Unlocking the Volt - A guide to Solar Chargers
The following is a detailed and in-depth look into solar chargers and their components, authored by our intrepid General Manager, Simon Alsop. If you're in the mood to be educated, grab a cup of tea and a bickie and settle in. If you just want a general waist-deep wade in the subject, feel free to skip the "Battery" section. On the other hand, if you're adventure-ready and strapped for time, jump right down to the "Product Reviews" section.
For a complete, easy-to-read comparison chart of all the solar chargers and their respective capabilities, click here.
Extended, multi-day bushwalks and treks are now often accompanied by an array of ever more power hungry battery powered devices for communication, navigation or entertainment, all of which may require recharging at some time during the trip when access to mains electricity is limited or impossible. There are a number of portable solar chargers available which utilise the free power of the sun to help with this to varying degrees.
Considerations when looking at portable solar chargers are:
- Size and output of solar panels
- Battery storage capacity
- Weight
- Price
Size and Output of Solar Panels
Measurement of Power
Power is measured in watts and the larger the solar panel, the more power it will generate. More useful solar chargers have solar panels of between three and seven watts. Though some solar chargers exist with a one watt panel, realistically they are a relic of a bygone era belonging to the iPod Mini and Nokia 6100 and just too small to cope with the demands of modern electrical gadgets.
Most lightweight style solar charges have panels which output around 5 volts so that they have the voltage differential necessary to charge devices or their own battery, which are mostly 3.7V Lithium Ion (Li-ion). As watts = volts x amps, a higher power panel can deliver a higher current flow at this voltage, giving it the ability to charge faster.
Solar Efficiency
A 3W solar panel in good sunshine for one hour will, in theory, provide 3 watt-hours (3Wh) of power. So, if left in the sun for ten hours it should be capable of generating 30Wh. The Watt-hour is the universal measure of battery capacity and will be discussed later, enabling one to work out the ability and speed of a solar panel to charge a battery.
The wattage stated by the manufacturer is a maximum and assumes best light conditions. Unfortunately solar panels are not very efficient. When charging a battery energy is lost through heat and, in the real world, the sun is rarely at the optimum angle and distance, can be momentarily obscured by passing clouds, is filtered through air pollutants or gets shaded slightly by tree branches. Maximum Power Point Tracker (MPPT) technology is used in some panels to help reduce this loss in less than perfect conditions; however, it can be expected that a solar panel will deliver only 25% of its rated power. So the 3W panel sitting in 10 hours of sunlight is likely to deliver not 30, but 7.5Wh, meaning much longer charge times.
What do they charge?
Solar panels do not store charge themselves but are used to charge a rechargeable battery pack which can then hold the charge for use at a later time. Some panels may be used to directly charge the battery of a device, such as a phone or iPod, though smaller panels are unlikely to generate enough current for the device to recognise, or the charge rate will be so slow as to be impractical. Despite this, it is recommended that you always connect the supplied battery pack between panel and the device you are charging as the battery pack acts as a capacitor to ensure even current flow. Some more modern devices can be sensitive to current flow.
Size Matters
The solar panels are the generators which convert the sun’s rays into electricity to charge batteries and devices. Whilst solar panels are clever technology, they are not miracle workers. Simply, the larger the solar panel the more electricity generated, and therefore the faster the charge or the greater the ability to charge higher capacities. This is an important practical consideration as there will always be a limited amount of day, or sunlight. Modern smartphones run their batteries down within a day, so it is not very useful if a solar charger takes longer than this to recharge it.
Batteries
Internal or external?
Portable solar chargers generally have a battery which the panel charges, either integral (built into the panel) or separate. As well as being charged by solar, all can also be charged by mains or USB.
There are three major reasons for the battery:
- The battery builds and stores a higher capacity than the panel can deliver directly, which can then effectively be more quickly unloaded into a device.
- The batter can be used to store energy for use when actually needed, when it is more convenient or when the sune is not shining.
- When the battery is fully charged, it acts as a capacitator between the solar panel and device being charged to ensure consistent flow of charge in changing sun conditions.
With integrated battery solar chargers, such as the Solarmonkey Adventurer, the solar panel’s job is to charge its own internal battery, and devices are always recharged using that storage. This makes the solar charger simple, neat and tidy, but a little less versatile. Because the charge to the device comes from the battery in the charger, there must be already sufficient power within it to begin to charge the device. If the integral battery is low or flat then there will be a delay before it has been charged by solar to a sufficient power (around 25-30% of its capacity) to begin charging the device. In this case the device is likely to continue to lose power for a while and, if it is flat, remain unusable itself for up to an hour or so.
Units with a separate battery are more versatile as the battery pack can be used separate from the solar panel and conveniently carried during urban travel as a backup, recharging from USB or mains. If the solar panel is capable of direct charging devices this can be relied on more in the bush, again leaving the charged battery pack as a backup. Most solar chargers can charge their own battery and a device simultaneously (though the Powermonkey Explorer cannot) and some, such as the Goal Zero Guide 10, can charge multiple devices at one time. Bear in mind, however, that as there is a limit to the power provided by the panels, the more devices added, the longer the charge time for each.
Types of battery
Lithium Ion (Li-ion) 3.7V batteries are the most common type used in solar chargers. Li-ion are lightweight, hold their charge for a long time and have a lifespan of around three years (ever notice how your smartphone charge doesn’t last as long as it did when new?). They do not need to be charged and discharged fully each time and should be stored with a partial (c60%) charge.
The Goal Zero Guide 10 is an exception in that its separate battery pack uses Nickel-Metal Hydride (NiMH) batteries. NiMH batteries are a little heavier, have a voltage of 1.2V and are used as multiples (in this case four) in series to create a higher voltage (of 4.8V). Their advantage is that they come in traditional battery sizes such as AA or AAA which can be used and interchanged in hand-held GPS, torches or Steripens. Traditionally NiMH batteries self-discharge at a much higher rate so are best used and charged regularly, though Goal Zero batteries and newer technology Sanyo Eneloop NiMH batteries have been developed with a very low self-discharge rate. They benefit from being fully discharged and charged occasionally and have a longer useful life of up to five years. As NiMH batteries are readily available, they are easily replaceable or added to in order to gain more storage capacity.
Battery capacity
When discussing relatively low powered batteries the common method of measuring battery capacity is “milliamp-hours” (mAh) (a millamp is one thousandth of an amp). It is not however the true measure of “energy capacity” (or power) of a battery as it doesn’t take into account the battery’s voltage. A better and more universal measure is the watt-hour, which does take voltage as well as current into account (Wh = Ah x V). Therefore two batteries of 2,200mAh each, but one with a voltage of 3.7V and one at 5V, will have different energy capacities, of 8.1Wh and 11Wh respectively. The battery capacities of the solar charger batteries can be found in the table at the end of this guide.
Solar charger battery capacities range from 8.1Wh for the Goal Zero Switch 8, to 33.3Wh for the Powermonkey Extreme. For a solar charger it is important that the battery size is balanced by an adequately sized solar panel, otherwise the long recharging times from solar will become unreasonable and impractical. Larger (and therefore heavier) battery capacities are best suited for shorter trips, or trips when the sun is less likely, so the battery can be pre-charged from mains and then may have enough capacity to cope with the demands over a few days without recharging quickly or fully. For longer trips where there will be an increased reliance on solar energy it is better to look at a larger solar panel, which is capable of recharging its (albeit smaller) battery, or batteries, quickly and regularly.
Battery charging speeds from solar
Taking into account the inefficiencies of solar panels discussed earlier, a rule of thumb is that it takes 1 watt of solar panel to charge 2.5Wh of battery capacity in ten hours of good sunshine. Using this rule of thumb, an approximate charge time can be worked out using this equation:
Charge Time from Solar (Hrs) = Battery Capacity (Wh) x 4
Solar Panel Output Rating (W)
This may result in a different time than claimed by the manufacturer, though is possibly more realistic for use in the real world. Charge times range from around six hours for the Guide 10 Plus to forty four hours for the Powermonkey Extreme, which uses a solar panel of less than half the size to charge a battery three times the size.
What devices can be charged, and how many times?
The battery packs in the portable solar chargers all have an output voltage of 5V, the ideal voltage to recharge the 3.7V Li-ion batteries used in most hand held devices such as iPods and smartphones. And so will at least partly charge them (depending on the capacity)A Canon DSLR battery, on the other hand has a battery of 7.4V and so cannot be charged by a 5V charger. Currently the only battery to be able to do this is the new Powermonkey Extreme battery which has a separate 12V output port to provide the necessary voltage differential.
Due to inefficiencies in charging (such as energy being lost through heat), it takes 25% more energy to charge a Li-ion battery to its full capacity. Therefore only 80% of the charging battery’s capacity will be used to charge a phone or iPod’s Li-ion battery. The equation below gives an indication of the number of times the device can be charged from one of these batteries:
Number of complete charges = Charging Battery Capacity (Wh) x 0.8
Device Battery Capacity (Wh)
Battery capacities of common devices are listed in the chart accompanying this guide along with number of charges which can be expected from each charger.
Device charging speed
The speed at which a device (phone, iPod etc) will charge depends not upon the charger but the device itself, which is designed to draw the amount of current it needs regardless of what is on supply. Even if its mains charger can provide 1,200mA, a smartphone only draws around 600mA. The USB outlet from a laptop delivers 500mA, which means that the phone will simply charge a little slower from a laptop than mains. The outlet currents supplied by the charger batteries are listed in the accompanying chart. Depending on the demands of the device, the slowest battery charge one could expect (Powermonkey Discovery at 500mA) would be the equivalent to charging from the USB on a laptop. All of the others should charge smartphones and iPods at the same rate as they would from the mains. All supply a current less than an iPad takes from the mains, so charge (or part charge) of an iPad will always be slower, though Goal Zero offer the fastest at 1,000mA.
Weight
Most solar chargers and their battery packs have a weight of between 400 g and 500 g. Carrying a smartphone, iPod, Kindle and GPS is likely to add another half kilo to your pack. Whilst weight is an important consideration if the charger is to be carried for a length of time, it is more important to choose a solar charger which is capable of charging what you want, when you want. Choosing a charger because it is 150g lighter than another could result in a kilogram of electronic gadgetry quickly becoming dead weight, which would have been better left out altogether.
Weights stated on packaging and in specifications can be misleading. The Solarmonkey Adventurer including its padded case weighs 50% more than stated, at 400 g, whereas the Powermonkey Extreme actually weighs 50% less, at 500 g!. Actual weights listed in the accompanying chart include solar panels, battery, cables and case, but leaves out any mains charging items. In effect, it represents the kit one would reasonably expect to take on an extended self-supported trip or hike.
Tips for using solar chargers
- Start off the trip with the charger's battery, and all devices, fully charged from mains or USB.
- Make the most of sunshine. Always be charging or topping up something with your solar panel.
- Position, position, position. When stationary, angle solar panels perpendicular to the sun and move them around to follow its track. When hiking, a panel still works better strapped to the outside of a pack than inside it.
- Avoid shade, even a little bit, falling on the solar panels.
- Change from the 'urban' approach to charging. Charge 'little and often', and don't allow devices to run down fully before plugging in.
- Solar chargers are even less efficient when behind glass.
- The most powerful solar panels are heavier, as are higher capacity batteries.
Product Reviews
Powermonkey Discovery
$94.95
Though not actually a solar charger, it is a useful backup battery for recharging electronic devices.
Sold at an affordable price, it is simple, small, compact and lightweight and has the decent capacity of 13Wh, enough to charge a smartphone twice.
The Discovery does not deliver enough current to charge, or partly charge, an iPad and needs to be recharged itself from mains, USB or car outlet.
Best suited: Urban travel, commuting, short trips and bushwalks which will involve only a couple of days away from a power source.
Goal Zero Switch 8
$119.95
A lightweight battery pack accompanied by a decent sized solar panel at a reasonable price. The Nomad 3.5 panel gives an output of 3.5W, which will charge the 8.1Wh battery pack in around nine hours. Goal Zero panels are very robust (check out Goal Zero “Power Wars” on YouTube) and are large enough to charge phones direct (note: Goal Zero’s website shows the Switch 8 with the larger Nomad 7 panel, which may be a change for the future in Australia)
The Li-ion battery is small and lightweight and can also be charged via USB or mains, cutting the charge time down to around four hours. It will charge a smartphone about 1.2 times, will give a 25% boost to an iPad and can be used separately from the solar panel as a day to day backup charge.
Best suited: Commutes, travel, extended trips and bushwalks where access to power source will be very limited. For one person with small number of small devices.
Powermonkey Explorer
$139.95
A small and lightweight kit which, due to the rapid development of and increasing reliance on technology, is already a relic from a bygone era.
The solar panel is cute but too small, at 1W, to be taken seriously. It simply cannot provide enough power, quick enough, to keep up with the demands and usage of a smartphone and it takes over twenty hours (nearly three days’ worth) of sunshine to recharge its own battery.
The battery is small and lightweight and can also be recharged from the mains or USB, which is faster. Though its capacity is amongst the smallest at 8.1Wh, it is still enough to fully charge a smartphone, so it could be useful as a day to day backup device.
The Explorer sits somewhat in no-man’s land. It is bettered, at significantly lower prices, by the Powermonkey Discovery as a travel backup device, and by the Goal Zero Switch 8 for more remote use.
Best suited: Urban travel and commuting. Anyone who still uses a Nokia 6100.
Solarmonkey Adventurer
$159.95
A neat, simple all-in-one solar charger. Significantly heavier than the advertised weight if the handy, protective case is used, though if that is left behind it is the lightest of the realistic solar chargers available. The Adventurer’s 9.3Wh battery is integral with the 3W solar panel and is easy to use; just open, plug in and charge. The battery will recharge a smartphone about 1.5 times and can be recharged itself via solar in about 12 hours (a day and a half) of quality sunlight or in around five hours via USB and mains.
The integral design means it is less versatile and more bulky to be used as a travel, commute backup as with other devices. With no option to charge devices directly from the panels (all devices are charged through the internal battery), there may be a significant delay in beginning to charge a device as the battery has to have a decent amount of residual charge before it can begin to pass it on. Though there is enough capacity to boost an iPad by about 30%, the delivery of the current is at only 700mA (iPads normally charge at well above this) and so the charge is likely to be very slow. There is no battery level indicator, other than a light which turns from green (‘some’ charge), to red (‘no’ charge). An oddly archaic measure for modern technology.
Best Suited: Single person use with small devices. Multi (3-4) day trips or bushwalks
Goal Zero Series - Switch 8, Guide 10, Venture 30, Nomad 7 Smart, Sherpa 50
$149.95
A very versatile kit made up of a powerful Nomad 7 (7W) solar panel and a separate Guide 10 battery pack which, unusually, takes NiMH AA sized batteries with a capacity of 11Wh. It has the quickest charge times of any of the chargers here and charges the battery pack in around 6 hours, making the use of solar realistic rather than just a nice idea.
The panel can also charge devices directly, again at a decent rate, with a smartphone taking four hours compared to three via the mains. The panel can charge multiple devices at once though this will slow down charge times to each. Goal Zero pride themselves on the robustness of their panels which is demonstrated on YouTube (“Goal Zero Power Wars”).
As the Guide 10 battery pack is made up of four AA NiMH batteries, these can be charged and used in other devices such as headtorches or GPS. It will also charge AAA NiMH batteries, in around two hours by solar. The four AA batteries will recharge a smartphone about 1.2 times and give a 30% boost to an iPad. Whilst this is not a particularly large capacity, the charge speed makes it less of an issue and the feature unique to the Guide 10 is that the battery capacity can be added to, easily and inexpensively, at $20 for another 11Wh (four extra AA batteries).
The Guide 10 battery pack can be charged from USB and mains (both slower than from solar due to a restriction on the current coming through the USB) and used separately from the solar panels for short trips and commutes as a lightweight, easily carried backup.
The solar panels are relatively heavy, which makes up most of the Guide 10 kit’s weight of 540g but its power and charging speed mean it can be used to charge devices for a number of people. Whilst the panels do have a 12V outlet it is not possible to direct charge a Canon 7.4V DSLR battery and the battery pack does not provide adequate voltage for that either. The Guide 10 would benefit from having a more sophisticated battery level indicator. Green and red LEDs are a crude measure making it difficult to see exactly how much charge remains.
Best Suited: Commutes and travel (Guide 10 battery pack) and (with Nomad 7 solar panel) extended trips and bushwalks where there will be unreliable or infrequent access to power source. Multi person (1-3) use depending on devices used.
$159.95
Since its release the Venture 30 has pretty much become the benchmark for lightweight adventure travel. And the battery on its own is great for people going about 3-4 days away from mains at a time. Its 29Wh, 7800mAh capacity meets the demands of modern devices like smart phones, GPS’s, GPS watches, iPads, eBooks, GoPro’s etc. But it is its rugged design, smart power regulation and the fact it can charge 2 devices at once that sets it apart.
The Venture 30 is extremely tough and you don’t have to be careful with it at all. You don’t need to keep it in a case as it is extremely shock resistant, dust proof and even without the convenient rubber bungs in the USB ports it still rates to IPX 6 – meaning water from any direction or quick dunks in the drink are fine. We tested this and ran it under a hose then plugged the iPhone straight in and away it went. The only reason to put it in a waterproof case would be if you’re diving or perhaps swimming through canyons or something. The other aspect about its ruggedness which needs a mention is how resistant it is to losing power in the cold. As you are probably aware, batteries hate being cold. On an alpine trip where the temps were regularly below 0C I tested this against a regular ‘city’ style spare battery of similar capacity. While at normal temperatures they both hold similar charges – the city one died after a couple of days just being in my back while the Venture was still full and performed the similar to at standard temps.
Power regulation is the other key to this battery. Not every device likes being charged at the same voltage and amperage as another. Smart phones and particularly Apple products require a specific current to charge them in the lowest time or even start charging them at all. The Venture 30 has a smart regulator built in which recognises which device is plugged in, then delivers the optimum current to charge it in the lowest time.
Best Suited: Those off the grid for 3-4 days that need a tough battery. Trekkers, climbers, alpine climbers, adventure travellers and commuters will still reap all the benefits of this product
$269.95
The Venture 30 battery paired with the Nomad 7 solar panel. If you are planning to be off grid for a little while – say more than 4 days – than the Venture 30 Solar kit is the way to go. All the above attributes of the Venture 30 battery pack with a 7W, Nomad 7 Solar Panel. In optimal sunlight the panel will fully charge the battery in about 8 hours but realistically with moving around and the occasional cloud etc. you’re looking at about 10-12 hours. As long as you continue to top-up your battery as you use it this becomes quite manageable as you will need about 3 hours of sunlight to boost it up after a Smartphone has been charged.
Best Suited: Those off the grid indefinitely that require a tough kit. Alpine expeditions, long treks or those who love travelling without plans and don’t know how long they might be away from mains power.
Powermonkey Extreme
$249.95
A solar charger which pairs the solar panel of the Adventurer with a high capacity battery.
The solar panel is the 3W of the Adventurer but without the integrated battery, making it lighter and more versatile. It is possible to charge devices direct from the panel.
The major feature of this charger is the size of its battery, the largest capacity of any of the chargers featured here, at a huge 33.3Wh. The battery can charge a smartphone up to 5 times and an iPad once. The large storage capacity also means that it is able to boost to a 12V outlet too, in order to charge a 7.4V DSLR battery just over three times. It is the only charger here to be able to do so.
Charging the battery from solar is a long process due to the mismatch between its capacity and the relatively small solar panel. The manufacturer claims that in optimum sunlight conditions it takes at least 22 hours, the best part of three full days of sunshine, which is not really a practical possibility. Charging from the mains will take around 10 hours, meaning recharging a fully discharged battery still requires a reliable power source for a significant time.
At 260g, the battery is not lightweight but it is probably acceptable, and the only real option, to the power hungry gadget freak who wishes to completely recharge an iPad.
Best suited: Travel and work where a number of power hungry devices are used and there may be times when a reliable power source are missing. Trips and bushwalks of up to a week with smaller devices. Three or four day bushwalks with a low probability of sunshine (winter or tree cover).
Goal Zero Nomad 7 Plus Smart Solar Panel
$169.95
This device approaches solar charging from a different angle. Up until now the standard procedure has been to pair a panel with a battery pack. Even if the battery pack is fully charged you run the panel through the battery then into your device. This makes the battery act as a regulator to even out the current as the current from solar panels fluctuates a lot as their angle to the sun changes, cloud cover or shadows fall on the panel. If you were to run a standard panel straight into an iPhone the phone would quickly come up with an error message as soon as the current dipped, then would stop charging.
The Nomad 7 Plus Smart Panel now comes with a smart regulator which firstly recognises which device is plugged into it, then delivers the consistent charge that it requires. If for some reason the panel is unable to deliver the required current due to cloud cover etc. then it turns off the charge. The panel continues to check the available sunlight constantly and as soon as it can deliver the required current it kicks back in again.
So what does this mean when it comes to using it in the outdoors? It means now that you can forego the heavy battery and as long as you are on top of charging your devices while there is available sunlight you can keep power to everything indefinitely. The panel itself is super skinny and super light weight at a minimum weight of 260 grams. Thought though would be needed as to whether to pair it with a Venture 30 battery if you want power at night or are expecting longer periods of bad weather.
Best Suited: Super weight conscious travellers, trekkers, climbers or adventure types who are confident they will get a reasonable amount of sunlight. Theoretically you could be off the grid indefinitely with this panel but pairing with a Venture 30 battery might be wise.
$499.95
This is the only option if you want to power a laptop, DSLR or any other 12V product. While it is not particularly small or light (1.4kg for kit) compared to the other options, chances are if you’re taking a laptop or SLR than you don’t really need it to be. Great for base camp style trips or if there is at least 2 people to share the load. The battery has a 58Wh (11V 5800mAh) capacity and the 13W panel will fully charge in 3 hours off the mains or 8-16 hours off the panel depending on quality of sunlight.
It features multiple power plug options to optimally charge the devices you have and an inverter can even be purchased separately so you can plug a standard 240V wall plug into it. Plug options include:
- USB port (output): 5V, up to 1.5A (7.5W max), regulated
- 6mm port (output, 6mm, green, hexagon): 12V, up to 8A (100W max), regulated
- laptop port (output, 7.4mm, orange, square): 19V, up to 5A (100W max), regulated
- sidecar port (chain, 9mm): 9-13V, up to 10A (100W max)
- AC inverter US (output, sold separately): 110V, up to 0.7 (75W max), 60Hz, modified sine wave
- AC inverter Int. (output, sold separately): 220V, up to 0.35A (75W max), 50Hz, modified sine wave
- charging port (input, 8mm, blue circle): 14-25V, up to 2A (30W max)
Best Suited: This kit is perfect for photographers or those doing work in the field that require electronic equipment. Also just as perfect for those travellers and trekkers who just can’t be without their devices – iPads, laptops etc. while on trail. Great for expedition base camp setups with a couple of people needing to use it.
For a full, easy-to-read comparison chart of all the solar chargers and their respective capabilities, click here. (link to pdf Unlocking_the_vault_comparison_chart_BG copy)