PWM and MPPT are 2 main ways for solar DC charge to the battery.
 

PWM, short for Pulse Width Modulation, is a technology to use a certain part of voltage from the PV modules, to charge according to the battery actual voltage. For example, if we use a 18V solar panel, to charge a 12V battery bank by a PWM controller, and supposed the battery now is on voltage of 12.8V, then PWM controller would take 12.8V from the panel to charge, but this voltage would change when the battery voltage changes, so we may also lose the part of PV voltage above the battery value. That’s why we always say PWM is lower in charge efficiency, and commonly we may have a rate of 70-80% as the PWM charge efficiency. Please notice that if you use 30 or 36V PV module to charge the 12V battery bank, then most of the PV voltages would be wasted because normally no higher than 16V PV voltage would be used for PWM charge on 12V battery system.

MPPT, short for Maximum Power Point Track, is a technology to track and use most of the PV power to charge the battery. There are several kinds of MPPT tracking methods, but all are leading to make full use of solar power. The difference is about the adjustment of the MPPT technology to different applications. As you can see, MPPT is to try to make full use of PV power, and this is the biggest advantage over the PWM. Considering of the unavoidable power loss, the MPPT charge efficiency is supposed to be over 95% averagely, and could be up to 98% in the peak time.

Many solar power system users may have headache about how to choose a controller for their solar power systems, PWM or MPPT? Commonly, we have a few guidelines for this:

• 1) If your system is very small, like PV lower than 100W, PWM may be better. However, if you system has a high demand on charge amount, you also can choose MPPT, like some solar light with high LED power.
• 2) If your PV array’s voltage is much higher than the battery bank, like 12V battery, but 30V PV system, then no doubt you need to use MPPT. Almost all thin film PV arrays (very high voltage) have to use MPPT.
• 3) If you use Sun Power cells assembled modules, then it’s better to use MPPT.
• 4) If your PV module voltage is very close to the battery voltage, like 30V module to a 24V battery bank, or 18V module to a 14.8V lithium battery, PWM charge may have an improved charge efficiency, sometime could be up to 90%. * please notice that the PV voltage shouldn’t be lower than the battery charge voltage all the time while it’s working.
• 5) If the PV voltage is very close to the battery bank voltage, then MPPT may not work very well, and in some cases like that, MPPT charge may never begin in the whole charge time.
• 6) If you still have no idea about how to choose, then please contact us.

Firstly, MPPT charge starts only if the PV with a voltage difference higher than the battery. Usually this voltage difference should be higher than 3V at least.

Secondly, all MPPT device may have a limit on max input voltage. For example, Helios’s MPPT controller VENUS-M2440 with a max input of 100Voc, so this 100Voc (PV open circuit voltage, not rated voltage or working voltage) is the limit, and you can not plug in arrays over this limit. * usually 100Voc means 75V around of working voltage.

Thirdly, if the voltage difference between the battery and PV is too much, then we may have a not small power loss, so we need to control the array voltage by connection more parallel and less series on panels. For example, for controller VENUS-M2440 (12/24V), if you have 2 panels of 36V, then it’s better to make them connect in parallel (36V), but not in series (72V).
However, it’s not really a big deal, the difference could be very small. If you find it’s not convenient to make the modules connected in parallel, then just do it by series.