I got a (relatively inexpensive) SmartYard LED Solar Spotlight as a gift, and have been playing around with ideas to extend the battery life. It uses a 3"x5" solar panel, and comes with a 3.7V, 1400mAh 18500 Li-ion battery.

There's extra room within the housing, so I originally considered the prospect of adding a second 18500 battery in parallel. However, several other posts here suggest there are multiple concerns/complexities associated with charging them in parallel, and with potentially unbalanced cells.

So now I'm thinking I could buy a single cell housing for an 18650 and use a (protected) 18650 instead of the 18500. The voltage is the same between the two, so presumably I'd get added battery life, with the only cost being it will take longer to charge.

Are there other issues I should be concerned about with this approach?

  • \$\begingroup\$ If the batteries are the same voltage, wouldn't it just mean it'd take longer for the panel to fully charge the 18650? Or is there a scenario where it could never get the battery to full capacity, even over a multi-day period? \$\endgroup\$
    – ktwd
    Jan 27, 2019 at 23:53
  • 2
    \$\begingroup\$ Please edit your post rather than commenting on your own question. It's worth mentioning, by the way, that 18500 and 18650 are both size codes--18mm diameter and 50.0/65.0mm length. If the device is normally able to have batteries replaced, and probably even if it isn't, it should be able to use any size of battery, as high-capacity 18500s may have a higher capacity than cheap 18650s, despite the size difference--so some variance needs to be considered in designing it. \$\endgroup\$
    – Hearth
    Jan 28, 2019 at 0:00
  • \$\begingroup\$ Sorry, the first answer was originally posted as a comment, so I was responding with a comment. \$\endgroup\$
    – ktwd
    Jan 28, 2019 at 0:04
  • \$\begingroup\$ "the only cost being it will take longer to charge" - it might be that all you will need is to ask the Sun to shine a bit longer. :-) \$\endgroup\$ Jan 28, 2019 at 2:16

1 Answer 1


The limiting factor may be the Solar panel and not the battery. 5Voc to 3.7V @ 30mAh in max sun.

So depending on LED load it may be 1:1 or 2:1 charge:discharge ratio per day full direct sun hours full sun:dark hours

If you live in a very sunny climate, it may help a bit but may never fully charge with double the battery capacity. If you get 4 good sun-hrs around noon and only get 1 hr of decent light, a better battery may help get 2h.

It's just a SWAG.

3x5" cells can be arranged in series or parallel to get different open circuit voltage, Voc 2.5,5,10 but the same VI power. From a quick search, I see 3x5" panels rated for 30mA on a 3.7V cell which is near ideal at 70 to 80% of Voc.

So 30mA charge rate is max at 100kLux full sun. Then only 3mA at 10kLux cloudy or a wide angle offset.

There may be 1 or 2 LEDs with series R selected for ~13mA each +/-50% depending on design then operating most of the time from 3.7 to 3.3 then dropping to 2.85V when LED off. The LEDs are about 2.8V + If*16 Ohms=Vf then + IfRs=Vbat Still this is just a SWAG. with 50% tolerances on any LED internal chip Rs.

  • \$\begingroup\$ Can you expand on "5Voc to 3.7V @ 30mAh"? Also, "depending on LED load it may be 1:1 or 2:1 charge:discharge ratio per day full direct sun hours full sun:dark hours" kind of lost me a little. I'm trying to learn more about home-hacking solar stuff, so sorry for the dumb questions. \$\endgroup\$
    – ktwd
    Jan 28, 2019 at 0:00
  • \$\begingroup\$ You can always cheat and string phone wire or magnet wire to the lights and run off 3.7V all night from an outdoor brick with 5V-2x diode drops (1N400x) Then everyone will say you a great solar light, it even works in the daytime (lol) \$\endgroup\$ Jan 28, 2019 at 0:12
  • \$\begingroup\$ Is the 30mA charge rate you mention the rate per hour? Does this imply that a 3000mAh battery would take 100 hours to charge? \$\endgroup\$
    – ktwd
    Jan 28, 2019 at 0:29
  • \$\begingroup\$ yes that's what I mean \$\endgroup\$ Jan 28, 2019 at 0:32
  • \$\begingroup\$ So in this scenario, if I added a second solar panel in parallel (thus keeping the output voltage the same), would the additional amperage cause any issues for the 18650 battery? Or would effectively doubling the mA output of the charger be just what was needed (given the same amount of sunlight in a day) to effectively double the battery life for a given night? \$\endgroup\$
    – ktwd
    Jan 28, 2019 at 0:59

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