Transform a 25-Watt Semi-Flexible Panel into a Practical Battery Charger


Sunlight may be free, but transforming it into usable electricity is not. I learned this back in 1965 when I bought some surplus silicon solar cells from dealer Herbach and Rademan (now H&R Company, Inc.). Those 0.75″×0.75″ cells cost around $2.49 each or $19.30 today according to the CPI Inflation Calculator.

In 1975, I used nine of those solar cells to make a charger for two AA penlight cells that powered my flashlight during cross-country bicycle trips. I placed the cells on a 3″×6″, ⅛”-thick acrylic sheet after they were soldered together, covered them with Silastic adhesive and placed a thin sheet of clear plastic over the Silastic. I added a blocking diode between the positive output and the positive battery holder terminal to keep the batteries from discharging through the solar cells at night. This provided a waterproof solar charger (Figure A) that works as well today as it did 42 years ago.


Figure A. The author’s DIY solar panel from 1975.

Advantages of Portable Solar Power

Today’s silicon solar cells are more efficient and cheaper than those I bought in 1965. They are also available in a wide variety of preassembled, waterproof, and even foldable and semi-flexible (Figure B) arrays with built-in blocking diodes. Semi-flexible arrays are much lighter than those installed in metal and glass frames. Non-silicon arrays are also available, but they don’t provide the efficiency and long life of silicon.


Figure B. Foldable and semi-flexible solar panels.

Portable solar power is ideal for charging phones and flashlight batteries during bicycle trips, hikes, and camping. It’s especially handy during extended power failures. But solar power also has drawbacks.

The Cost of Portable Solar Power

Cost is the major drawback of portable solar power. As these words are being typed, my iPhone SE is consuming 5.3 watts (0.0053 kW) while being charged through a Kill A Watt power monitor. This phone requires 2.5 hours to be fully charged from empty. Assuming the power consumption during charging is constant (mine declines as it charges), a full charge will consume 2.5×0.0053 kW or 0.01325 kW/hrs.

According to the U.S. Energy Information Agency (, in May 2017 the average cost of electricity in the 50 states was 13.02 cents per kilowatt-hour. At this price, fully charging my phone every day for a year costs only 62.97 cents.

This test is the first time this phone has been charged by line current since spring 2016, when I began charging it from a lithium power pack charged by a 25-watt, semi-flexible solar panel with a combined cost of more than $150. I consider this a reasonable expense for hikes, camping, and emergencies.

Operational Problems

Solar power works only when the sun is shining, even when thin cirrus cloak the sky. But thicker clouds significantly reduce maximum energy production from a panel. Another consideration is that solar panels should be pointed toward the sun for best results. This is a good excuse for me to get away from my writing desk several times a day to adjust the position of my panels. Another issue is that solar panels can become very hot, especially during summer days. While I’ve not yet been able to fry an egg on a solar panel, I’ve come close (Figure C). Lying a panel on grass on a hot day will bake the plants (Figure D). The heat problem is why rechargeable battery packs with built-in solar chargers may not be a good idea, at least during summer.

Solar Charging Cameras, Mobile Phones and Tablets

Compact panels like the foldable array in Figure B are equipped with a USB port for directly charging devices that have a USB power port. While these panels can be easily carried in a backpack, they must be used with caution to avoid overheating the device they are charging. Advertising photos sometimes show these panels adjacent to or even behind a phone, which means the phone’s internal battery pack might overheat. Instead, it’s best to place the battery or device being charged away from both the panel and direct sunlight.

Indirect charging is when a solar panel charges a battery pack that later charges various devices. For the past year, an ATOTO Ultra UPS Power Source has resided on the left side of my desk, where it is now charging my phone and a tablet computer. Every 10 days it is recharged outdoors with a 100-watt solar panel. While the ATOTO is apparently no longer available, the Chafon CF-UPS018 346WH Portable Power Supply is a very similar unit with better capabilities. I also have several compact lithium power packs with built-in charge controllers that can be charged by 15- to 20-volt solar panels. While some of these are no longer available, many USB-compatible battery packs are available that can be charged by solar panels with a USB output. Figure E shows my 25-watt panel charging a power pack during a hot summer day when the temperature was 98°F. When the power pack reached a temperature of 115°, I moved it indoors.


Figure E. Connecting a solar panel’s wires to the power plug of a storage battery.

A DIY Portable Solar Charger

When traveling, I carry a foldable solar panel suitable for charging devices and battery packs with a USB port (Figure B). On long trips and at my home office, I use 25-and 100-watt semi-flexible panels designed to charge high-capacity battery packs at 15 to 20 volts. The 25-watt panel has been modified to charge a RAVPower Model RP-PB14 lithium battery pack, a ChargeTech (expensive) and an ANKER Mobile Power 79AN20L (discontinued). Like most high power panels, the 25-watt panel came with positive and negative cables terminated with weatherproof MC4 connectors designed to interconnect two or more panels in series or in parallel. Here’s how I transformed the 25-watt semi-flexible solar panel into a practical battery charger (Figure F):



Figure F. A thick layer of insulation will protect a battery pack from the heat of a solar panel.

» Clip off both power leads three inches from the plastic terminal at the top of the panel and remove 0.5″ insulation from the end of each wire.

» Use a multimeter to determine the polarity of the leads while the panel is exposed to weak sunlight or indoor incandescent lighting. Mark the upper corners of the panel with the respective polarities (+ and -).

» Slide 1″ lengths of heat-shrink tubing over each of the panel wires.

» Remove 0.5″ insulation from both ends of a pair of 16″ red and black #12 stranded wires and insert the wires through the two uppermost eyelets from the back side of the top of the panel (red = + eyelet and black = – eyelet).

» Observe polarity and solder the new wires to the two solar panel wires.

» Slide heat-shrink tubing over the soldered connections and apply heat.

» Solder the free ends of the red and black wires to a power plug compatible with the power input socket on a compact battery pack designed for solar charging at 15 volts–20 volts (not USB). Or, to guarantee compatibility, clip a 16″ section of the power brick output cable supplied with the battery pack and solder it to the red and black solar panel leads.

Caution: Observe polarity.

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Transform a 25-Watt Semi-Flexible Panel into a Practical Battery Charger

About The Author

Muhammad Bilal

I am highly skilled and motivated individual with a Master's degree in Computer Science. I have extensive experience in technical writing and a deep understanding of SEO practices.

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