Weird things have been happening in the world of portable electronics. Smartphones and tablets are now as powerful as the PCs of just a few years ago, and they have managed to reduce their power needs at the same time. Despite this, a modern flagship smartphone struggles to give you 24 hours of battery life with just moderate use. They are certainly better than smartphones from five years ago, which would be dead in half a day, but keeping charged is still a bit of a pain in the behind.

The mediocre battery performance is driven by a couple of things. Lithium batteries have definitely improved by leaps and bounds, but mobile hardware developers have opted to increase performance rather than extend battery life. There’s also a (rather silly) drive to make phones as thin and light as possible. Personally, I think we’re already as thin and light as anyone needs, but apparently smartphone companies won’t rest until you can chop carrots with the things.

This situation leads to plenty of battery-related anxiety, which has created a market for what’s now called a “power bank”.

Power Bank

What’s This Now?

Power banks are energy storage devices. In other words, they are portable batteries. Inside a power bank is a big lithium ion battery. At least, it’s big compared to the batteries that you get in phones. This battery is accompanied by a built-in charge controller and of course some ports for putting power in and then taking it out again.

Thanks to cheap and capacious battery banks, there isn’t much charge anxiety left in the world. Even better, battery banks themselves have started to gain more and more great features.

Why Does this Matter for Solar Users?

That’s all wonderful, but why should people interested in solar energy care about an external smartphone battery? I suspect you already know the answer, but for those in the back of the class I’ll explain it anyway.

Traditionally, we’ve used 12V lead acid batteries as a way to store solar energy collected while out and about. That’s still something you want to do, but mainly for things like camper van and boat batteries.

However, lithium ion batteries are incredibly useful thanks to their low weight and size, with the added bonus of high energy density. Not to mention that power banks are already set up to power your personal electronics. Times have changed and the sorts of mobile devices we want to keep alive while away from civilization work best with the humble power bank.

When you use a solar charger that directly provides USB power to charge phones and tablets, the charging stops as soon as a cloud comes between it and the sun. Obviously, when it’s dark it won’t work either. Power banks let you collect solar power so that you can use it later, when direct solar energy is not available.

The third main reason to combine solar chargers with power banks is the simple fact that you need to give up your phone while it is charging. You can take a power bank with you while leaving one to charge up. This way you don’t have to leave your phone with the solar panel. So what should you consider when looking at power banks that go with your solar charger?

Filled to Capacity

The first and most important thing you need to look at is the capacity of the power bank. This is generally listed as mAh or milliampere hours. You’ll note that this is the same unit the batteries in your devices are listed in.

So, in theory, if your phone has a 2500 mAh battery and the power bank is rated for 10,000 mAh, you should be able to fully charge it four times, right? Actually, that would be a little optimistic. For one thing, there’s some power lost when transferring electricity from one battery to another. There’s resistance in the wires and other components, which ends up creating heat instead of moving electrons where they are meant to go.

On top of this, the voltage of lithium ion batteries is different from the voltage that USB ports deliver power, which means that at a higher voltage you are getting fewer mAh from the battery. The circuit board inside the power bank has to convert the 3.7 volt lithium cells to 5 volts. That process eat some power and the device converts it back to 3.7 volts to charge its battery. On top of this, the usable capacity goes down as the voltage increases. Some power bank makers will list the capacity at 5 volts in the specs or even on the bank. That’s closer to the truth than the advertised capacity, although it doesn’t take some of the inefficiencies I mentioned above into account.

When it comes to picking the right capacity for your needs, I actually think you should buy the biggest power bank you can afford, provided that it will still fit into the available space. Otherwise you’ll end up with multiple smaller banks that need to be charged in turn.

solar power bank

Passing Through

Many solar chargers only have a single USB port, which means if you need to charge your phone right now, then you’ll have to unplug the power bank and swap them out. Some power banks have the ability to pass power through to another device, so the bank is plugged into the solar charger and then your phone plugs into the bank.

This is also useful if you want to charge two banks at the same time. This is not the most efficient way of getting things charged, but it is convenient! Whenever I can, I look for power banks that have pass-through ability.

Any Port in a Storm

The number of ports that a power bank offers can be a very important factor in deciding if it’s for you. Most larger power banks will have at least two USB ports. These ports are also not all made equal. One port is often a fast-charging port which will deliver power at a higher amperage, which means whatever you are charging will be done more quickly. Many USB-powered devices don’t support fast charging, so you might as well plug them into the other port. That second port is useful for things like USB fans and lights.

Sizing it Up

It’s not just the storage capacity that matters, but also the volume, weight, and dimensions. Although the actual lithium cells should weigh more or less the same if they have the same capacity, their shapes and sized can differ.

What’s more, the enclosures can be made from various materials and also be any size and shape. When picking out a power bank, you should keep in mind where you’re going to carry and store it. A power bank you have to leave at home is worthless; then again, if you have a power bank too big for your bag, it’s just as worthless.

Safety First

If you haven’t heard, lithium ion batteries are actually pretty dangerous. Because they hold so much energy, it also means they can be volatile. If they are punctured or otherwise physically abused, these batteries tend to explode.

They can also go bad if you overcharge them, which is why good power banks contain protection circuitry to make sure the battery doesn’t burn your house down. This is why you should avoid cheap power banks from the Far East. Only buy power banks that have US or EU safety certification. The few dollars you save buying deathtrap batteries just isn’t worth it.

On a good product you’ll get several sorts of protection. Overcurrent protection ensures that just the right amount of electricity gets shoved into the power bank. Overvoltage protection makes sure that the voltage stays in the right range. Finally, a good battery will have overcharge and discharge protection, making sure that the total charge in the battery stays within safe parameters.

In the context of solar charging, it’s also important that you shield your power bank from direct sunlight. An overheated lithium battery is pretty bad news. Likewise, your power bank should not get wet unless you buy a ruggedised model. Even then, during charging any waterproofing is compromised by the in-use port.

Who Wants to Live Forever?

Every lithium battery has to give up the ghost at some point. This is usually expressed as the total number of charge cycles the battery can go through before losing capacity. Typically a good quality battery will last about 500 to 1500 charge cycles. That might not sound like a lot, but if you completely used up a power bank every day and recharged it would be about two to four years before the battery starts to degrade and lose capacity.

Usually the battery will not fail all in one go, but will keep less and less charge as time goes by. This is the limit of current commercial lithium ion technology, so if you plan on making heavy daily use of the power bank, then keep that in mind. Your power bank won’t just suddenly die when it reaches the limit – it will slowly lose capacity until it is useless. That’s actually a good thing, since catastrophic failure isn’t a selling point.

Storing Power Banks

Lithium ion power banks are pretty good for long-term storage. The self-discharge rate for lithium batteries is pretty low. The right way to store these batteries is to keep their charge between 30 and 80 percent. So charge the battery up to 80% and then check on it every six months, charging it up to 80 percent again.

Be sure to store it somewhere that does not reach extreme temperatures, whether high or low. So no direct sunlight and not in a place that gets to below freezing. Moisture is also a no-no, as is high humidity. Basically, if you wouldn’t be comfortable then neither would the battery.


Power Bank Safety

Lithium batteries are incredibly safe, which is actually impressive given that this is one of the most volatile consumer battery technologies on the market. If it weren’t for international safety standards and protection technology built into power banks that meet those standards, I wouldn’t go near the things!

Despite this, lithium batteries do flame out from time to time. Whether it’s an accident or a poorly designed product, like the Galaxy Note 7, when lithiums go bad they do it with style. You need to be aware of the various things that can turn your power bank into a small explosive. Crushing, piercing, too much heat, and general abuse can all lead to a fiery end.

Traveling with Power Banks

Leading on from that, when you travel with your power banks you need to take some precautions. For example, don’t leave your power bank in a hot car, in direct sunlight, or any place where it will be knocked around.

When you take your power bank with you on a plane, you absolutely should not leave it in your checked baggage. The low air pressure can cause all sorts of issues, including making it blow up.

Apart from that, traveling with a power bank is pretty straightforward.

Filled to the Brim

That’s just about all the things I can think of you need to know when it comes to power banks. Combine them with a good solar panel and your portable power needs away from the grid will be completely covered!