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I have \$3.7V\$ Li-ion battery \$3800mAh\$ and Solar panel \$U=5.5V, I_{max}=290mA\$. Battery will supply arduino with sensors taking \$100mA\$ current. How long will it take to charge the battery from the solar panel?

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    \$\begingroup\$ You can't really say how long it takes because it depends very much on the amount of light you can get. But with li-ion batteries its always a good idea to use a dedicated charging IC to prevent overcharging the battery (which can get quite dangerous). \$\endgroup\$
    – 0x6d64
    Commented Jul 1, 2015 at 12:29
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    \$\begingroup\$ Did you mean to use \$100mA\$ instead of \$100mAh\$? AFAIK, \$mAh\$ isn't a measure of current. \$\endgroup\$
    – tangrs
    Commented Jul 1, 2015 at 12:45

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It will depend on how you're charging the battery (i.e. how your charging circuit will take the input power and use it to charge the battery) and how much sun light you actually get.

In general, the rule is to divide the battery capacity by your charging current to get the time (in hours) to fully charge the battery.

Assuming your solar panel is able to maintain a constant 290mA output current and your charging circuit uses a linear constant current regulator to charge the battery, you should expect the charging time to be at least $$\frac{3.8Ah}{0.29A - 0.1A} = 20\text{ hours}$$

However, the above calculation makes a few assumptions which may not likely hold up in reality. It is probably best to test and measure in a realistic environment.

Lastly, as indicated by several people in the comments and answers, ensure you're using proper charging and safety circuitry for your li-ion batteries. Li-ions should almost never be charged directly by a power source.

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Charge current: 290 mA - 100 mA = 190 mA Time to charge: 3800 mAh / 190 mA = 20 hours

This is assuming the solar panel can deliver 290 mA the whole time !

Note: You MUST use a charger circuit, something like this to prevent your battery from overcharging, getting too hot, or even EXPLODING !

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  • \$\begingroup\$ Sun will take a break in between :) \$\endgroup\$
    – User323693
    Commented Jul 1, 2015 at 14:13

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