Determine voltage and current for given I-V curve

Question

Excuse me for my noob question, I'm new with solar panels.

I'm trying to understand what value of current\voltage a charge controller gets in a given moment.

Lets say that on given temperature and radiation, the solar panel has its I-V curve:

My question is, again, how can you tell what current\voltage the charge controller gets? Does it according to its resistance?

I know that if the load has resistance of R=Vmp\Imp, the solar output maximum power. And a MPPT module can change resistance in order to get the maximum power.

What happens in case of charge controller and 18650 battery only? how do I know what Vx and Ix the charge controller gets? I think that the circuit (charge controller + battery) has constant, resistance. will it able to get to maximum power at all?

Example of charge controller- TP4056

Hope my question is clear.

Many thanks :)

Answer 1

My question is, again, how can you tell what current\voltage the charge controller gets?

It depends on how smart it is. Let's start with dumb.

Suppose the battery is discharged, so the charge controller decides to go with full charge current. Unfortunately, there is not enough sunlight, or the solar panel is too small, so the controller tries to draw more current than the panel can provide, and voltage collapses. Depending on the controller, it may shutdown or reboot, not charging the battery, or just use whatever current is available, which won't give you the maximum power point, but it'll still charge the battery, which is better than nothing.

Next, suppose the battery is fully charged, so the charge controller decides to stop charging current. Voltage will rise to the maximum the solar panel can provide, and that should not exceed the maximum supply voltage of your chip. Otherwise, it will fry.

So it is possible to charge a battery with a solar panel and a dumb controller, provided the panel's maximum open circuit voltage won't fry the controller, and the controller can tolerate its supply voltage sagging when it tries to charge with more current than the panel can provide.

If the panel's maximum output current is lower than the battery's maximum charge current, the controller can be as simple as a hysteretic comparator that monitors battery voltage, and switches a MOSFET to connect the battery to the panel when it needs charging and the panel is producing current. That won't give maximum power, because the panel's output voltage is equal to the battery voltage, but it'll work.

If the controller is smart, it'll use a switching converter to run the panel at its maximum power point. Basically it will try to draw just enough current from the panel so the voltage doesn't sag, and adjust depending on illumination. Then, the switching converter can convert, say 12V 1A from the solar panel into 4V 3A (minus efficiency) to charge the battery.

A common mistake is to use, say, a 12V panel with one LiIon battery and a linear charger. In this case, the panel will either output around 12V if the charger doesn't draw enough current to make the voltage sag, and the extra volts will be turned into heat in the linear charger. And if the charger draws enough current to make the panel voltage sag, then... it'll drop, and the 12V panel becomes a 4V panel.

An evolution of this mistake is to use, say, a 12V panel with a 12V to 3.6-4.2V switching converter to charge a battery. Now suppose there is enough light to get 100mA out of the panel at maximum power. If the charger draws 100mA, then great, it'll output 4V at 300mA. But if it tries to draw 200mA, the panel's voltage will drop... and you'll get 4V at 100mA, or it will shut down.

Basically, if you use a panel voltage that's several times higher than your battery voltage, and a buck converter, then you need it to be smart enough to not draw too much current from the panel to keep it operating at its maximum power point.

Answer 2

Determine voltage and current for given I-V curve

It depends what load is connected to it. Google "load line" for a method of graphically comparing the IV curve for the source (solar panel) and the load to determine the operating point.

how do I know what Vx and Ix the charge controller gets?

You need to know the actual values of \$I_{max}\$, \$V_{oc}\$, and (to be accurate) the exact shape of the solar panel's IV curve. This is not a fixed curve; it will change depending on how the panel is illuminated.

Then you also need to know the input I-V curve of the charger device. This will depend not only on the construction of the charger but also on the state of charge of the battery being charged.

Connecting this panel directly to that charger is probably not a great idea because the panel, for most of its useful operating range, operates as a constant-current source. But the charger is designed to be driven by a constant-voltage source between roughly 4.2 and (maybe) 6 V (although the maximum isn't specified).

If \$V_{oc}\$ is just slightly over 5 V and \$I_{max}\$ is over 1 A, it might work...but since the panel output is variable depending on illumination and the load is variable depending on the battery's state of charge, it's not likely to be at all reliable.