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Build A 12/24V 3-Stage Solar Charge Controller

Are you building the ultrasonic anti-fouling unit for your boat? You will need a solar panel and a charge controller to keep the batteries topped up. Or are you thinking of a large solar panel for your caravan or 4-wheel drive? Again, you will need a solar charge controller. This is the one to build.

by John Clarke

This charge controller is suitable for 12V panels up to 120W and 24V panels up to 240W. It incorporates Maximum Power Point Tracking (MPPT) and 3-stage battery charging. It works with any 12V panel from 40W up to 120W (3.3-10A) and can also be used with 24V panels in the 80W to 240W range, in conjunction with a 24V battery.

Wouldn’t it be nice if you could just wire a solar panel (or panels) to a battery or two and leave it at that? Unfortunately, for all but the smallest panels, this is a very bad idea. The battery will be overcharged on sunny days and on cloudy days the battery may not charge at all, even though the panel is capable of harvesting energy. So there is no choice – you need a charge controller.

This Charge Controller is suitable for charging Flooded Lead Acid, Gel-Cell (Sealed Lead Acid or SLA) and AGM (Absorbed Glass Mat) type batteries. Ideally, any battery used in a solar system should be a “deep discharge” type. Car batteries are not deep discharge types and are not suitable.

Ultrasonic anti-fouling for boats

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Fig.1: The current/voltage curve for a typical 120W solar panel. Maximum current, with the output shorted, is Isc and maximum voltage, with the output open circuit, is Voc. For best efficiency, the panel is operated at its maximum power point.

We have already mentioned the Ultrasonic Anti-fouling unit for boats (SILICON CHIP, September & November 2010). This must run continuously to protect the boat hull from marine growth and for those without shore power, a solar panel and charge controller is the only solution. For this application we recommend, at minimum, a 12V 40W panel with a 12V 12Ah SLA battery.

For continuous anti-fouling, the circuit draws an average of about 200mA. Over a 24-hour period this amounts to 4.8Ah or 60Wh per day from the 12V battery. This means that if a 40W panel generates full power for 1.5 hours or longer each day, this is enough for the anti-fouling unit to operate. However, if you are also concerned about automatic operation of bilge pumps etc, a 40W panel would be a good choice. The reason we have specified a larger panel and battery than strictly necessary is twofold.

First, for a boat installation, you cannot orient the panel for best efficiency. If you are on a swing mooring, the boat’s heading will constantly change according to wind direction and even if it didn’t, you would still install the panel to result in minimum windage and this means that it must be installed horizontally. The same comment generally applies to a caravan installation. Second, you need a bigger panel to cope with sustained periods of bad weather when there is little sun.

In Australia, we receive a yearly average of five peak sun hours per day. Seasonal monthly breakdowns are available at http://www.yourhome.gov.au/technical/fs67.html#siting

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Fig.2: this block diagram shows how the microcontroller (IC1) monitors the battery and panel voltages and the current. It also shows how the switchmode step-down circuit for battery charging is arranged. When Q1 is on, current (i1) flows through inductor L1 and into capacitor C2 and the battery. When Q1 switches off, the stored energy in L1 is fed to the battery via diode D2 (current path i2).

MPPT & charge optimisation

Given that the solar panel is mounted horizontally, it is most important to collect as much energy as possible from it and this is where the Charge Controller’s MPPT (Maximum Power Point Tracking) comes in.

As shown in Fig.1, for a typical solar panel exposed to full sunlight, the output ranges from maximum current when the output is shorted (Isc) to maximum voltage when the output is open circuit (Voc). For a typical 120W 12V panel, Isc is 7.14A and Voc is 21.8V. But the maximum power from a 120W panel is at 6.74A and 17.8V which is hardly a suitable match for a lead-acid battery.

If we were to connect that 120W solar panel directly to the battery, the charge current would be about 7.1A at 12V (85.2W), 7.05A at 13V (91.7W) and 7A at 14.4V (101W), ie, much less than the 120W available from the solar panel at 17.8V.

By contrast, MPPT keeps the solar panel current and voltage at the maximum power point while charging the battery, even though the battery voltage is lower than the solar panel voltage.

This is achieved by an intelligent switchmode step-down voltage converter. To see how this works, refer to the block diagram of Fig.2 below. Current from the solar panel flows through diode D1 and Mosfet Q1. When Q1 is on, current (i1) flows through inductor L1 into capacitor C2 and the battery. This stores energy in the inductor’s magnetic field.

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