I am interested in building my own MPPT charge controller, but need quite a bit of help understanding each part of an MPPT charge controller.
The first part is solar panels in series creates higher than battery voltages. From what I have been reading about MPPT chargers is there is a DC to DC converter that steps down this voltage to a certain voltage.
Does the output voltage of the DC to DC converter ever change unless the input voltage falls below the output voltage?
What is the difference between using PWM to lower voltage to a predefined state and using a DC to DC converter?
- With PWM, you can change the "On time" of the pulse (Period) which in turn increase / decrease amperage, then you can change the distance between pulses (Duty Cycle) to adjust voltage correct?
The second part is the charging the batteries. I am not going to go into this until I understand the first part. Most of the time, if I ask 5 questions, only 1 will get answered and then the rest get lost in all the posts and then never answered....
All help is greatly appreciated!
A max power point charge controller works by using a DC-to-DC converter to accept power from the solar panels (or other power source) at a higher voltage than the output load and trades away extra voltage for increased current in the down conversion. This lets you run the power source at a voltage where it produces substantially more power than it would if its output voltage were held to the charging voltage of your battery pack. The extra power from running the power source this way is far more than the losses from the DC-to-DC converter, so you get a substantial improvement. (In some cases a controller is designed so the power source voltage is several times the battery voltage: As long as you're down converting anyhow, why not reduce your wiring losses with the same mechanism? B-) )
A solar panel's open-circuit voltage is the number of cells in series times the bandgap (which decreases with higher temperature). Its short circuit output current is proportional to the light intensity (photons create free electrons). It output voltage is the open circuit voltage minus resistive losses from current through the wiring - mainly the metalization on the cells (which increases slightly with temperature), and also minus the voltage drop of any series protective diode, as long as the current doesn't approach the short-circuit current. (If the load tries to pull more, the voltage will drop percipitously until the short circuit current is not exceeded.)
Panels are designed with enough cells that the open circuit is substantially higher than the load battery they're expected to drive. This is so they will still drive essentially full current into the batteries through the wiring resistance, even if the bandgap voltage is degraded by high panel temperature. That means there's usually considerable additional power available for an MPPT controller to harvest by running the panel output at a higher voltage.
The output voltage of an MPPT controller is controlled by the charging voltage of the batteries. There are no options (except for things like limiting the charging rate to prevent battery damage, cutting off charging when they're full, or occasionally continuing to charge them past full, to drive an equalizing charge into them.)
MPPT controllers for permanent magnet wind generators have a complex job to do: A wind generator's generated voltage (equivalent to a panel's open circuit voltage) varies with RPM. The current drawn puts a torque load on the shaft, which lowers the RPM, but gradually, so the response is not immediate. Meanwhile the torque from the blades varies with wind speed, RPM, and yaw direction, and the relationship is not simple, because it's' due to the complex aerodynamics of the blades. The alternators also have a maximum current above which they will burn out - but must be kept under load (and slowed down into stall by deliberate overload) so they don't overspeed and tear themselves up.
A typical MPPT controller for a windmill cycles the output load voltage slightly, slower than the mill's reaction time, sees which side of the cycle (higher or lower than setpoint voltage) produces more power, and walks the load voltage in that direction. But it does it within the constraints of: Limit the battery voltage / amount of charging, limit the current from the mill by overloading it to slow it down (yes that momentarily RAISES the current).
For a solar panel array as the generation, the problem is simpler, because the response is immediate. (Also: The panels can be safely shut down by shorting them. This dissipates the power they would have generated as heat in the panels - but that's what happens if you leave them sitting out in the sun unconnected, too, so it doesn't heat them any hotter and thus damage them.)
Unfortunately, though the solution for doing an MPPT controler for solar is simpler than wind, I don't know exactly what it IS. B-b