MPPT Solar Panel Controller Tutorial

MPPT stands for Maximum Power Point Tracking. A MPPT solar panel controller converts the power from the solar panel to maximize the current entering the battery or batteries. As this "Power Point" changes continuously, the controller must be intelligent to "Track" the maximum power point.

Anyone who has solar panels knows that they never get the watts specified for the panel. The specifications for the panel are under ideal test conditions which we never get on our boats, so we just accept the losses.
Although many losses are due to non ideal conditions, significant losses can be due to the mismatch between the panels and batteries that can cost 20% to 40% loss. With MPPT controllers we can minimize these losses. In a strange way MPPT controllers work best when sunlight is poor and batteries are low, exactly the conditions where they are most needed. We will see why below.

So why is there a mismatch between panels and batteries and what causes the losses to vary?

Although the panels are rated for 12V systems, their actual voltage is much higher, from 16V to 36V. The power rating is the product of the voltage and current. For example 10 amps at 18 volts = 180 Watts.
Batteries however have a voltage of 10.5 volts when discharged, to 12.8 volts when charged.
So in the worst case, charging low batteries when maximum power is required, the panel provides 10.5 x 10 = 105 watts  of charge from a 180 watt panel. At 12 volts, when the battery is reaching towards full charge, the panel produces 120 watts, still a long way from 180 watts.

So why are 12 volt nominal panels designed to give a much higher voltage resulting in the mismatch?

Well, the panels are rated for 180 watts at full sunlight at a particular temperature. Normally around 72 degrees F. Full sunlight can also generate significant heat, especially in summer in warm climates, with panels that happen to be black, which absorbs heat. As the temperature goes up, the panel voltage drops, a great reason for keeping panels cool. The panel voltage can drop by a few volts due to temperature. If the panel voltage drops below the charging voltage of about 14 volts, efficiency goes to 0, yes nothing, in full sunlight! So leeway that you may never need is built into the panels.

Now some of you may be wondering why batteries need about 14 volts to charge, yet in the calculations above, I used 10.5 to 12 volts for power entering the battery. Simply, the power absorbed by the battery is based on the battery voltage (10.5 to 12.8) while the 14 volts is required to "push" the current in. Technically the internal resistance of the battery accounts for the difference.

So how does a MPPT charger work?

The MPPT charger takes the panel voltage and converts it to the optimum voltage to push the maximum current into the battery. So although the panel in the example above is rated at 10 amps, a good MPPT controller will be pushing more than 10 amps into the battery. You can get 20% to 35% power gain in winter and 10-15% in summer.
Your gain increases with the difference between panel voltage and battery voltage. So once again, keep the panels cool by sailing in cool weather with bright sunlight, rather difficult to arrange, or ensuring good ventilation around the panels.

When do you get the most benefit from a MPPT controller?

When  batteries are discharged and when cool temperatures and hazy days keep your panels cool.
In cold sunny climates, you can actually get more power than specified for your panels, as the voltage will go up if the temperature is less than the panel's specified temperature. Of course this increase is theoretical unless you have a MPPT controller.

So what's inside a MPPT controller?

A MPPT controller has a microprocessor that takes measurements continuously to control electronics so as to maximize the charging current. It reads battery temperature and electrical characteristics and uses them as control parameters. The panel voltage is transformed to the optimum battery charging voltage by converting the solar panel  DC voltage to high frequency AC, which is then transformed in a high frequency transformer and converted back to DC which,  is fed to the batteries. This conversion process has losses, but they are acceptable when considering the savings.

Why is a trickle charger included in the 250W MPPT controller?

The trickle charger is to keep the engine starting battery charged. Typically the MPPT controller charges the main battery bank, while the engine battery is charged by the alternator. If the engine is not used, the starting battery will discharge. The trickle charger is useful especially when the boat's electrical system does not have a way of feeding charge from the main bank to the starting battery. Boats having 500 watts of solar panels typically have such a controller for the starting battery, so this feature is only included in the 250W controller.

What is a low voltage disconnect?

The 250W MPPT controller has a low voltage disconnect feature. This feature switches off power to a circuit if the battery voltage falls below 12V. This protects batteries from total discharge if loads are inadvertantly left on. This circuit should NOT be used to drive an invertor and obviously the bilge pump and burglar alarm circuits must not be included in this load. The controlled circuit must have a maximum of 15 amps load.