Quote:
Originally Posted by hellosailor
All conventional regulators are "dump" regulators. That is, any time the panels are putting out more than 14.4 volts (the exact voltage will vary) the panels literally DUMP the extra voltage. Either into a dump load, or burning it off as heat. Thrown out, goodbye, wasted.
An MPPT controller is a specialized type of PWM controller. It does not put out pure DC, the way a dump controller does. It puts out pulsed DC, like PWM controllers do. And actually, pulsed DC can be 10% more efficient than plain DC at charging the batteries, because it does not boil the electrolyte, does not cause constant microbubbles, does not drive up the internal resistance and actually slow down charging the way excess pure DC does.
But MPPT goes one step further. Since pulsed DC is basically a kind of AC power, the MPPT controller can adjust the pulses, put them through a transformer and capacitor, and instead of throwing out the excess voltage, they convert it ALL into exactly the right amount of voltage for the battery AND extra amperage. Think of it as an instant recycling program for that "wasted food" that the others throw out.
As a result the MPPT controllers gain about 10-15% over the PWM controllers, and over the plain dump controllers.
Some MPPT controllers actually do not use a "three stage" charge logic. Instead of doing the first bulk charge at 14.4 volts, they will look at your battery (they have to be programmed for it) and supply just 1/2 volt more than what your battery is actually at. All the rest of the power gets turned into amperage. As the battery voltage rises, every minute or so they readjust and again, supply just 1/2 volt more at the maximum amperage. This actually charges the batteries faster than plan "bulk, absorption, float" logic does.
So yes, MPPT (beware the counterfeits online) should gain you 10-15%, perhaps a little more. It may be cheaper to add another panel--if you've got room for it. It also means that you can recharge 10-15% faster, if there are gray skies or nightfall coming in.
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Well, that is a very unscientific explanation. The truth is something different.
First of all, a
solar panel can be considered a
current source, not a voltage source, that said with rising radiation the
current rises somehow proportionally while the voltage stays almost constant. The inner resistance of a battery is usually much lower then the inner resistance of a solar panel or even a grid of panels. So if you connect a battery, that is not full yet, to a solar panel directly, the voltage of the panel will drop to the voltage of the battery and the panel will yeld all current that is produced at the moment with this voltage in the voltage/ current diagram. If the Vmp is relatively close to the Absorption voltage of the battery chosen (+2...3V), then the panel operates almost at its peak capabilities and there is no energy loss at all. You can easily shortcut a solar panel and measure the Isc, the current is always between 0A at Voc (open circuit) and Isc at V=0V (short cut), the power output is V*I between this two ends and has a maximum at Vmp * Imp.
That is exactly what PWM controller do. They connect the panels directly to the battery and yeld the maximum amps of the panel at a given battery voltage. Due to heat, panel voltage and current degrades, that is the reason for having 2-3V higher Vmp on the panels. A PWM controller would beat in normal conditions (sun shine, hot weather) with the right panels chosen any MPPT.
The pulsing starts at the point, when the battery voltage climbs over the Absorption set point. Then the PWM controller turns the solar off and on to reduce the current output. A MPPT controller would also start to waste energy at that point, otherwise the battery would start boiling.
So why pay more for an MPPT then?
First of all, you can use any panel with an MPPT, as long as the voltage is much higher than the battery voltage, then the solar current to the controller will be much lower and you can use smaller cabling, additionally you may have advantages in partial shading, where the bypass diodes short shaded strings and use the other cells with full amps on a lower voltage set point. This is something an MPPT can handle well, itsearches always for the maximum possible power output between battery voltage and Voc of the solar strings connected.
This comes with a tradeoff of course. The MPPT controller must transform incomming variable voltages and currents to the voltage of the battery and there are some losses by heat during the transformation ( reason for the heat sinks ). This energy is really burned.
However, MPPT controller have advangages in cloudy conditions and shading and outperform in the long run the PWM controller by 5...15%, but also cost at least double as the PWM.