In a healthy battery, ions flow freely between the anode and the cathode, and back again. Batteries degrade mainly because the surfaces of these electrodes become encrusted with oxidised electrolyte, and because other “parasitic” reactions follow on from that. Apple says that the lithium-ion batteries in its iPhones lose about 20% of their capacity after 500 charge cycles; most manufacturers rate their devices at about 300-500 cycles. Every time you recharge your laptop, you shave a few seconds off its battery life.

Many of us have the idea that it’s better to charge your phone all the way up, and then to use it until the battery’s nearly dead. This is quite wrong. Lithium-ion batteries don’t need to be fully charged; in fact, a high voltage stresses the battery. Most of the time, you’re better off charging it to 80% and then plugging it in again if it gets below 50%. Don’t let it drop to zero, and only charge it to 100% once a month or so. Leaving it to charge overnight is also bad. It keeps the battery in a high-tension state, wearing down the chemistry within. Batteries hate high temperatures, so don’t leave your phone in the sun, or your laptop on a bed with its cooling vents blocked. And if you’re not going to use a device for a bit, try to leave it charged to about 50%.
(The Week 15 December 2018)