1
\$\begingroup\$

I slapped together a bunch of spare DIY electronics in an attempt to make an automatic plant watering system. The goal was for it to be solar powered, something to set and forget. Just add water.

Yesterday was my first time testing it. When I manually check the battery voltage, everything attached, in direct sun, the voltage displayed is only the battery's voltage. If I disconnect the solar panel and check its voltage alone, it is 14.3V. At this rate, the battery will never charge right?

For what it is worth, if the solar panel is connected, but the battery is disconnected. ESP32 is running etc. the voltage is 3V-5B.

What I should do to correct this? Is the solution a larger panel? Maybe some sort of charge controller or something else?

Hardware

Basic wiring

\$\endgroup\$
9
  • 2
    \$\begingroup\$ I hope that's not a lithium ion battery. What battery is it? \$\endgroup\$
    – Justme
    Jun 21 at 19:48
  • 3
    \$\begingroup\$ All the necessary information needs to be in your question here, not buried in comments on another web site. Please edit the question to add it. \$\endgroup\$
    – Finbarr
    Jun 21 at 20:34
  • 1
    \$\begingroup\$ Between the solar panel and the battery you need a MPPT controller/charger to suit your battery chemistry, not a diode. Google awaits your enquiry. (Shopping recommendations are off-topic here.) \$\endgroup\$
    – Graham Nye
    Jun 21 at 20:54
  • \$\begingroup\$ And whatever the battery voltage X is when empty, it will not jump to 12V immediately when it is charging. Voltage will slowly rise from X to 12V after long period of charging. Assuming the 3W 12V panel gives constantly out 250 mA (which it never would), it would take 17.6 hours of charging a 4400mAh battery from empty to full capacity and voltage (likely never if voltage is not enough). So, battery voltage is what you measure. It won't suddely get any higher when panel is connected. \$\endgroup\$
    – Justme
    Jun 21 at 21:09
  • \$\begingroup\$ The reason I went with a 3w panel is because the device consumes very little. A smarter person than myself calculated that the energy in was greater than used. Given how it sleeps, sometimes doesn't pump water, etc. I know voltage is not a direct indicator of battery life. But yesterday, the voltage was 11.9v. Today, at about the end of the sun intake window, it was 11.3v. So it appears to not be taking in more than it is putting out. What is my best course of action? Plug it into a wall? \$\endgroup\$ Jun 22 at 1:28

2 Answers 2

1
\$\begingroup\$

Yes, you need a "charge controller", but a specific type -- a MPPT charge controller. MPPT = Maximum Power Point Tracker. From googling MPPT, "An MPPT, or maximum power point tracker is an electronic DC to DC converter that optimizes the match between the solar array (PV panels), and the battery bank or utility grid." So, the problem is that, as the solar irradiance from the sun changes due to clouds, or the angle of the sun, or what part of the season it is, the solar panel will change it's voltage output, and also change the maximum power that it is capable of. Power is V*I, and when V=0 then power is zero, and when I=0 then power is zero, so somewhere between open circuit and short-circuit is the best point, where you get the max power out of the solar panel (which changes at any given moment). It turns out that the max power point is at the knee of the graph, when the V-I curve starts to plummet (but as we said, as clouds go by, that curve moves around all the time). Study the datasheet for a big solar panel for what you would use for the roof on your house, and it should give you the V-I curve. It won't directly apply to your 3W panel, but the principles will be the same. So, if you buy an MPPT charge controller for your 3W panel, that should do it for you to get started. We don't do product recommendations here but going on over to the EEVblog forum, you should be able to ask your question there.

All of the following curves come from the following datasheet for a 12 volt 100W solar panel found here:

https://renogy.com/content/RNG-100D-SS/RNG-100D-SS%20G3%20Datasheet.pdf

So, for the following curve (considering only the highest curve):

enter image description here

If your DC-DC converter tries to extract the energy from the panel like this:

enter image description here

you get only about 5V x 5.2A = 26 Watts.

If your DC-DC converter tries to extract the energy from the panel like this:

enter image description here

you get only about 24V x 1A = 24 Watts.

But if you get an MPPT charge controller, it will extract the energy from your solar panel more like this:

enter image description here

giving you somewhere about 20V x 5A = 100 Watts.

So you need an MPPT charge controller between your 3 Watt panel and your 12V battery.

Here is an Amazon search where you can find some candidates that should work for you.

\$\endgroup\$
5
  • 1
    \$\begingroup\$ Thank you for your suggestion and being so definitive. Since I've started this solar project, I've received a lot of vague advice. I didn't know a solar charger was mandatory here. I will purchase such a charge controller that suits my batteries etc. and report back. \$\endgroup\$ Jun 22 at 3:32
  • \$\begingroup\$ @baldest_programmer -- This one jumps out to me. You're welcome for the suggestion. And if you liked my answer, please upvote it. And if it is the best answer, please mark it with a checkmark as well. We work hard for those! And that would be great if you would report back. Not enough people do that. May things go well in your project. \$\endgroup\$ Jun 22 at 4:06
  • 1
    \$\begingroup\$ Today, I received and installed an MPPT solar charger for the project. I went with an MPPT board module. ( SD30CRMA 12v ) I did not ask anyone about it. : } I spliced the board in behind the diode of the solars cable and installed it into the orange case. Nice and clean. I did a very short test - sun was going down. Before the charging board, with everything connected and running, solar @ 14v. The overall highest system voltage was only as high as the batteries voltage. Bad. \$\endgroup\$ Jun 28 at 0:19
  • 1
    \$\begingroup\$ But after installing the board. The battery alone was at 11.2v. However, this time, the system read 12.2v. I think it's charging! I guess it's time to start the long test. I reduced the battery pack by half, or down to 2200 mah. Thanks for helping me with my solar project! \$\endgroup\$ Jun 28 at 0:19
  • \$\begingroup\$ @baldest_programmer -- You're welcome! \$\endgroup\$ Jun 28 at 0:49
-2
\$\begingroup\$

When a battery is connected to a solar panel, the solar panel voltage is the battery voltage.

  1. This is a characteristic of electrical systems. When you connect things together, they have one voltage.

  2. Batteries are designed to maintain a specific voltage, regardless of current. Panels are designed to maintain a specific current, regardless of voltage.

The current from a panel depends on the amount of sun. The panel voltage is the result of that current. When open circuit, the current, from the sun, leaks back through the solar cells: the sun is still providing the same current, and the voltage is still the result of what is connected (just the panels, no external load.)

The voltage of a battery depends on the electrochemistry of the elements. When short-circuited, all the voltage is still there: the chemistry is the same, but the voltage is being used in the battery to move ions through the battery fluid/gel.

When you connect a battery to a solar panel, you get the battery voltage, and the solar panel current.

In real life, the battery voltage rises slightly, and the panel current falls slightly, because in real life the chemistry is more complex, and the solid state physics is more complex. Unless the panel is far too big for the battery, the voltage is regulated by the battery, and unless the panel is designed for a much different battery, the current is set by the sun.

Regardless of battery voltage, if current is going through it, it is charging (or discharging.)

\$\endgroup\$
5
  • 1
    \$\begingroup\$ I feel like you are simplifying too much when you say that solar panels maintain a constant current no matter what. Depending on where they are on their V-I curve, they can sort of look like a constant voltage or a constant current or neither. I realize that you are trying to avoid confusing the OP with complexity. But lots of people read answers here and this one could confuse a lot of people about how solar panels work. \$\endgroup\$
    – mkeith
    Jun 28 at 6:16
  • \$\begingroup\$ I work in the Solar industry, explaining battery charge controllers to people. The number of people confused by oversimplification of this particular issue is far smaller than the number of people confused by accurate description. My experience does not match your feeling. I don't know why the common intuitive understanding is that a charge controller should control battery voltage, but because of that strong prior misconception, basic operation of battery and panel needs to be very clearly and very simply stated to correct common error. \$\endgroup\$
    – david
    Jun 28 at 8:05
  • 1
    \$\begingroup\$ Well, you can explain that the panel voltage and battery voltage are different without saying that the panel is a fixed current source. My objection stands. \$\endgroup\$
    – mkeith
    Jun 28 at 16:29
  • \$\begingroup\$ Your objection is based on your feeling. My answer, which already includes the 'real life' exception which fully meets your objection, is based on experience as an educator working in the industry. \$\endgroup\$
    – david
    Jun 29 at 9:17
  • 1
    \$\begingroup\$ David, I know how solar panels work. I am familiar with the V-I curve of solar panels. I know how MPPT charge controllers work, etc. Your answer contains factually incorrect information with no hint or suggestion that there is more to the story. It is extremely misleading. That is the basis of my objection and downvote. I don't work in the solar industry but I have been an electrical engineer for over 20 years and have met my share of confused people. \$\endgroup\$
    – mkeith
    Jun 29 at 16:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.