My objective is to build a Circuit which will draw current from Solar Panel, recharge a battery as well as run the load even if voltage from solar panel goes too low.

I have made the following circuit, where V1 is a Solar Panel 3-10 V output, BAT1 is a Rechargeable Li-ion battery, LOAD is a submersible pump which draws about 100 mA.


simulate this circuit – Schematic created using CircuitLab

Will the circuit perform my objective?

  • 1
    \$\begingroup\$ What are you using as a charge controller for the lithium cell? If your answer is anything but "a dedicated charge controller" then the answer is no. \$\endgroup\$ Dec 9, 2016 at 22:26
  • 1
    \$\begingroup\$ What is the battery's capacity and max charging current, and the solar panel's voltage and current at max power output? What is the purpose of D3, and what type of LED is it? \$\endgroup\$ Dec 10, 2016 at 0:39
  • \$\begingroup\$ @BruceAbbott My battery's capacity is 1 Ah, dont know the max charging current. Solar Panel's Rating is 5V 3W. D3 is to indicate whether the solar panel is providing any voltage. \$\endgroup\$ Dec 10, 2016 at 6:56
  • \$\begingroup\$ @insta I am trying to do that! Can you suggest some edits in my circuit or a different circuit to do that? \$\endgroup\$ Dec 10, 2016 at 7:02
  • \$\begingroup\$ @aniruddha.sarkar I added an answer \$\endgroup\$ Dec 12, 2016 at 14:58

3 Answers 3


You need to replace the regulator with a regulator + charge controller. A very common choice is the TP4056, which is available packaged on convenient little modules all over eBay for a few dollars (just search for TP4056).

Feed the output from the regulator into the "input" side of the charge controller, and drive your circuit from the output side, with a battery connected across the output terminals. Use protected lithium cells which will auto-cutoff on excessive discharge.


There are several problems with your circuit:-

  1. A '3.7V' Lithium-Ion cell can be safely be charged up to 4.23V, but no higher or it will explode! Your voltage regulator is set to produce 5.0V, and D2 drops 0.5~0.9V depending on charging current, which could put up to 4.5V on the battery. However D3 drops about 1.7V at normal operating current (assuming it is a red LED, other colors may be higher) leaving only 2.8V so the battery will never get charged.

  2. Your 5V 3W solar panel should be able to deliver 600mA at 5V in full sunlight. However at 600mA D1 drops ~0.7V and U1 drops ~1.8V, so the regulator output voltage would only be about 2.5V. At low current the solar panel's voltage may rise to ~6.5V and the regulator will have lower voltage drop so its output voltage may be sufficient, but the battery would charge very slowly.

  3. Most linear regulators require input and output bypass capacitors to prevent unwanted oscillation. The LM317 is better than many, usually only needing 0.1uF across the supply input. If using a different regulator you should follow the manufacturer's recommendations.

D1 is redundant and should be removed. U1 could be replaced with an LDO regulator such as the MIC29150 which drops less than 0.2V at 600mA.

You should connect battery negative directly to the solar panel so that charging current doesn't go through the LED. This could result in the battery getting up to 4.5V, so you must reduce the voltage regulator's output voltage until the 'open circuit' voltage after D2 is below 4.23V.

Your circuit doesn't include over-discharge protection. To prevent the battery from being damaged if for some reason the solar panel cannot maintain sufficient charge, you might consider adding a PCM (Protection Circuit Module). This will also protect against over charging and accidental short circuits.

  • \$\begingroup\$ I added a PCM, removed D2,D3, and R3. The circuit is working but the charging rate is too low. Is there any method to increase the rate? \$\endgroup\$ Dec 11, 2016 at 17:53
  • \$\begingroup\$ What charging current are you getting, and what is the voltage at input and output of the LM317? What happens if you remove D1? \$\endgroup\$ Dec 11, 2016 at 17:58
  • \$\begingroup\$ @Bruce_Abbot Its night here pal, cannot check. Will see tomorrow. \$\endgroup\$ Dec 11, 2016 at 18:01

I think you misread the pump specs.

Did you calc 37 Ohm?

Most motors have winding DCR resistance of are 8~10% of the rated V/I so surge current is increased on start this V+/DCR

300mW pump? maybe 450mW?

I suggest you pump is 4.5V @100mA rated thus start current is 800 to 1000mA

Your design too inefficient with all the Silicon diode drop and the V PMT would be around 8V or 80% of the Voc (open cct) so it will not generate less than 50% of rated max PV power at 3.9~4V. Choose a voltage limiter of 3.9V and drive from LiPo direct. Then make sure load is removed at 3V with hysteresis to 3.3V.


Your LDO is set for 5V . If the LiPo reaches 4.4V, if std. chemistry, it may explode from critical self damage. Max 4.15V to LiPo for longer life.

  • \$\begingroup\$ I connect a mili ammeterbetween D4 and SW1 whenever I start the circuit. The reading never goes above 100 mA. \$\endgroup\$ Dec 10, 2016 at 6:49
  • \$\begingroup\$ does that method measure peak surge or DCR? \$\endgroup\$ Dec 10, 2016 at 10:58
  • \$\begingroup\$ I believe the current \$\endgroup\$ Dec 10, 2016 at 11:15

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