The world, maybe more now than ever before, runs on battery power. So many everyday things – from your watch to your car to the cellphone you’re reading this on right now – are fueled by these little containers of stored electricity, that it’s easy to take the whole situation for granted.

Until, that is, the worst happens: your battery dies. Because as well-designed and long-lasting as a battery is, they all have a finite lifespan – and understanding why that is requires a little explanation of how batteries work at all, let alone why they stop.

How do batteries work?

Big picture, we all know what batteries are for: they release electricity on demand, so we can power things like cell phones or low poly mid-life crises. But how exactly do they do that?

Well, first things first: a battery isn’t, as you might have once imagined, just a cylinder filled with unused electricity sloshing about inside. “You cannot catch and store electricity,” explained Antoine Allanore, now a Professor of Materials Science and Engineering and Metallurgy at MIT, back in 2012. “But you can store electrical energy in the chemicals inside a battery.”

There are a huge number of different types of battery, but in all of them, the idea is the same: a chemical reaction takes place, which moves electrically charged particles from one terminal – the anode – to another, called the cathode. 

Depending on what you want to use your battery for, these terminals will be made from different chemicals. A lithium-ion battery – the rechargeable type you find in smartphones or electric cars – will use some form of lithium compound for the cathode, and (usually) some kind of graphite as the anode. In an alkaline battery – the kind you probably actually think of when you hear the word “battery” – the cathode is made from manganese dioxide, and the anode is zinc.

When the battery is connected up to a circuit, electrons move through the external wires – that’s what creates the electrical charge – while ions travel through the third crucial component of the battery: the electrolyte, which separates the anode and cathode. 

“These two reactions happen simultaneously,” Allanore said. “The ions transport current through the electrolyte while the electrons flow in the external circuit, and that’s what generates an electric current.” 

Why do batteries stop working?

Assuming your battery is alkaline rather than lithium-ion, the reason it eventually stops working is simple. The chemical reaction producing the current has to stop once it runs out of reactants – that is, when the zinc has fully reacted with the manganese dioxide. At that point, it’s flat.

“Because the chemical reactions will only start when the terminals are connected, an unused battery can sit on a shelf for a year and still pack plenty of power,” explains the Open University. “However, a battery only contains a fixed amount of reactants, and, once these have been used up, the chemical reactions stop – the battery is dead!”

However – and as anybody who’s owned a car in bad weather will already know – there are a few ways to speed up the process of your battery dying. Temperature is a big one: cool down a battery, and the chemical reactions inside will occur more slowly, producing less current than a battery at a higher temperature. Go far enough down the thermometer, and there comes a point where the amount of current being generated just isn’t enough to power your doodad, and the battery is functionally, albeit temporarily, dead.

Equally, letting a battery get too hot is a surefire way to… well, literally to ensure fire, sometimes. Let a lithium-ion battery overheat, and they can – quite notoriously, in some cases – fall prey to thermal runaway: a “phenomenon in which the lithium-ion cell enters an uncontrollable, self-heating state,” explains safety science organization UL Research Institutes, which “can result in extremely high temperatures, violent cell venting, smoke and fire.”

Batteries will also die faster if they’re leaky – a real and potentially quite dangerous phenomenon which occurs when some kind of break in the battery container allows its chemical components to escape. The reason this shortens battery life is pretty self-explanatory: there’s less reactant inside the battery, so it runs out faster.

Finally, there’s the thing you always suspected, but shook off thinking it couldn’t really be true: yes, your smartphone really does lose charge faster when its battery is already low. This is because the battery’s voltage drops as it discharges, and much faster right at the end of its life – your device, on the other hand, continues to require the same amount of power. 

Since power is equal to current times voltage, the only way to maintain the same level is for the amount of current to increase when the voltage runs low.

What about rechargeable batteries?

This does all raise the question, though: what’s going on with rechargeable batteries? Surely, if batteries die because they run out of reactants, then that’s the end of it, right?

Well in fact, the solution is both pretty simple and strikingly clever: you just run the reaction backwards.

“In a rechargeable battery, electrons and ions can move either direction through the circuit and electrolyte,” explains the Department of Energy’s Office of Science. “When the electrons move from the cathode to the anode, they increase the chemical potential energy, thus charging the battery; when they move the other direction, they convert this chemical potential energy to electricity in the circuit and discharge the battery.” 

“During charging or discharging, the oppositely charged ions move inside the battery through the electrolyte to balance the charge of the electrons moving through the external circuit and produce a sustainable, rechargeable system.”

Now, rechargeable batteries can still fail permanently – and why exactly that happens is an area of ongoing research, since the reasons tend to be chemistry-specific. Part of the problem is that the chemical reaction can’t be 100 percent reversed, and so gradually, the internal chemical structures of the battery simply change from what they were originally.

 “All batteries show performance losses during their service lives that involve a progressive decrease in capacity […] and increase in internal resistance, leading to voltage decay and loss of power,” notes one 2016 review into the phenomenon.

Like disposable batteries, though, exactly how fast this degradation happens can be “substantially different […] depending on storage or usage conditions,” the authors write. So basically, if you want to extend the battery life as much as possible, treat ‘em right.

And last of all: once your batteries are well and truly gone, don’t just throw them out. “Batteries can contain metals such as mercury, lead, cadmium, nickel and silver, which can pose a threat to human health or the environment when improperly managed at the end of their service life,” points out the Environmental Protection Agency.

Chuck them in landfill, then, and they’re basically just going to sit there poisoning anything that gets too close – and if that’s, say, the drinking water, then that might just include you.

Your best bet, therefore, is to recycle them, with the precise method depending on what kind of battery you’re dealing with. “Certain batteries should NOT go in household garbage or recycling bins,” the Agency points out. 

Oh – and about those exploding lithium-ion doozies: that can still be an issue, even if they’re dead. “To prevent fires from lithium-ion batteries, tape battery terminals and/or place batteries in separate plastic bags and never put these batteries in household garbage or recycling bins,” the Agency advises.

Overall, though, “waste batteries can always be recycled,” they say, “or taken to household hazardous waste collection points.”  

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.