (LEDs1-3) 1 3mm red LED (LED4) 1 3mm orange LED (LED5) Capacitors 3 4700µF low-ESR 16V PC electrolytic 2 2200µF low-ESR 25V PC electrolytic 1 100µF 16V PC electrolytic 1 10µF 35V PC electrolytic 6 100nF MKT polyester 2 10nF MKT polyester 1 470pF ceramic Resistors (0.25W, 1%) 1 100kΩ 1 1.5kΩ 1 68kΩ 3 1kΩ 1 33kΩ 1 470Ω 1W 2 22kΩ 1 330Ω 1 8.2kΩ 3 100Ω 2 4.7kΩ 1 10Ω 3 2.2kΩ 1 0.01Ω 3W resistor (Welwyn OAR3-R010FI) (Element14 Cat. 120 0365) Parts For 24V Operation 3 1000µF low-ESR 35V PC electrolytic capacitors (instead of 3 x 4700µF 16V) 2 470µF low-ESR 63V PC electrolytic capacitors (instead of 2 x 2200µF 25V) 1 51kΩ 0.25W 1% resistor (instead of 22kΩ) 1 47kΩ 0.25W 1% resistor (instead of 22kΩ) 1 1kΩ 0.25W 1% resistor (instead of 100Ω) pin 7 and the ground track. These remarks apply to the revised PCB which is coded 14102112. If you are changing the original PCB, the IC pads themselves need to be bridged to the adjacent ground track that runs beneath IC1 and connects to pin 5. Note that a solder masked PCB will need to have the solder mask scraped off where the solder is to be applied. Note also that before these charger options can be used, the microcontroller (IC1) must be programmed with the revised software 1410211B.hex. New Mosfet Mosfet Q1 is now a SUP53PO6-20 instead of the original IRF9540. As well, for the 12V version, DSSK 380025B double-Schottky diodes can be used in place of the original MBR20100-CT double-Schottky diodes used for D1 and D2. This new Mosfet has an on resistance of 0.0195Ω (just 19.5 milliohms) compared to 0.1Ω for the IRF940. This will bring about a substantial reduction in the heat dissipated in the Mosfet. Under the same conditions when using a 120W solar panel, the SUP53P06-20 should dissipate about 2W compared to 10W for the IRF9540; a reduction of 8W. Additionally, the DSSK 38-0025B diodes have an approximate 66% lower forward voltage compared to the original MBR20100-CT diodes. The respective forward voltages are 0.48V and 0.72V at 25°C and at the typical operating current with a 120W solar panel. At 100°C, the diode drops are 0.4V and 0.62V respectively. Typically, power dissipation for D1 will be 4.82W for the MBR20100-CT and 3.22W for the DSSK 38-0025B diode at a current of 6.7A (the typical current from a 120W solar panel at its maximum power point). For D2, expected power dissipation at 10A for the MBR20100-CT would be 3.6W at a 50% duty cycle and 2.4W for the DSSK 38-0025B. So overall power dissipation in the Mosfet and fast recovery diodes when charging at 6.7A will be around 7.6W instead of more than 18.5W in the original circuit. The DSSK 38-0025B diodes are rated at 25V and are not suitable for use with a 24V solar panel or battery. We do not intend giving a full circuit description in this article. Those readers who want the complete circuit description and constructional details should refer to the original article featured in the February 2011 issue. This siliconchip.com.au Using 24V Batteries & Solar Panels The Solar Charge Controller can also be used with 24V batteries and 24V solar panels. However, this requires some component changes to the circuit and these are indicated on Fig.3. The changes are as follows: (1) The 22kΩ resistor at pin 3 of IC2a is changed to 47kΩ, the 100Ω resistor feeding ZD3 is changed to 1kΩ and the 22kΩ resistor at the AN0 input of IC1 is changed to 51kΩ. (2) The 2200µF 25V low-ESR capacitors are all changed to 470µF 63V low ESR types, while the 4700µF 16V low-ESR capacitors are changed to 1000µF 35V low-ESR types. (3) The number of turns for L1 is increased from seven to 10. Note that the dissipation in Q2 will rise to around 500mW but suitable heatsinking is already provided by the case. Several set-up changes are also required: (1) The voltage at TP1 (set by VR3) must now be the battery voltage x 0.15625 (instead of 0.3125). (2) The voltage at TP2 for temperature compensation must be half that set for 12V operation. For example, for 38mV/°C compensation with a 24V battery, TP2 should read 1.9V (not 3.8V). THERMISTOR COVER EACH CONNECTION WITH 2.5mm DIA HEATSHRINK TUBING COVER THERMISTOR AND CONNECTIONS WITH 6mm DIA HEATSHRINK TUBING Thermistor installation The MPPT Charger case runs quite warm during bulk charging when using the original IRF9540 Mosfet and MBR20100-CT diodes. For this reason, the thermistor should not be simply connected directly to the screw terminals in the box as it will provide a false temperature reading. For correct temperature monitoring, the thermistor should be mounted in contact with the battery being charged and connected to the MPPT Charge Controller using a length of figure-8 wire. This applies even when the revised Mosfet and diodes are used since the case will still get warm. The thermistor is soldered to the ends of a figure-8 cable, with the leads insulated using 2.5mm diameter heatshrink tubing. The thermistor and siliconchip.com.au INDUSTRIES PTY LTD Now manufacturing the original ILP Unirange Toroidal Transformer - In stock from 15VA to 1000VA - Virtually anything made to order! - Transformers and Chokes with Ferrite, Powdered Iron GOSS and Metglas cores - Current & Potential Transformers DYNE Industries Pty Ltd TO MPPT CHARGE CONTROLLER THERMISTOR INPUT TERMINALS FIGURE-8 CABLE Fig.5: to obtain accurate readings, the thermistor must be mounted in contact with the battery being charged and connected to the MPPT Charge Controller using a length of figure-8 wire. can be accessed on our website for a fee or you can obtain the back issue from SILICON CHIP. DYNE Keep Cable Resistance Low When this unit is used with a 120W panel, the charging current to the battery can be as high as 10A. For this reason, the cable resistance between the Charge Controller and the battery should be as low as possible, otherwise voltage losses will affect the changeover from the bulk charge to the absorption stage of charging. To minimise these voltage losses, mount the charger close to the battery and use heavy-duty cables (see the February 2011 article for details on wire gauges for different lengths). sheathed soldered connections are then further overall covered in 6mm diameter heatshrink tubing, as shown in Fig.5. The opposite end of the wire is passed through a cable gland mounted adjacent to the thermistor screw terminals in the box and secured SC into the terminals. Ph: (03) 9720 7233 Fax: (03) 9720 7551 email: sales<at>dyne.com.au web: www.dyne.com.au Silicon Chip Binders REAL VALUE AT $14.95 PLUS P & P H SILICON CHIP logo printed on spine & cover H Buy five and get them postage free! Price: $A14.95 plus $10.00 p&p per order. Available only in Aust. Silicon Chip Publications, PO Box 139, Collaroy Beach 2097. Fax (02) 9939 2648 or phone (02) 9939 3295 & quote your credit card number. March 2012  95