Clarification regarding misconception of the purpose and connection of a MPPT

An MPPT allows you to take advantage of the mathematics behind power conversion and allows maximum power delivery from the panel.                                                        

Let’s look at a typical solar panel (the values will be adjusted to make reading/maths easier).

• Open circuit voltage = 22 Volt (Voc)
• Power point voltage = 17 Volt (Vmp)
• Short Circuit current = 10.1 Amp (Icc)
• Max Charge current = 10.0 Amp (Imp)
• Max Power = 170 Watt

What do these values mean?

Open circuit voltage: If the panel is exposed to sunlight while the panel is not connected to anything the voltage will be 22 Volt.

Power point voltage: If we connect the panel to an MPPT the panel will be kept around 17 Volt (depending on the temperature of the panel, angle of sun…) as long as the MPPT has a flat battery to charge. This is the point at which panel voltage multiplied by panel current will equal max wattage (not max amps or max voltage but the combination of the two).

Short Circuit current: If the panel + and panel – touch this current will flow. The panel will be at 10 Amp when the panel voltage is at 17 Volt, if the panel voltage is forced lower the current will increase as the voltage decreases, when the panel voltage reaches 0 Volt, the current will be equal to 10.1 Amp.

Max Charge current: As long as the sun gives you 1000 Watt/m² and the panel is kept at 25 °C (P.S. panel temp rises when current is flowing) and the panel voltage is kept at Vmp then the Imp will = 10 Amp.

Max Power: Vmp x Imp = 170 Watt

What Happens When You Charge a Battery.

Remember that power cannot be created out of nothing and cannot be destroyed so input = output so Vbat x Ibat must = Vpan x Ipan.

With the MPPT the panel voltage is controlled independently from battery voltage and the panel voltage is kept at Vmp allowing Imp to flow, with PWM the panel is connected to the battery while charging, forcing Vpan to equal Vbat, a current slightly higher than Imp will flow, because the panel is directly connected to the battery the current must.

So here you can see that the PWM can give you as low as 100 Watt from a 170 Watt panel under full sunlight condition and a max of about 140 Watt in ideal conditions (battery Volt, temp, radiation …).

The MPPT will always give you a 170 Watt in ideal conditions (temp, radiation …) no matter what the battery voltage is.

So now that you know the first and foremost importance of an MPPT we can discuss some added advantages.

It is true that an MPPT helps to improve system efficiency by allowing a higher panel voltage to be used, because power stays the same and power = V x I it means that the panel array current will decrease, resulting in less voltage drop in the cable, resulting in less power loss in the cable. What is important here is that this is only true if you keep the cable / conductor diameter the same when or as you increase the voltage, if you use thinner cable because the current is less than you’re only saving on system installation cost and not gaining any efficiency. This gain in efficiency is normally almost insignificant when compared to the 1st most important reason for using an MPPT gain of up to 70% increase in power (normally around 30% because battery voltage does not always stay at 10 Volt).

So my point here is, do not use an MPPT to increase panel voltage, increase panel voltage to make your installation easier and more economical but what is very important to note is that there is a limit to how much you can increase the panel voltage on the MPPT.

Why is this so?

An MPPT (all step-down MPPT battery chargers) uses a Buck regulator circuit to do the power conversion, these circuits do not operate at max efficiency when the input to output voltage ratio is very high, try to not exceed a ratio of 1:4. This is especially true on 12 Volt systems. For examples is you charge a 12 Volt battery do not connect 120 Volt of panels to the MPPT, the ratio = 1:10 the MPPT will give you a “High Panel voltage” Warning meaning that the MPPT is now operated out of spec. For a 12 Volt system your max Charge is 15 Volt so use a panel array with a power point voltage (Vmp not Voc) between 15 Volt and 60 Volt (1:4 = 15V x 4 = 60V), then your MPPT will be happy and you will get maximum power transfer efficiency from your MPPT. In essence your MPPT has 2 paths where the power flow, 1 path steps down and divides the voltage and the other increases and multiplies the current. If your ratio is 1:10 then each circuit has to work 10 times harder than what it would have worked if the ratio was 1:1, having too dived the voltage by 10 and boosting the current 10 fold (turn 1 Amp from the panel into 10 Amp being charged into the battery), the difference in efficiency between 1:1 and 1:4 is minuscule and not worth stressing about but when the ratio becomes greater than 1:6 then the efficiency is notably lower.

Which solar charge controller: PWM or MPPT?

1. What they do

The PWM controller is in essence a switch that connects a solar array to a battery. The result is that the voltage of the array will be pulled down to near that of the battery. The MPPT controller is more sophisticated as it will adjust its input voltage to harvest the maximum power from the solar array and then transform this power to supply the varying voltage requirement, of the battery plus load. Thus, it essentially decouples the array and battery voltages so that there can be, for example, a 12V battery on one side of the MPPT charge controller and a large number of cells wired in series to produce 36V on the other

2. The resultant twin strengths of an MPPT controller

a) Maximum Power Point Tracking The MPPT controller will harvest more power from the solar array. The performance advantage is substantial (10% to 40%) when the solar cell temperature is low (below 45°C), or very high (above 75°C),or when irradiance is very low. At high temperature or low irradiance the output voltage of the array will drop dramatically. More cells must then be connected in series to make sure that the output voltage of the array exceeds battery voltage by a comfortable margin.

b) Lower cabling cost and/or lower cabling losses Ohm’s law tells us that losses due to cable resistance are Pc (Watt) = Rc x I², where Rc is the resistance of the cable. What this formula shows is that for a given cable loss, cable cross sectional area can be reduced by a factor of four when doubling the array voltage.

In the case of a given nominal power, more cells in series will increase the output voltage and reduce the output current of the array (P = V x I, thus, if P doesn’t change, then I must decrease when V increases).

As array size increases, cable length will increase. The option to wire more panels in series and thereby decrease the cable cross sectional area with a resultant drop in cost, is a compelling reason to install an MPPT controller as soon as the array power exceeds a few hundred Watts (12 V battery), or several 100s of Watts (24 V or 48 V battery).

3. Conclusion

PWM

The PWM charge controller is a good low cost solution for small systems only, when solar cell temperature is moderate to high (between 45°C and 75°C).

MPPT

To fully exploit the potential of the MPPT controller, the array voltage should be substantially higher than the battery voltage. The MPPT controller is the solution of choice for higher power systems (because of the lowest overall system cost due to smaller cable cross sectional areas). The MPPT controller will also harvest substantially more power when the solar cell temperature is low (below 45°C), or very high (above 75°C), or when irradiance is very low.