Should we unplug our chargers each night?

If you own a smartphone of any description then you're probably familiar with seeing the battery warning level light flick on at some point during the evening. It's now become standard practice for phones to be charged up at least once every 24 hours, and the easiest time to do that is overnight when they're sat still doing nothing but waiting for the morning.
Is this really the most sensible option, though? Are we causing permanent damage to our handsets by keeping them charged for much longer than they need to be? (Assuming you get more than two hours of sleep each night, that is.) And just how much unnecessary power is being drawn by our chargers while we're snoozing away oblivious?

The charger facts

The good news is that a lot of progress has been made in the efficiency and performance of smartphone chargers in recent years, thanks to organisations like the International Energy Agency and the manufacturers themselves.
The IEA's One Watt Initiative helped get standby power usage down below a single watt and then below half a watt in 2013 for many electronics. What does that mean? Well, when your charger is plugged in but not connected to a device (while you're out at work say) it's only using the equivalent of a few pence worth of energy over an entire year.
That's assuming you're using a phone and charger made in the last few years and abiding by current regulations, of course; but the trend is moving in the right direction.
As Cambridge professor David MacKay points out, that half a watt equates to saving one hot bath's worth of energy every year if you unplug your charger when it's not in use. Multiply that by everyone who owns a smartphone and you get a lot of hot baths - but only a very small percentage of the energy that those people are using in total.
Of course not using energy is better than using it, but as Professor MacKay puts it: "Obsessively switching off the phone charger is like bailing the Titanic with a teaspoon. Do switch it off, but please be aware how tiny a gesture it is."

Overnight charging

That half watt figure goes up by a factor of ten (give or take) when your phone is actually plugged in to the charger, but we're still talking about a small amount of energy use overall.
A 2012 study at the Lawrence Berkeley National Laboratory found that an idle charger drew 0.26 watts on average; this figure goes up to 3.68 watts when a phone is attached and charging, and drops down to 2.24 watts when the phone is attached and fully charged. Overall, the cost is a handful of pounds over 12 months.
Npower puts the figure at £3.50 a year for the average household. Some quick and very rough calculations suggest that if all UK phone users unplugged after charging, enough energy would be saved to power a town the size of Canterbury - once you look beyond your own energy bill it suddenly it starts to take on more significance.

The Energy Saving Trust reckons that the average household can save £50-80 a year by unplugging all of the devices that are on standby or connected to the mains during the night, that includes smartphones, but also microwaves, television sets and routers.
As smartphones only take an hour or two to charge, the EST suggests doing it right after work rather than overnight. Even if your charger is plugged in and connected to nothing, it's pulling a small amount of energy from the grid.

Lithium ion batteries

Every smartphone on the market is now fitted with a rechargeable lithium ion battery. If these batteries are overcharged or completely drained, they can become very volatile and dangerous, which is why modern chargers are designed to stop those two things from happening: they will actually cut out the power once a phone reaches 100%.
Your phone is therefore getting 'trickle charged' through the night - dipping very slightly then getting topped up again. That's one of the reasons why it's important that you use the charger that came with your phone whenever possible, otherwise there's the risk of causing unnecessary damage to it.

MicroUSB has standardised the system somewhat, but it's still not worth the risk of something catching fire. If you're using the supplied charger and cable then much of the power management will be taken care of for you.
Besides the issue of energy drain, keeping a phone topped up at 100% for longer than necessary degrades the battery slightly faster. The difference isn't huge, but there is a difference: lithium ion batteries prefer not to be fully charged if possible.
Shallow discharges, where the battery drains for a short time and then is charged for a short time, work better than leaving it on charge constantly - it's a bit like someone going for an occasional walk rather than sitting slumped on the sofa all day long.

The smarter battery future

As we've already pointed out, a lot of great work has already gone into making chargers as reliable and as energy efficient as possible, whether or not a smartphone is attached to them, that's another reason not to cut corners when it comes to getting replacement chargers.
Technology developed by Californian startup Qnovo is promising to reduce the time it takes for devices to charge and increase the lifespan of batteries at the same time. We're also seeing batteries that can be recharged from your body's movement, doing away with power sockets altogether.

In the not-too-distant future, charging times could eventually get down to just a few minutes, which would make overnight charging a thing of the past. So while unplugging your phone after charging (and unplugging the charger too) can make a small difference to the health of your battery and your electricity use, it's a problem that's already close to being solved.
That's good news for the planet, the device that sits in your pocket and your household energy bills. In the meantime: consider changing up your nightly charging schedule.