I have built a voltage regular circuit with LM317. Its input is connected to my solar panel.

As in many circuit examples, there is a voltage divider to bias the adjust pin. Also, I have connected a diode on output to prevent backward current flowing into voltage divider.

When no load is connected on output, I measure the output of my solar panel, it is also the input of LM317. It is generally over 18V. I set the output voltage to 8V by using a pot, and it works perfectly. In this connection, due to the voltage drop on diode, I get about 7.2V at the end.

Now, when I connect my 6V lead acid battery (it says 7.20V - 7.50V for charging), I measure the input of LM317, and see that it drops to about 4-5V and stays there. I don't have much experience with lead acid batteries, but some days it works perfectly, and output stays really high, other days, battery doesn't charge at all and above situation is seen. What is causing this? Is it normal?

P.S. After charging batteries, I use it to light hallway with white LEDs.

.. After a while ...

I used a 12V power adapter last night as substitution of solar panel, and measured voltage at different points. I saw the same effect. That means this effect is not about solar panel.

  • \$\begingroup\$ On the bad days, are you still connecting the lead-acid battery with the correct polarity? The diode that I guess is in series with the output of the LM317 won't protect against reversal of the load, only against the load injecting current. \$\endgroup\$
    – fgrieu
    Commented Feb 25, 2014 at 18:00
  • \$\begingroup\$ Today, it was rainy, but light still. When battery wasn't connected, input was about 13V. Everyday I measure input and output at least 10 times, so I am sure it is correct polarity. I am using diode thinking that if output voltage drops below battery's voltage, battery may lose its charge on the voltage divider. The weird thing is it waited from morning to evening, despite trying to charge it, it also lost 0.1V. \$\endgroup\$
    – tcak
    Commented Feb 25, 2014 at 18:11

1 Answer 1


A solar panel is not a constant voltage, or constant current source. It can be thought of as a constant power source with maximum rated voltage and maximum rated current. The power is relative to the light hitting the panel, the voltage is maximum with no current, and drops as current is drawn from the panel.

If you are using a 10W panel, and it's in its full rated sun exposure, you'll get 10W out.

If you draw 1A in that situation, the voltage will be about 10V. If you draw two amps, the voltage will be about 5V.

If your battery is full, you probably aren't going to draw much current, so the voltage is higher.

If the battery is nearly empty, it will draw a lot of current, and it will cause the panel's voltage to drop.

In your specific case, what you're finding is that the panel can't provide full charging current all the time - whether that's due to less than full sun exposure, or a low-charge battery depends on the situation.

However, you can still use this system, even though the voltage is low. If you disconnect the battery and measure its voltage, then connect it to the charging system and measure the voltage at the battery, you'll find that the attached voltage is higher - the battery is accepting current from the system, and is charging. It isn't charging as fast as it could be, but that's due to the panel's limitations.

If you want to learn more about this, and what professional solar charging systems do in order to handle this effect, do a search for MPPT circuits - maximum power point tracking. The solar panel is most efficient at a certain voltage and current for a given sunlight input, and these circuits attempt to track that maximum point so you get as much power from the panel as possible.

Also, note that SLA batteries are very forgiving. It may be that you can eliminate the voltage regulator, and just use the diode in the circuit. This will increase the voltage at the battery since the regulator drops 1.5V-3V depending on load, and thus charging efficiency. Given that you're having a hard time keeping it charged, I'd expect the solar panel is unlikely to damage the battery, but check the panel's maximum current at 7.2V and see if the battery can accept a constant trickle charge of that rate.


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