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# How long does it take to charge a 500mAh battery with 5V 80mA?

#### BhanuShashanka

Jun 21, 2023
6
Hello guys,

I have a battery of 3.7V 500mAh. I want to charge it with a 5V 80mA Solar Cell. How long does it take to charge the battery? after charging can I use the battery to charge my 4000mAh phone? if yes, how much percentage will it charge? Please help, thank you.

#### kellys_eye

Jun 25, 2010
6,514
The battery is 1.85Whr and the charge can deliver 0.4Whr so (assuming zero losses and 100% charge from zero) it would take 1.85/0.4=4.625 hours.

With expected losses and other inefficiencies I'd plan for at least 8 hours.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,800
Assuming the battery is fully charged and the charger has 80 % efficiency the battery can charge the phone to
500 mAh / 4000 mAh * 0.8 = 10 %

#### dave9

Mar 5, 2017
1,190
There is no simple equation, because (presuming your 3.7V is Li-Ion) you're starting with a solar cell rated for max output, which you will not achieve in sequential hours over the amount of time it would take, then needing a charge controller to not damage the Li-Ion 500mAh battery through overcharge, and then after fully charged, incure another loss boosting the 4.2V (of the 3.7V cell fully charged) up to 5V to charge the phone.

I guesstimate that to fully charge your phone from a completely drained state, and accounting for limited daily solar power production, it will take at least half a month, or the better part of a couple days just to charge the 500mAh battery, depending on the weather.

You'd be better off with a larger solar panel, a larger storage battery, or just using a hand crank generator instead.

#### bidrohini

Feb 1, 2023
200
Charging time (in hours) = (Battery capacity in mAh) / (Charging current in mA)

Charging time = 500mAh / 80mA ≈ 6.25 hours

The formula doesn't account for factors like charging efficiency, losses, and variations in solar cell output due to weather conditions. In real-world scenarios, the charging time may be longer.

As for using the charged battery to charge your 4000mAh phone, let's calculate how much percentage of the phone's battery can be charged:

Percentage of phone charge = (Battery capacity in mAh) / (Phone battery capacity in mAh) * 100

Percentage of phone charge = (500mAh / 4000mAh) * 100 ≈ 12.5%

So, the fully charged 3.7V 500mAh battery can charge your 4000mAh phone by approximately 12.5% before it gets depleted.

#### dave9

Mar 5, 2017
1,190
^ Except you forgot the losses boosting the 3.7V battery to 5V for USB input to the phone, then the losses charging the phone battery.

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