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Well I thought it would be either 12 or 18 but... I have a battery and a solar panel so I thought I'd just try it.

I actually measured the voltage to be 9.7 before hand (btw it's an old lead acid car battery) and then I measured the volts on the solar panel and it was 20 +- a few (bc clouds and what not) then I hooked up the positive to positive and negative to negative, and measured 10.2 volts. Why not 9.7 or 20!?

I'm theorizing that this is because the battery put a drain on the solar causing that things volts to drop, so not 20. And the solar is boosting the battery causing it's volts to increase so not 9.7... and of course the internal resistance of the battery is causing the whole thing to slide towards the lower end of the scale. Well is anything I said even making sense? Are there other considerations? Or is my original expectation right and it should read 20v and the solar setup is anomalous? lol.

Ps I took a few physics classes and even a circuits class years ago so if your answer gets technical I may still be able to follow.

Thanks in advance.

Edit: I drew this picture which is a simplified circuit diagram of my setup (voltages shown are only approximations):

enter image description here

If you get rid of the battery between points a and b you read 20, if you get rid of the solar panel you read 9, my question is simply why 10 and not 9 or 20? What calculation results in a 10? Is it an average, subtraction? In other words it doesn't look like it is simply related to either of the starting voltages. :/


marked as duplicate by user273872, Community Sep 19 '16 at 19:52

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  • $\begingroup$ This is not a duplicate. In this case we are not connecting 2 voltage sources (batteries) together, we are connecting a voltage source to a current source. $\endgroup$ – hdhondt Sep 21 '16 at 10:12

Solar panels are actually constant current sources. They supply a constant current regardless of what is connected to them, up to a maximum voltage. In your case that maximum is about 18V.

When you connect the panel to the battery, it will supply its nominated current, as the battery's 9.7V is below the 18V of the panel. The panel would supply that same current even if you shorted it.

When the battery receives that current, it's voltage rises, and will keep rising until it reaches the 18V of the panel. Of course, a 12V battery will have died well before that.

The moral of the story? Do not connect the solar panel to the battery like that. You should connect the panel through a circuit that will limit the voltage to something the battery will tolerate.

  • $\begingroup$ So I should read a voltage starting at 9.7 and increasing till it gets to 18? I still don't understand why. Updating question.. $\endgroup$ – user273872 Sep 19 '16 at 19:03
  • $\begingroup$ The thing is that, as soon as you connect the panel to the battery, its voltage will suddenly jump, to 10V in your case, simply because it is receiving charge. From there, the voltage will start to increase as the battery continues to be charged. The battery determines the voltage you measure, and that voltage is in turn determined by the current the panel injects into it. $\endgroup$ – hdhondt Sep 20 '16 at 10:00
  • $\begingroup$ But then you still have two different emfs across points ab (10v and 18v). My confusion is why the volt meter reads one and not the other. Thanks for trying to help me btw. $\endgroup$ – user273872 Sep 20 '16 at 18:43
  • $\begingroup$ Because the solar panel is not a VOLTAGE source, it is a CURRENT source. Its voltage automatically adapts itself to the voltage the battery presents when the charging current is flowing through it. To rephrase, your panel is not an "18V panel", it is an "xA panel". It will deliver its x amps at whatever voltage is needed by the circuit, from 0V to 18V. $\endgroup$ – hdhondt Sep 20 '16 at 22:56

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