The magic word is MPPT ok four words (Maximum Power Point Tracking). An small explaination of the MPPT principle is nessecary.
Small warning !
First of all on Ebay a lot of vendors seem to use MPPT as a sales agrument without their controller actually having a true MPPT function. If a change controller seems to good to be true price-wise, you might have a regular PWM (Pulse Wide Modulation) charger.
How to recognize ?
The maximum PV-input voltage is usually a lot higher that the working voltage on the controller, this defines the MPPT range. On 12 or 24Vdc installations, this can be up to 100Vdc, on the higher 48V installations even upto a 150Vdc. Prices vary from 100 to 300+ dollars depending on maximum power.
To get the maximum power from your solar panels into your batteries a MPPT charge controller is the best solution. A bit more money but if the solar setup gets bigger it really makes a difference.
When we look at a solar PV panel specifications a few things are mentioned:
- Wp = Rated Power
- Vmp = Rated Voltage
- Imp = Rated Current
- Voc = Voltage at Open Connection
- Isc = Current at Short Circuit
The Wp (Rated output at maximum point) = Vmp x Imp, so you’ll see on a 100Wp panel 17.3V x 5.78A (as example). Now the battery is only 12Volt. In reality a fully charged battery can be 13.8V and rise to 14.4V during float charging.
Lets start with a regular solar charge controller.
The voltage that comes from the panel is simply cut-off by the charge controller on the battery voltage. The maximum current stays the same. For example your battery is a bit discharged and holds about 13V. Power from the panel that actually goes into the battery therefore is 13V x 5.78A = 75.1W. When the battery is really drained at 11.5V it gets worse just 66.5W from the 100Wp panel goes into the battery.
What about the MPPT solar charge controller.
That controller is an DC-DC converter with a variable rate. So it takes the 100Watts from the panel and converts it to the battery voltage. If the battery holds the 13V from before it will convert 17.3V x 5.78A into 13V x 7.69A and supply the full 100Watts from the panel by using a higher current. The 11.5V drained battery gets even 8.69A (11.5 x 8.69 = 100) and no power will be lost. Of course this is all theory but MPPT controllers have upto 98% efficiency.
Higher voltage !
One more neat trick of MPPT controllers is an input voltage range that is much higher than regular controllers. Not on every controller out there so read the specs before you buy. Some controllers (for example) can take upto 150V PV input voltage.
To get more power we use 2 panels of the forementioned 100Wp. These can be connected in series that doubles the voltage and keep the same current (34.6V x 5.78A) or connect parallel to each other, that doubles the current and leaves the voltage in tact (17.3V x 11.56A). Look for more on this at our PV-panel page.
On a regular controller they would be connected in parallel since we already found that extra voltage generated is cut off at the battery voltage by the charge controller. On the MPPT controller it doesn’t really matter, the output will be the full 200Watts either way, simply because of the MPPT-logic.
But connecting the panels in series has one more advantage on a MPPT controller. If early in the morning or on a bit of a clouded day the outut voltage of the panels drops below battery voltage lets say 11 Volts, than the regular controller stopt charging. Now the two panels in series on the MPPT controller would still output a total of 22 Volts and keeps charging, win-win.