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12/24V MPPT Solar Charge Controller While we know that our revised MPPT Solar Charge Controller will be welcomed by many readers who had requested improvements, there are bound to be other questions, like “Can we now push the revised design to handle more solar panels?” The answers could be yes, no and maybe, as demonstrated by this selection of recent questions from readers. . . 96  Silicon Chip Q I intend to build the MPPT regulator to try this technology out at one of my radio repeater sites.     All my sites operate at 12V but it seems there could be a further efficiency gain by using an MPPT regulator with a 24V panel. The original article outlines some component changes to swap the whole system from 12V to 24V so it seems simple to leave the battery side components for a 12V battery but change the solar side components for 24V. This leaves the inductor which may require a higher value of inductance to operate in 24V to 12V mode. Is this possible or is the inductor likely to saturate if used in this way? A It is unlikely that higher efficiency would be obtained using a 24V solar panel with a 12V battery. The high efficiency of MPPT is achieved because the solar panels are operated at their maximum power point for charging and because the panels are at a higher voltage than the battery voltage. A 24V panel would be running at about 36V when delivering its maximum power and this would need to be stepped down to between 12 and 14.4V when charging the battery. Compared to the typical step-down from 18V when operating at the maximum power point for a 12V panel, we would expect that the charger will have greater losses when operating with a 24V panel. The charger would certainly work with a 24V panel and 12V battery although L1 will need to be larger at 20uH with 14 turns on the former. This core should still be suitable for use without any danger of saturation. Overall, we would not recommend purchasing a 24V solar panel on the off-chance that it will deliver any better overall efficiency than would a 12V panel with the MPPT charger. On the other hand, if you already happen to have a 24V panel then there is no reason why you cannot press it into service in the way you suggest. Q I tried improving your solar MPPT project to increase the current output it can handle     from a bank of 24V solar panels up to 25A. I removed the existing driver and P-channel Mosfet and substituted a Toshiba TLP350 driver with an isolated drive power supply plus a high-power N-channel Mosfet. However, I am not getting a proper output from the current amplifier and it is varying around an 18V average with large spikes during to switching. Could you help me fix this issue and suggest a suitable current amplifier stage. I am using a 75mV 50A shunt for the current sensing. A The MPPT charger was not designed for 25A. The input and output low ESR capacitors, diodes, the PCB tracks, inductor L1 and fuse etc are simply not rated for this current which is about double that of the original design. You have increased the Mosfet and shunt ratings but left everything else unchanged. We are also not aware of how you have arranged the TLP350 Mosfet driver which is an opto-isolated device but its supply would need to be isolated (which you apparently have done) and 18V above the source of the Mosfet, ie, 18V above the battery voltage. The shunt you are using is 1.5milliohms (75mV/50A) and so with 18V at the output of IC2b and its amplifying by about 45, the shunt must have have 400mV across it and so 266A is flowing. Clearly something is wrong there and it may be due to the shunt being higher in resistance than it should be. If you have used interconnecting leads between the shunt and PCB then the lead resistances may account for a higher shunt resistance; that would cause the higher shunt voltage. As we have indicated elsewhere in these pages, a higher power MPPT Charger Controller will require a complete re-design, not just a few component changes. siliconchip.com.au Q&A Q I built the MPPT Charge Controller and I notice that the Absorption and Float voltages are     quite different from what is expected, even when all the voltages are set with calibrated meters. The NTC thermistor has about 2.2V at a temperature is 25 to 30°C. Instead of setting the volts divider ratio near 0.3125 for the PIC ADC input, it seems to work correctly with a ratio of 0.32. Any comments? A The battery charge voltages are for a temperature of 20° C. Initially set VR2 for 0V so that the temperature compensation has no effect. A temperature of 30°C will change the battery charging voltage by 190mV due to your 1.9V setting and the 10° C rise above 20°C. So the MPPT charger is probably working correctly with the 0.3125 divider. Another possible cause for the discrepancy is the 5V supply which must be at 5.00V, set by VR1, for correct absorption and float voltage calibration. Note also that the compensation setting for VR2 will affect the absorption and float voltages with temperature above or below 20°C. When VR2 is set for 0V, the temperature has no effect on the absorption and float voltage. siliconchip.com.au Q I am building a 60ft high antenna tower to provide a better 3G modem signal. The tower     will be some distance from the house and no mains power is available. The tower electronics requires 12V at 1.4A and to cover days without sun, a 120Ah battery is required. Your 3-stage MPPT Solar Charge Controller will be used to charge the battery, but I need to use larger solar panels, ie, 12V 180W. Can this be achieved simply by changing D1, D2 and Q1? A Use the DSSK 38-00258 diodes and SUP53P06-20 Mosfet of the revised MPPT Charge Controller when using a 180W panel. The inductor core should be OK but the windings should comprise two lengths of 1.25mm wire wound together rather than just using one 1.25mm wire. You should also thicken the PCB tracks for the current carrying sections with a layer of solder. The low ESR capacitors should be increased, with the 2 x 2200µF 25V changed to 3 x 2200µF 25V and the 3 x 4700µF 16V to 5 x 4700 16V. The fuse rating needs to be increased to 15A. The 0.01Ω 3W shunt resistance for current measurement will be suitable but the output from IC2b should be reduced to avoid voltage exceeding 5V at pin 2 of IC1 with full current from the panel. This can be achieved by placing a 4.7kΩ resistor between pins 2 and 5 of IC1, forming a divider with the 2.2kΩ resistor from IC2b’s output. Q I would like to use the MPPT Charge Controller to upgrade my present solar power installation.     I have 180W of panels on the roof (3 x 60W) and at times I see up to around 14A at peak sun conditions on cool days. I am adding another single 120W panel and wonder if the unit can be upgraded so that I can run all four panels in parallel through the controller. If not, what sort of issues would you see arising if I used two of these MPPT chargers feeding into the common battery bank, with a single 120W panel on one and the other looking after the three 60W panels? Even at the higher price it still works out cheaper buying and building up two commercial units that are available with around 10A capacity but not adjustable in terms of voltage settings and do not have the neat features designed into your unit. From my experience with solar setups I cannot see any real problem operating two units in parallel but I have not yet tinkered with an MPPT type so am unsure if there are any traps for the unwary. A While the 120W MPPT Charge Controller can be upgraded to 180W (see previous answer) it is not practical to go much above this – certainly not to a total of 300W (as you propose) without a lot of re-designing. For example, the low ESR capacitors would need to be increased to suit the higher current and the P channel Mosfet would be need to be paralleled with added gate drive circuitry. Similarly, the two diodes (D1 and D2) would need to be be changed for higher rated types and the PCB tracks thickened along with using a 15A fuse. The inductor would require less turns, from 7 to 6. However, paralleling two chargers should not cause problems. During charging both Charge Controllers will be delivering power to the battery and will be using MPPT with the solar panels. This is provided that the battery voltage falls below the voltage that each charger needs to begin the bulk charge. This normally happens since the batteries tend to be discharged at night. When full charge is reached the charger with the highest cut off voltage will be the one that continues charging and this will deliver the absorption charge. Similarly during float, the charger with the highest float voltage will dominate the float charging process. This is not important since the absorption and float tend to be at lower current than bulk charging. SC March arch 2012  97