It's a familiar story. You buy a new phone, and for a while the battery is good. You take your phone out in the morning, spend all day at work, and still have more than enough charge left for the journey home. But fast forward a year or so and everything's changed. You can practically see the battery meter tick down—or at least you could if you hadn't completely dimmed the screen in a futile attempt to keep your music playing just long enough to reach your front door.

Clearly, something has changed. But why is it that smartphone batteries seem fail so quickly?

In some respects, the problem is that smartphones require more power than ever thanks to more complex processors and larger screens, while batteries are getting smaller as companies push to make phones thinner. But that's a general trend. We're interested in why your specific battery doesn't work as well as it used to.

Smartphones, like many portable electronics, use Li-ion batteries because they're much lighter than other batteries and hold their charge for a long time. They work by moving Lithium ions between two electrodes – a Lithium-Cobalt Oxide cathode and a Carbon (graphite) anode. When you charge the battery the ions collect on the anode, and when you discharge it (to power a device) the ions move back to the cathode. This process is called 'cycling', and it's an accepted part of Li-ion battery design.

If the cycling process were 100% efficient, your battery would never get worse. But as you've probably guessed, it isn't. Each time you charge a battery, a film of Lithium atoms remains bonded to the anode, which reduces its capacity. If the ions can't move, they can't transfer their charge and therefore can't deliver power. The next time you charge it, another layer is deposited.

Once that process is replicated a few hundred times, you'll start to see a noticeable drop in amount of power a battery can store. An increasingly thick layer of immobile Lithium (in the form of Lithium Oxide and Lithium Carbonate) collects on the anode, obstructing interaction with the graphite.

But that's not the whole story. The inefficiencies of cycling a battery cause a constant but gradual decline in capacity. But if you think your battery's capacity has dropped off suddenly and without any obvious cause you're not necessarily imagining it.

Just as the anode can get covered with a film of material by the charging process, the cathode can also develop a similar layer because of something called electrolyte oxidation. The hotter a battery gets (or the higher its voltage), the quicker and more damaging the reaction is. The cathode's reactive abilities are immediately impeded, causing a sudden and irreversible capacity loss – one which is more immediately noticeable compared to the nature cycle of charge and discharge.

The end result is that the lithium-ion reaction used to deliver power can no longer happen properly, and the battery won't retain or deliver as much power as it did when it was new. Essentially, the reason your smartphone battery stops working is because the electrodes inside have gone rusty.

But knowing why your battery keeps dying is only half of the problem. How can you use this information to protect your phone's battery life?

One important thing to do is avoid exposing your battery to extreme temperatures. Heat above 35 degrees Celsius noticeably accelerates the cathode's decline. Battery capacity is also diminished at low temperatures, but this is (usually) a temporary effect. For ideal performance, keep your battery at a temperature between 16-22 degrees Celsius. This may mean removing any phone cases during charging and even unplugging it during charging if it gets too hot.

Speaking of charging, here's another tip: don't charge your battery to 100%. It might seem counter-intuitive, but unless you need a full charge for portability's sake the battery will fare better on a partial one. High temperatures actually damage the battery by raising the voltage, and when you charge a battery to 100% the voltage is also raised to its limits.

Unlike some rechargeables, Li-ion batteries are not negatively affected by a partial charge, so ideally you should prevent a full charge occurring unless you really will need that extra few percent. You'll stretch the battery's lifespan as far as possible if you maintain the charge between 20-80% whenever possible.

It's worth noting that you should also keep devices partially charged during storage or periods of disuse. The batteries need to retain some power to keep their internal protection circuits active, and allowing them to fully discharge (a 'deep discharge') will destroy its ability to hold charge at all. On a day-to-day basis, Li-ion batteries prevent this from happening by claiming to be empty while they still have some power left, but they can fail if they're then left to self-discharge beyond that point. If you store a device for a long period of non-use, charge it to around 50%. This keeps the voltage low (which protects the cathode) but retains enough capacity to keep the protection circuits active for potentially months.

Unfortunately, these techniques only prevent a battery from losing its efficiency. There's not much you can do to rejuvenate an aging battery without specialist equipment—but at least you can now stop yours getting any worse!

This post originally appeared on our UK site.