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Here’s the truth behind the biggest (and dumbest) battery myths

Yes, keeping your phone on charge overnight is bad news for its battery. And no, you don't need to turn off your device to give the battery a break. Here's why

For an object that barely ever leaves our palms, the smartphone can sometimes feel like an arcane piece of wizardry. And nowhere is this more pronounced than when it comes to the fickle battery, which will drop 20 per cent charge quicker than you can toggle Bluetooth off and give up the ghost entirely after a couple of years of charging.

To make up for these inadequacies, we’ve made all kinds of battery myths. Whether it’s avoiding leaving your phone on charge overnight, or powering off to give the battery a little break, we’re forever looking for ways to eke out a little more performance from our overworked batteries, even if the method doesn’t make an awful lot of sense.

To help sort the science from the folklore, we asked a battery expert to give their verdict on some of the most pervasive myths, explain the science behind the rumours and, just maybe, offer us some sage advice on extending the life of our smartphones.

Even when your battery is at 100 per cent, there’s still room for some more charge

There is more juice in your smartphone battery than the percentage displayed suggests, but if you used that juice you’d end up dramatically reducing the overall lifespan of the battery. At the crux of this problem is a delicate trade-off played by manufacturers. Increasing the available charge within a battery reduces the number of times that battery can be charged and discharged without being damaged internally. To make batteries last for hundreds or thousands of charge cycles, manufacturers place limits on the amount of juice that batteries can discharge.

To understand why, you need to know a little about how batteries work. The guts of most lithium-ion batteries, like the ones in smartphones, laptops and electric cars, are made of two layers: one made of lithium cobalt oxide and the other of graphite. Energy is released when lithium ions move from the graphite layer to the lithium cobalt oxide layer. When you charge up a battery, you’re simply shifting those lithium ions back the other way – out of the lithium cobalt oxide layer and back to the graphite.

This is where we get to the problem with battery life and charge cycles. Shift too many of those lithium ions out of the lithium cobalt oxide layer, and the whole structure of the layer messes up. “The atomic structure of the material actually falls apart if you remove all that lithium,” says Kent Griffith, a researcher on energy storage at University of Cambridge.

So while it is possible to charge a battery beyond 100 per cent, the only way to do that is to pull out more of those crucial lithium ions. “It’d be like pulling all of the supports out of the floor of a building,” Griffith says. You could get the lithium ions out, but good luck putting them back once you’ve messed up that internal structure.

That’s why manufacturers set limits on the amount of charge in their batteries. Most of the time, they’re set so only around half of the lithium in the lithium cobalt oxide layer is removed during one full charge. “Your battery could give you more charge if you went beyond removing half of the lithium, but you wouldn’t be able to do that very many times.”

Having Wi-Fi and Bluetooth on in the background is a big drain on battery life


Apart from the screen, one of the biggest drains on battery life is the energy your phone wastes trying to find and connect to Wi-Fi or data networks. If you’ve ever noticed your battery plummeting while on a train, it’s probably because your device is working overtime to connect to a mobile network. “If you can connect to something stable, like if there’s WiFi on the train, it’s probably better to connect to that,” Griffith says. Reducing screen brightness and the time it takes your phone to go to sleep are also easy ways to extend your battery life.

Using an unofficial charger damages your phone


Not all phone chargers are created equal, and that could have a damaging effect on your phone’s battery life. Chargers have all sorts of controls that limit the amount of current delivered and stop it charging when the battery is full, but some off-brand chargers might not have such rigorous safety settings.

And if too much current is delivered to a battery, that could mean ripping out too many of those lithium ions and leading to the same kind of degradation you read about earlier. That’s not to say that all off-brand chargers will be this bad, Griffith notes, but you’re still probably better off sticking with an official model.

Charging your phone through your computer or laptop will damage the battery


If anything, charging a little more slowly is probably good for batteries, Griffith says. This goes back to those lithium ions again – are you sensing a theme here? The more slowly you charge a battery, the less strain that’s put on lithium ions and the structures accepting them, and the less potential damage to the battery. That’s why manufacturers place limits on devices so they don’t charge too quickly.

Powering off a device occasionally helps preserve battery life


This one is a myth too, but not a completely unfounded one. Before the lithium-ion battery became ubiquitous, the nickel metal hydride battery was the rechargeable battery of choice. In those batteries, it was impossible to get an accurate reading of the battery charge level without fully discharging and then recharging the battery. “If they were half discharged and recharged, you’d lose where you were. So you’d have to fully discharge to keep track,” Griffith says.

In Lithium-ion batteries, that’s no longer the case. Modern batteries are capable of reading their state no matter their level of charge, and when your device isn’t in use the strain on the battery is almost the same as if it was off altogether, so you wouldn’t be giving the battery much of a break if you turned it off anyway.

Batteries perform worse when they’re cold

False (mostly)

Actually, the opposite is true. “Using your battery in cool temperatures and keeping your battery cool is much better for battery life,” says Griffith. Exposing your battery to high temperatures is a much more likely way to end up reducing its overall life. “You don’t want your battery to be hot. You don’t want it to overheat when it’s charging, you don’t want to leave it in the sun or in your car.”

But why do batteries hate the heat so much? The reason has to do with the liquid electrolytes that fill the gaps between the lithium cobalt oxide and graphite layers (remember them?) and stop the two components from touching. This is what the lithium ions travel through when they’re shuttling between the two layers, so it’s pretty important for the battery structure.

At high temperatures, these liquid electrolytes start to break down, causing the battery to degrade over the course of just a few hundred charge cycles. This is a major issue for electric vehicle batteries, which often spend much of their day sitting out in bright sunlight. For your smartphone, however, as long as you usually keep it at around room temperature you’re doing okay.

It’s possible that your phone might be a little slower in cold temperatures, and that’s because those lithium ions move slightly slower which means the battery might not be able to provide as much power to components if it’s really cold outside. The change is generally only slight, however, and isn’t linked to any permanent damage to the battery.

Leaving a charger plugged in at the wall and turned on wastes energy

False (well, maybe a tiny bit)

With phone chargers and other ‘dumb’ cables that just have a wire, they probably aren’t drawing any energy at all if there’s no device plugged in. When it comes to TV or laptop cables – or any charger that has a big ‘brick’ attached to it – these are a little more clever as they often draw a small amount power while they’re essentially waiting for the TV or other device to boot out of standby mode. In the past, the energy draw of these devices was up to 10 per cent of the average household’s energy bill but recent regulation changes mean that these now draw a relatively tiny amount of energy.

You should let the battery get all the way down to 0 per cent before recharging


Strangely enough, batteries are under the most strain when they’re fully charged or completely empty. The real sweet spot for a battery is 50 per cent charge as that means that half of its moveable lithium ions are in the lithium cobalt oxide layer and the other half are in the graphite layer. This equilibrium puts the least amount of strain on the battery, and extends the number of charge cycles it can withstand before degrading.

So really, if you were super-keen on keeping your battery living as long as possible, you should keep its charge between 20 and 80 per cent. This means that it spends as little time as possible with lots of lithium ions crammed into either layer, a situation which causes the layers to expand, putting physical strain on them. “But if you did that you’d only be getting about half as much charge every time you used it,” Griffith says. Maybe not, then.

Charging past 100 per cent will damage your battery

True (but not for the reason you think)

This one is closely linked to the above myth. Charging your phone so it stays at 100 per cent overnight isn’t great news for the battery, but that’s not because you’re cramming in more charge than it can handle. A ‘trickle charge’ mechanism cuts off the charger after the phone has reached 100 per cent charge, and only tops up the battery when it drops down a little.

The problem is that you’re keeping the charge level at 100 per cent which, as we know from the previous myth, puts the battery under a certain amount of strain. “It’s not good,” Griffith says, “but the battery manufacturer has set [the battery’s limits] it so it shouldn’t be harmful.”

This article is part of our WIRED on Energy series.

May 23, 2020

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