Measuring voltage and current from PWM solar controller

Projects General Guidance
1

I use arduino and ADS1115 to measure voltage of battery and solar panel of PWM solar charger controller using 2 voltage dividers for both sources.

To have a corrrect measurement, ADS1115 GND must be connected to GND for both sources (I think).

The problem is when I turn on the switch of battery first, the voltage of solar panel got a reading and showed incoming voltage even I did not turn the switch for solar panel.

I suspect that it caused by the way that I connected both voltage dividers together and to ADS1115. But that is only my assumption.

I like to know what is the best method or circuit to get accurate reading from each voltage divider and at the same time it does not interfere other voltage divider?

You might say why not take PWM solar controller that can produce the measurement and send it to arduino. The problem is I got another big problem. My solar controller does not show current reading. Also, I have an experience where the serial module in solar controller blew up but the controller was still working, fixing the serial module in the mainboard is very hard and time consuming.

I hope somebody can give a great suggestion. Thanks.

2

My best suggestion would be to provide us with a properly annotated schematic. Sorry, but that's the language of electronics.

3

Beware that many inexpensive solar charge controllers use the positive side of the solar panel and battery as the common, or "ground" connection.

In other words, the battery negative and solar panel negative are not directly connected.

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4

Note that regulation of a solar charger is usually in the ground line. Meaning, solar(-) is not the same as battery(-). There can be several volt between batt(-) and solar(-). Panel(-) is negative during the day, and positive during the night. I think only three things are important.

  1. solar panel current
  2. load current
  3. battery voltage

Panel voltage is meaningless.

You could fry the ADS when any input voltage gets outside it's supply/GND.
Leo..

5

From several testing, I assumed that too. If it is true, I think having both voltage dividers grounded together is not a good choice.

6

Correct.
That will short the regulator of the charger, connecting the panel directly to the battery.

As said, forget about panel voltage.
It's meaningless, because solar is essentially a current source.

Something like an INA219/226 are much better to measure voltage/current high-side.
High-side = in the + line.
Leo..

7

Actually I use ACS758 for current.

Now I understand that all GNDs are not connected together in solar controller.

Actually, I am very particular to have all measurement from solar panel, battery and load. I think for my circuit, I have to put load reading a way from it with assumption that load measurement is the same with battery.

So, is there a way to measure solar panel voltage and current without having its GND connected to GND of measurement circuit?

8

Voltage is relative to battery(+).

But why do you want to know panel voltage. Generated solar power is more relevant, and that is the product of battery voltage and panel current. From that you can calculate panel voltage, but why.
Leo..

9

It sounds like you don't have much experience with solar panels. After some experimentation, you will learn that the open circuit panel voltage is nearly independent of illumination (above some minimum level of illumination), while the short circuit current flow is linearly proportional to the illumination intensity.

That is why solar panel voltage is of nearly zero interest when measuring the performance of a setup.

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