A mobile phone is about as useful as its battery life: it wouldn’t really be a mobile phone without a charged battery after all. But while most of us protect our phones with sturdy cases, we don’t seem to invest as much effort in protecting our phone battery’s lifespan. Today the vast majority of mobile phones (and tablets and laptops) use lithium-ion batteries. Lithium-ion offers many advantages (its higher energy density means smaller batteries, which in turn mean lighter, sleeker phones, for example) but these batteries still inevitably degrade with use. Anyone who has used a mobile phone for many years will probably notice that its battery life seems to get shorter over time, perhaps lasting only half a day even after a full charge. That’s because every time your phones lithium-ion battery charges and discharges energy it loses how much charge it can hold.