I'm need to calculate the how long it will take to charge a battery using a solar cell. I am not taking any charge curves into account, I just want to get a close answer before I run the experiment. I have attached the battery specs as well as the solar specs. I tend to get lost as I go down the rabbit hole with the calcs. Could someone please help.

My Battery calcs:


1600mAh / 13.44W *4.2v = 5hrs (im not sure if this is even close)

My solar calcs: wh=(mAh)(v)/1000 1600mAh*4.2/1000=6.72wh 6.72wh/0.000198W = ?

I am not an EE and I feel like im all over the place. Thoughts?

Battery Characteristics

Solar Characteristics

  • \$\begingroup\$ divide the battery watt_hours stored by the solar cell output power(watts) \$\endgroup\$ – analogsystemsrf Nov 26 '19 at 2:05
  • \$\begingroup\$ So would you agree that the battery Wh = 1600mAh (4.2V) /1000 equates to 6.72Wh? \$\endgroup\$ – Bryan Cunningham Nov 26 '19 at 2:10
  • \$\begingroup\$ What charging circuit do you intend to use? \$\endgroup\$ – Bruce Abbott Nov 26 '19 at 17:09

If this is a Lithium battery my answer does not apply.

The battery capacity is 1600mAh.

The solar cell can supply 42mA.

\$t=\frac{1600mAh}{42mA} => 38h\$

This is a very rough estimate, and can be used for both charge and discharge times.


With LiPo batteries, you don't want to exceed the battery cell voltage, 3.7V in your case. When the batteries are near full charge, and not pulling much current from the solar cells, then the solar cells you have selected will attempt to keep pushing current into the battery by increase their voltage towards 6.2V. That's double the battery cell voltage and will most certainly damage the battery. You would need a battery charge controller to handle this. Google "lipo charge controller" and you'll see lots of options.

The mechanism described also happens with Pb, NiCd and NiMH but they are more tolerant of it. Lithium batteries are not. However any system designed to last will want a charge controller designed for the battery chemistry in use.

  • \$\begingroup\$ This is in fact a lithium battery. what is the determining factor here? What does that calc look like with lithium? \$\endgroup\$ – Bryan Cunningham Nov 26 '19 at 2:11
  • \$\begingroup\$ what happens if we use two of the solar cells and wire them in parallel? \$\endgroup\$ – Bryan Cunningham Nov 26 '19 at 2:37
  • \$\begingroup\$ What happens when you need to charge up in 1 day and account for >50% losses due to beam off axis loss then you need 10x PV charge rate. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 26 '19 at 2:40
  • \$\begingroup\$ The main purpose is to keep the battery at a full capacity after the battery is charged by USB. At that point the solar would allow trickle charging to keep the li-po battery at full capacity. I am just curious to see how long this solar cell (parallel)would take to re-charge this battery. \$\endgroup\$ – Bryan Cunningham Nov 26 '19 at 2:59
  • \$\begingroup\$ Consider a week or so. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 26 '19 at 4:23

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