Solar charger

The use of personal electronic devices in the outdoors continues to grow, creating demand for portable power solutions to recharge these devices. Portable solar panels have become the preferred power source.

This article will help you match the portable power technology that’s best suited to your adventure, travel and power needs.

Shop REI's selection of portable power devices.

The Power Supply Chain

By understanding the basic components of the power supply chain, you are well on your way to finding the right technology for your needs.

In short, you use or recharge your electronic device using power stored in a portable battery, which was received from an initial power source. Listed in actual order:

Power
SOURCE

Power
STORAGE

Power
USE
Energy source to charge the battery pack

 

A portable battery pack to store energy

 

Electronic device using stored energy

Power Sources

Several sources of power can be used to charge up a power storage unit (battery pack) or your electronic device directly. The most common sources, from fastest to slowest:

  • AC power (wall outlet)
  • 12V DC (car charger)
  • USB (any source: wall, car, computer or even the BioLite Wood Burning CampStove)
  • Solar panel
  • Kinetic motion
  • Hydrogen fuel cell

To recharge your portable power pack when away from wall outlets, vehicles or USB output, you'll want a battery pack that can be recharged by solar power or another means.

Solar Energy

Solar charger

This is the most common off-the-grid source of power available. Consider using a solar panel for supplemental energy if your electronics use will exceed the available power stored in a supplemental portable battery.

Solar energy is your best option for long trips when multiple recharges of your devices are desired.

Tips on Choosing Solar Chargers

Several popular solar panel options are available:

  • Panels only, either rigid or semi-flexible
  • Panels with integrated storage batteries
  • Panels with independent (separate) storage batteries

 The main variables to consider:

Surface area: The larger the solar panel, the more sunlight it collects and the faster it gets converted to power stored in a battery. A smaller panel, though easier to pack, takes longer to charge a battery. Large surface area is also best for conditions such as cloud cover or the low-angled, low-intensity light in winter, or when logistical constraints limit how long you can have it exposed to sun. Charge times can vary from 4 to 16 hours of sunlight for the same battery, depending on surface area and light conditions.

Output capacity: Solar panels are rated in watts. The higher the number, the more electricity is generated during a given time period.

Semi-flexible or rigid panels: Semi-flexible solar panels can be folded or rolled up for easy transport and open up to provide a greater surface area than many rigid panels. Consider, too, a solar panel’s attachment options so you can securely attach it to your backpack, bike, kayak or tent.

Shop REI's selection of solar chargers.

Hydrogen Fuel Cells

The most recent innovation in portable power devices is to create electricity from hydrogen (a chemical reaction creates the electricity). This requires the use of a consumable hydrogen cartridge. The voltage it generates is low, but it is available anytime.

Power Storage (Battery Pack)

Independent power storage

A solar energy producer is best used to charge up a storage battery rather than the electronic device directly. Why?

These power sources may not contain any circuitry to regulate the flow of electricity into your electronic device, potentially leading to damage. Check the manufacturer’s recommendations; some solar panels offer a regulated USB output for direct charging of small electronic devices.

  • An integrated storage battery offers all-in-one convenience of a power generator to create power and a battery to store it, for later recharging use.
  • An independent storage battery offers the flexibility to leave the generator (e.g., solar panel) at home or at base camp, and take only the battery.

If you are buying a solar panel that comes with a storage battery, pay attention to the battery specifications. This may be the most important piece of the package.

Key Specs for Portable Battery Packs

Storage capacity: This tells you how much charge the battery pack can store for use later to recharge your devices, before it in turn needs recharging.

By comparing the storage capacity of a portable battery to that of the battery in your device, you can get an idea of how many recharges you have available. This is usually stated in milliAmp hours (mAh) or Amp hours (Ah). For example, 2200 mAh = 2.2 Ah. Watt hours (wh) is another measure of capacity. To convert watt hours to mAh: (Wh /Volts) x 1000 = mAh

Tip: Transferring energy from one battery to another is not 100% electrically efficient. Thus, a power pack rated at 2000 mAh will not fully recharge a 1000 mAh electronics battery twice.

To recharge or top off your electronic device, you need a battery pack with enough storage capacity and enough output voltage to move energy into your device. 

How much is enough? Find out the storage capacity of the battery in your device. For electronics with built-in batteries, check the technical specs to find the battery capacity. If your device uses only replaceable AA batteries, this is not an issue. Choose a battery pack that features these.

Power output: The output of the charger (measured in volts) must be equal to the input battery voltage requirement of your electronic device. If it’s lpwer, you may drain your device’s battery instead of charging it up. Oops!

Most portable battery packs will charge small personal electronics like cell phones and mp3 players, but relatively few can recharge a laptop battery.

Small electronic devices that can be charged with a USB cable need a 5V output rating.  Larger electronic devices that require a DC power input (for example, laptops) may need a 12V – 24V output rating. This is available from the largest portable battery packs.

Battery technology: Portable battery packs vary in not only capacity and output, but also in technology, size and weight.

  • NiMH (Nickel Metal Hydride) batteries are likely to be rechargeable AA or AAA batteries. A battery pack that uses these is a convenient option for devices that use replaceable batteries like a GPS, camera or headlamp. Instead of recharging your device, you simply swap out the batteries.
  • Lithium-ion and lithium polymers are the most common type of portable battery packs, being the same technology as the built-in batteries in personal electronics.
  • Lead acid batteries are large and heavy but provide the greatest power capacity and output.

Output connectors and adapter tips:

  • If you buy a solar panel with an included battery—either integrated or independent—any necessary connectors between panel and battery will be included.
  • If you buy a solar panel and battery individually, note what output connector it has and its suitability for charging your device directly (if recommended) or connecting to a separate battery pack. Your options may include a USB (standard, mini, micro), a connector (with a selection of adapter tips) or DC output with voltage control.

Power Use

How do you find the voltage input rating for your electronic device? 1) Look in your owner’s manual under technical specifications, or 2) Look for the voltage rating printed or stamped as “DC output” on the wall outlet charger that you likely got with your device. Then make sure your portable battery pack can deliver at least the same voltage.

Listed below are battery capacity and input voltage examples for selected electronic devices. Keep in mind that internal battery capacity varies between makes and models.

Personal Electronics

Internal Battery Capacity

Voltage Input Rating

GPS with rechargeable
lithium battery

2000 – 2400 mAh

3.7 – 7.4V

MP3 player

600 – 1000 mAh

3.7 V

Cell phones

850 mAh

3.7 V

Smartphones – power input using a brand-specific tip

1000 – 1700 mAh

3.7 V

Smartphones – USB power input

1700 – 2100 mAh

5 V

E readers / tablets – USB power input

1250 – 4400 mAh

5 V

Netbooks, laptops – DC power input

3500 – 6600 mAh

8.5 -19.5 V

Practical Considerations

Still deciding? Here are some REI staff expert tips that can help:

Trip duration: For a weekend trip, a battery pack may be plenty. However once it is used up, it becomes unproductive weight. Off-the-grid trips of a week or more are likely to need a solar charger or other generator.

Tip: Fully charge your devices and power pack right before your trip. Even if you did so a couple of weeks earlier, it doesn’t mean they are still holding a full charge. 

Mode of travel: For a solar charger to be effective, it requires lengthy exposure to direct sunlight. Some activities like cycle touring and sea kayaking are well suited to “on-the-go” solar charging. Backpacking in open country can be suitable if you can orient the panel to face the sun. But if you are hiking under a deep forest canopy, there is not much point in strapping a solar charger to the top of your pack.

Battery type: If your electronic devices use only AA batteries, consider a solar charger that can boost up a spare set of batteries while others are in use in the device. This way you can rotate 2 sets of batteries instead of carrying spares.

Tip: Avoid completely draining a device’s battery before recharging it.

Size and weight: All chargers take up space and add weight. Is it worth it? Do you need unlimited power no matter what? Or is one emergency charge enough? The bigger (and heavier) a power pack, the more storage capacity it has to provide multiple charges. For solar chargers, consider their built-in attachment points: Can they be tied down or staked out so they don’t disappear in a wind gust?

Battery packs offer a supplement, not a guarantee. Do not assume that these products will bring your device’s battery up to a full charge. 

All rechargeable batteries have a usable life that is usually measured in discharge/recharge cycles. Not all manufacturers provide this figure, but you can assume a battery pack will have a minimum life of 500 cycles, with up to 1,000 cycles being common.

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