MPPT Charge Controllers
'Should I get a MPPT controller? Are they are more efficient?' We hear these questions a lot. And yes, they are more efficient. On the other hand they often do not live up to their promises of 'up to 40% more' yield.
How Does MPPT Work?
A Maximum Power Point Tracker (MPPT) controller constantly calculates the optimum charging voltage. Sometimes, the voltage coming from the solar array is higher than needed to charge the battery. Unlike conventional regulators, the MPPT then converts some of that excess voltage into current (Amps) for charging.
This process is an excellent technological advancement when compared to other regulators. Then again, there are few situations where the voltage from the solar array is indeed higher than needed.
Assumption: you have a 12 volt battery bank and a 12 volt solar array. A MPPT controller is most effective when the difference between the battery voltage and the solar array is at its greatest:
- Your panel voltage is high, AND
- Your battery state of charge (SOC) is very low.
Re: 1) Solar panels are rated at 25°C cell temperature and a typical nominal 12V panel produces 17.5 volt at maximum power (Vmp) @ 25°C. Quite deliberately, solar panels are usually positioned in full sun which increases cell temperature significantly and therefore reduces the panel voltage (the hotter the cell, the lower the voltage). So unless you live in a place with icy winds (like the South Pole), the panel voltage will hardly ever be equal to 17.5Vmp.
Re: 2) While solar batteries or deep cycle batteries have the potential to be deeply discharged, you are better off not to. The more often you deeply discharge your battery the shorter its lifespan. It is therefore recommended to charge up your batteries as soon as possible to avoid your SOC to ever be very low.
When is a MPPT controller effective?
MPPT controllers are always slightly more efficient than their PWM counterparts. However, the difference in cost is more than slight. In most situations we therefore recommend that you add an additional solar panel than worry about a few potentially lost Amps.
Nevertheless, some MPPT regulators offer one significant advantage over conventional regulators: the ability to accept a higher input voltage than the battery bank (e.g. 60V panel array on 12V battery bank). While this is not the most efficient solar system design it may be feasible for situations where the solar array is a long way from the batteries. The cable from the solar array could be a lot thinner (and therefore cheaper).
MPPT regulators have come down in cost significantly and are now used in nearly all of RPC's off-grid solar installations.