Electronic – Understanding working of MPPT charger

lipompptsolar cellsolar-charge-controller

I am trying to understand working of MPPT charger for LiPo battery (3.7V nominal) and 6V 2W solar ‎panel so that I can select right charger for my project.

After searching and reading lot of related material on the net, my understanding is that, while charging, in all light conditions, the voltage from solar panel will remain ‎more or less constant, but the available current will change.Thus at the Input side of charger we are ‎getting more or less same voltage but varying current as the light changes.

‎ At the output side of charger, voltage must be higher than the battery voltage (to be able to charge ‎it). Thus charger will keep on monitoring battery voltage and keep the output voltage higher than that.

But as the light condition changes, the available current from the panel changes, and so does the MPP ‎‎(maximum power point), and the charger adjust the out put voltage to the MPP voltage (to get most ‎of available power for charging). But still this voltage will be higher than the battery voltage.‎

Please correct me if I am wrong.‎

Best Answer

A solar panel is a current source over most of its characteristic; the voltage it shows is "set" by what you connect to it.

When you connect a battery to it, the voltage will be set by that battery; connect a charger to it, and the voltage will be set by the input impedance of that charger. This voltage may be nowhere near the voltage at the MPP; for instance, a 5V battery wouldn't be a good match for a 12V solar panel.

The idea of an MPPT is to keep the panel producing maximum power under all circumstances. The MPP is where V * I is maximal, or the maximum rectangular area that will fit under the panel's I/V characteristic, about 10W in this example:

MPP graph

(Image source - my website: Using a solar panel for USB charging)

Suppose you connect a 5V-output DC/DC converter to a solar panel; it would work fine, but it would set its input impedance (by varying the PWM) to a point that doesn't use the full power of the solar panel (the "you are here" point), only about 3W of the available 10W.

By varying the input impedance, you can arrive at the MPP. The optimal way of doing that is by varying the input impedance of the DC/DC converter and measuring if more power is delivered at its output.

A simpler (but less efficient) way is setting a fixed point on or near the MPP, say 16V, and reduce the input current of the DC/DC converter (by varying its input impedance by varying the PWM) when the panel voltage drops below that 16V. This method if often used these days for small applications.

In both methods, the input of the DC/DC converter is regulated, not its output, the output behaving like a current source.

The charger is a separate part of the system, and has its own regulation for correctly charging your batteries, using the output of the DC/DC converter.