How to Solar Charge Lithium Ion Batteries for Smart Phones




About: After being laid off in 2009, I got rid of my car to save money. The difficult transition from a car to a bicycle led me to start Solarcycle and design USB chargers that keep smart phones alive.

Please click below to visit our Kickstarter project page for Solderdoodle Pro 2.0, which uses the same circuit as our solar charger, and pre-order a production model!

This Instructable is a tutorial on how to solar charge lithium-ion batteries for smart phones. These images were taken from presentation I created to show how the Solarpad worked. The inspiration for this design was from my trip to Portland, OR and living a car-free lifestyle. My phone would run out of power constantly while riding my bike and I invented Solarpad to keep my phone charged while on my bike.

Step 1: Why Do Most Solar Chargers Underperform?

Right now the market is saturated with cheap solar products from China that have crappy batteries and solar cells. Usually there are no serial numbers on the batteries and the solar panels are coated with cheap epoxy that absorbs precious UV radiation that makes up most of the solar energy before it hits the solar cell underneath. This over saturation has caused solar products to become disposable and unreliable, which is the opposite of what is need for rugged outdoor use like on a bicycle.

Also, the competitive pricing has caused manufacturers to overstate the performance of their products, which has lead to customer confusion and mistrust in the solar industry. You'll see on Amazon that an 18650 battery is rated even higher than what Tesla is using, which is hard to believe. You'll also see on Amazon a solar panel that claims to be powerful enough to charge two iPhones, but in reality it can barely charge one iPhone.

Step 2: The Solution: Careful Solar Panel and Battery Selection

Solarpad uses a high-quality, custom Suntactics solar panel, which has a fluoropolymer film with the best light transmission, precision cut and tabbed high-efficiency solar cells, and a high-strength substrate. The Panasonic NCR18650B battery uses a high-efficiency, high-power charge controller from Maxim and uses the same battery that's used in the Tesla electric car, which is the best battery of its kind. Even the built-in USB cables consist of thicker gage wire, which allows the electricity to flow more efficiently. The Solarpad Kit is engineered to squeeze the most energy possible from the sun's rays in to your smart phone. No short-cuts that compromise power or efficiency are taken, period. A fully charged battery will recharge a dead iPhone from 0% to 100% in 3 hours even after being unused for a month! Other similar sized solar chargers can only charge a Smart Phone from 0% to 70% and take over 4 hours to recharge. Much larger smart phones, like the Samsung Galaxy S4, have a battery that's twice the size of an iPhone battery, but a fully charged Power Core can still provide a Galaxy S4 a 0% to 70% charge.

If you notice, the charge curve of a Lithium-Ion battery is NOT linear. It starts out at a trickle charge if the battery is drained until it gets to a minimum voltage. Then, the maximum charge current is reached up to about 80% charge and then it goes back to trickle charging until 100% charge. Due to this charge scheme, doubling the current to a Lithium-Ion battery doesn't reduce the charge time by 1/2, it reduces the charge time to 1/3 because the trickle charge at the beginning and end of the charge cycle doesn't change.

Step 3: Lithium Ion Charging Details

Whatever Lithium-Ion battery you choose, make sure that the battery manufacturer provides the charge profile for it and select the charge controller charge curve to match the battery recommended charge curve.

If the manufacturer doesn't provide this data, DO NOT use them because unknowingly mismatching the charge curves may destroy the battery and cause burns.

Using evaluation boards from charge controller companies helps with designing the circuit because you don't have to create a custom PCB.

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    4 Discussions


    4 years ago on Introduction

    Do you have any suggestions for a charge controller?

    What are the two white cables coming from what I assume is the grey battery pack?

    I really enjoyed the instructable and a 1400mAh battery is exactly what I'm looking for in a battery pack. The information about the charge profile was great; but would of loved to of seen more details about choosing an eval board.

    1 reply

    Reply 3 years ago on Introduction

    With 1400mAh I would suggest not pulling more than 500mA current unless you want to try and manage the heating battery issue. Anything above .5C current (multiply this number by the capacity of your battery) and most batteries will need to have temperature monitoring and possibly active cooling.

    If you're running below .5C and you don't need to drop the battery voltage below 3V, most charge controllers can handle this. You really need to be careful when going below 3V on the battery and above .5C current draw.


    4 years ago on Introduction

    This system has a charge and discharge current of 2Amps max to and from the battery.


    4 years ago

    Which is the maxium current?