I have a nightlight with a 25mA 3.3V LED powered with four 1.2V rechargeable AA batteries using a 48 ohm resistor. How do I hook up a 4.5V 1w solar panel (Radio Shack #2770048) to charge the batteries during the day? At night in 8 hours there is only about .2v drop in battery voltage.

NEVER MIND... Answer found in Mims Mini Notebook Vol. III page 140.


  • \$\begingroup\$ You don't, unless you want to eventually kill the batteries. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 16 '14 at 22:25
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    \$\begingroup\$ It is not that easy. You would need to charge them as 2+2 or individually, with power point tracking to get some efficiency. \$\endgroup\$ – venny Sep 16 '14 at 22:40

There are some ICs on the market that are actually made for this exact purpose (charging batteries from solar panels having a lower voltage), they are basically switching step-up (buck) converters with current limiting that are able to operate with low input voltages. See for example http://www.st.com/web/en/catalog/sense_power/FM142/CL1810/SC1517/PF251161 (probably a bit overkill for your situation, as it can charge at 1.8A...).

  • \$\begingroup\$ Ale, Thank you very much for your help. I ordered the IC which was only $4 but since I am new in this hobby I also ordered an evaluation board which set me back $85. Education is not always free. Dell \$\endgroup\$ – dell weston Sep 19 '14 at 22:19
  • \$\begingroup\$ @dellweston: good luck with your circuit! I think having an eval board can be very useful, also because the IC needs some external components (coil, resistors, capacitors, diode: see Fig. 2 on the datasheet). Don't forget to properly set the output voltage and current of the evaluation board (I suppose this is possible, didn't check) so that they match your batteries. \$\endgroup\$ – Ale Sep 20 '14 at 0:06

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