Originally Posted by cschlessman
One thing I keep running into is folks who don't like MPPT. I always struggle to explain the advantages but know they are many. I ran into this article which articulates the plus side of MPPT charge controllers. It si you choice as to using them or not, currently the only draw back is cost. But with increase charge that is usable to the batteries I feel its an acceptable cost. Read and enjoy.
Cost is not the only disadvantage, there are also conversion losses, but they can be offset by the greater power applied when the batteries are low.
The advantages in the article in the link about higher voltages at lower current and less "IR" losses in the wire don't really apply on a Skoolie with an average of 1 to 4 PV panels. In big array with 8 to 24 panels, wiring them in series makes a big difference. A Skoolie should have a proper installation with either MPPT or direct connect: short, heavy wires from the panels to the controller to the batteries.
HandyBob preaches only drawing down a small percentage of the battery capacity, and eliminating "IR" losses in the wires from the direct-connect PV by keeping them short and oversized. This works for him. But if you actually deep-discharge your battery bank, an MPPT charger will give you more charging output.
The power output of a photovoltaic panel is rated on a curve, from maximum current when shorted out to zero voltage, to maximum voltage at no current. The maximum power point is less than maximum current and slightly less than maximum voltage.
Let's take a theoretical 180-watt panel for 12 volts. The short-circuit current might be 12 amps at zero volts, which would be flat-line consistent at lower loadings and might begin to taper off at say 14.5 volts. The open-circuit voltage might be 22 volts at zero amps. And the rated output might be 10 amps loaded down to 18 volts, for the 180-watt specification. (These are round numbers to make the math easier, not an actual panel spec.)
If the batteries are run down to "zero capacity," which is specified as 10.5 volts, the direct-connect charging amperage in full sunlight is the 12 amps, and the wattage is 126 watts. Assuming the MPPT converter has a 5% loss, the MPPT charge with the panels running at 18 volts would be 180 watts less 5%, or 171 watts, giving 16.3 amps charging current at 10.5 volts. This is a 35.7% increase in charging current to fully discharged batteries.
(If you over-discharged your batteries to 6 volts, the direct-connect still provides 12 amps but only 72 watts, the MPPT would put out about 28.5 amps.)
When the batteries come up to 11.5 volts, the direct-connect's 12 amps is now providing 138 watts of charging. The MPPT is still 171 watts, but now that provides less than 15 amps. At 12.5 volts, the 12 amps of direct connect is now 150 watts, and the advantage of MPPT has dropped to 14%. At about 14.25 volts, there is now zero MPPT advantage, as the 12 amps (which is beginning to taper off) makes the same 171 watts as our MPPT example with the 5% conversion loss. Any battery charging voltage above that and the MPPT conversion losses actually cost available charging power.
The bottom line is that large arrays with panels in series can be efficient with MPPT as per the article. With small arrays, batteries that are never deeply discharged may not see any advantage due to conversion losses. But batteries that are actually deep-cycled will see probably 30% more charging without adding more panels when using MPPT conversion. With direct-connect, the more you need a charge, the less output power you will receive from your loaded-down panels.