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All About Portable Solar Panel Battery Chargers for Laptops

portable-solar-panel-battery-chargers-for-laptopsChoosing a solar charger for your laptop computer isn’t easy. Solar chargers for mobile phones are simple. They support USB 5 volt output from the solar panel or the power bank (built-in or plug-in your own), at 1 amp (5 watts) or 2 amps (10 watts). This works because almost all new phones can be charged from a USB port.

Unfortunately solar chargers for laptops are more complicated. A laptop draws 10 to 20 watts or more (the supplied power brick will be rated at a higher wattage, so don’t use that as your laptop’s power rating), at voltages ranging from 12 to 19 volts. Current solar products for laptops aren’t well developed – you might need to mix and match your own solution.

So the first step is to use a power meter, such as the Kill A Watt, to measure how much power your laptop actually draws. Measure this with the battery fully charged already, so you don’t get confused by the additional power used to charge the battery.

Solar Power Configurations

There are unfortunately, many system configurations to choose from.

(1) Sun —> Solar Panel —> Laptop

This is what most people will have in mind when they think of a solar charger. While such a system can work, the solar panel will have to be powerful enough to power the laptop directly, and have the correct output voltage (and electrical connector – plug or tip).

Many solar panels output voltage that is unregulated, that changes depending on the brightness of the sunlight. If you do choose this configuration, check the solar panel’s output voltage, and that it is regulated.

(2) Sun —> Solar Panel —> Solar Battery —> Laptop

Electrical power from the solar panel is weak and varies depending on how much sunlight the panel gets (depending on the time of day and the cloud cover). The solar battery stores or concentrates the electricity from the solar panel over the course of the day, so that it can push out the stored energy at high power when you need to charge your laptop.

The solar battery’s output will need to match the laptop’s input voltage (and have the correct electrical connector – plug or tip – to fit your laptop).

A “solar” battery is a normal rechargeable battery, we’ll call it a solar battery to differentiate it from other batteries.

(3) Sun —> Solar Panel —-> Solar Battery —> Inverter —> Power Brick —> Laptop Computer

If your laptop’s input voltage is not the same as the solar battery’s output voltage (usually 12 volts), you’re going to need an inverter to convert to AC volts, then plug in your laptop’s AC power brick.

This isn’t ideal because the extra equipment eats up some power and adds weight, but often you don’t have a choice. This is the most flexible configuration, useful when you need to power more than one type of laptop. This is not a system you’ll want to carry in your backpack every day, but does make sense on a vehicle or a rural outpost.

With an inverter, you can run almost any standard electrical device, including a CPAP (breathing device for sleep apnea sufferers). For CPAP use, check that the battery is big enough to power the CPAP for the whole night (calculate the watt hours), and that the solar panel is powerful enough to recharge the battery within a day.

Now, let’s look at each component in detail. We’ll use Goal Zero’s products as an example. Not because they are necessarily the best but because they have a comprehensive family of products.

Solar Panels for Laptops

solar-panels-for-laptopsLead-acid batteries can be trickle-charged with a low current, so the solar panel doesn’t need to be very powerful. Of course, a trickle charge can take days to recharge the solar battery. So if you want to use the battery to power your laptop for a few hours every day, you’ll need a powerful solar panel.

Prices range from $50 to $100 and up (depending on power), not including a solar battery. Brands include Goal Zero, Brunton, Powerfilm, Powermonkey, Instapark, Voltaic, Joos, Anker, StrongVolt, SolarAid, and Battery Tender.

Portable solar panels are typically the size of a DVD cover or a sheet of paper when folded. They contain 2 to 4 panels that fold out to cover a larger area. There is no built-in solar battery. They are often paired with the manufacturer’s own solar battery but as along as they have a 12 to 15 volt output, you can use most lead-acid batteries. Rated power ranges from 4 to 16 watts and higher.

If your laptop draws 20 watts, a 10 watt solar panel should theoretically be able to charge the solar battery for 1 hour of laptop use for every 2 hours of sunshine. The actual figures will be worse. You won’t always get 10 watts from the solar panel, and there will be some power loss in the inverter and when charging the laptop’s battery. A margin of 50% should be safe – 1 hour of laptop use for every 3 hours of sunshine in the example above.

Solar Batteries for Laptops

A 40 watt-hour battery should theoretically power a 20 watt laptop for 2 hours. There will be some power loss in the inverter and in charging the laptop’s battery, and you don’t want to fully discharge the battery to preserve the battery’s lifespan (for lead-acid batteries), so aim for 4x the watt-hours that you need.

If you want to run a 20 watt laptop for 4 hours off the solar battery, you’ll need a 20 x 4 x 4 = 320 watt-hour battery.

(To calculate the watt-hours, multiple the amp-hours by the voltage).
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(1) Dedicated Solar Batteries

Solar panel manufacturers often sell solar batteries that are designed to work with their solar panels. The battery can be lead-acid or rechargeable lithium. You buy the solar panel and solar battery from the same manufacturer and just connect the cables. This is convenient but you’ll still need to check that the power, voltage and energy capacity (watt-hours) are enough for your needs.

One example of such a matched solar battery is the Goal Zero Sherpa 50 (58 watt-hours), which can work with the Goal Zero Nomad 13 (13 watt) solar panel. The Sherpa 50 contains a built-in charge controller (see below), so you don’t need to get a separate charge controller. There’s also a 19 volt output for laptops and an optional plug-in inverter.

For more watt-hours, Goal Zero has their Yeti series of solar batteries (Goal Zero confusingly calls them “generators”) that go up to 1250 watt-hours. For the average person, a dedicated solar battery is definitely the way to go.
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(2) General Purpose Lead-Acid Batteries

You can use a generic maintenance-free (sealed) lead-acid replacement battery as the solar battery. Such batteries are available for electric lawn mowers, jump starters or uninterruptible power supplies (UPS). Make sure that the battery is “deep cycle” which means that it is designed to be discharged to a low level with minimal damage.

Connectors should be a standard car cigarette lighter socket or Anderson Power Pole, or you might need to solder your own connectors. Battery capacity is typically 7 to 18 amp-hours at 12 volts, which works out to 84 to 216 amp-hours.

Lead-acid batteries need a charge controller to prevent overcharging. Don’t connect the solar panel directly to the lead-acid battery.

Sun —> Solar Panel —> Charge Controller —> Solar Battery —> Inverter —> Laptop

This is a relatively complicated solution, only for DIY enthusiasts who have some electrical knowledge.

Portable DC-AC Inverters

The inverter is the easiest part of the system to buy. Inverters have been around for years – designed for use in cars – and there are many good models available. Just check that the maximum output watts is sufficient for your laptop.

For powering electronics such as laptops, you’ll want to get an inverter that outputs “pure sine wave” electricity, not “modified sine wave.” These are more expensive but will result in fewer compatibility problems.

Solar manufacturers such as Goal Zero, often combine an inverter with their battery products.

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