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I have a 5V 1000mA solar cell which I have connected to two 18650 3.7V 3400mA batteries in parallel. The solar positive comes in through a 1N4007 diode (cathode toward battery)and then a latching relay before going to the positive of the batteries. The latching relay disconnects the solar cell when the battery is charged to 4.2V and reconnected when the battery discharges to 3.4V. The batteries are connected to a 5V boost regulator, powering an Arduino Pro Mini which for most of the time is in sleep mode. Every two hrs the Arduino wakes up and performs some tasks for about 3 minutes. During this time the batteries also power an ESP8266-01 through a 3V voltage regulator for about 2 minutes. When not connected to the solar or at night the power drain is about 1/420V in 2hrs. However when exposed to sunlight (in Tucson, AZ in April) sunlight is about 30º off directly overhead it only gains 1to2/420V in a 2hr. period. It used to get more, about 4/2hr period. I am at my whits end as to why it is not charging more. It should be. I am attaching a schematic of my wiring. (sorry I'm not an electrical engineer so schematic might not be that great.) Can anyone see where I have done something wrong keeping the solar from charging more or some way I can get it to charge more?

Wiring of solar to arduino

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  • \$\begingroup\$ "...at night the power drain is about 1/420V in 2hrs" - '1/420V' is not a power measurement. What does it mean, and how did you get it? \$\endgroup\$ Apr 21 at 5:03
  • \$\begingroup\$ You’re severely limiting the output of your solar panel by using a 1N4007 in series. I would suggest using something with a much lower voltage drop like a Schottky diode, or preferably no diode at all and using a real charge regulator. Powering the relay also eats up precious solar energy. Using a purpose-made charge controller will be much better for the longevity of your batteries. \$\endgroup\$
    – StarCat
    Apr 21 at 6:17
  • \$\begingroup\$ @StarCat Is he limiting the current by much? It is only the current that charges his battery. The extra voltage available is wasted anyway, is it not? (There is no buck converter.) \$\endgroup\$ Apr 21 at 14:45
  • \$\begingroup\$ To be honest, I am not trying to diagnose your problem due to the poor quality of the schematic. The diagonal lines crossing components and other lines in the lower right hand corner simply makes me want to look away. You may get more help if you clean up your schematic. There is a schematic editor available for you use in the tool bar of the edit window if you want to try it. If you do decide to edit schematic, all the lines that merely go to headers (i.e. J2) are unnecessary, and provide unnecessary clutter as well. \$\endgroup\$ Apr 21 at 14:48
  • \$\begingroup\$ @MathKeepsMeBusy with the diode in place, the peak voltage of the solar panel is only just above the charging voltage of the batteries. As soon as the amount of sunlight drops below maximum, the voltage will drop below the charging voltage and the batteries will stop charging. \$\endgroup\$
    – StarCat
    Apr 21 at 15:33

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