by Chris Woodford. Last updated: March 21, 2020.
Power to go—aren't batteries brilliant?
The trouble is, they store only a fixed amount
of electric charge before running flat, usually at the most
inconvenient of times. If you use rechargeable batteries, that's less
of a problem: click your batteries in the charger, plug in, and in a
few hours they're as good as new and ready to use again. A typical
rechargeable battery can be charged up hundreds of times, may last
you anything from three or four years to a decade or more, and will
probably save you hundreds of dollars in buying disposables (so it's
brilliant for the environment too). But exactly how well your batteries
perform depends on how you use them and how carefully you charge
them. That's why a decent battery charger
is as important as the batteries you put into it. What is a battery charger and how does
it work? Let's take a closer look!
Artwork: Why use hundreds of batteries once when you can use one battery
hundreds of times by refilling it with electric charge? Rechargeable batteries cost a bit more to start with, but,
treat them carefully, and they'll save you a fortune over the years that they last.
They're much better for the environment too.
What are batteries and how do they work?
Photo: Ordinary batteries (like this everyday zinc-carbon battery)
are not designed to be used more than once—so don't attempt to recharge them. If you
don't like zinc carbon batteries, don't start trying to recharge them: buy rechargeable ones to begin with.
If you've read our main article on batteries,
you'll know all about these portable power
plants. An example of what scientists refer to as electrochemistry,
they use the power of chemistry to release stored electricity very
What happens inside a typical battery—like the one in a flashlight?
When you click the power switch, you're
giving the green light to chemical reactions inside the battery.
As the current starts flowing, the cells (power-generating compartments)
inside the battery begin to transform themselves in startling but
entirely invisible ways. The chemicals from which their components
are made begin to rearrange themselves. Inside each cell, chemical
reactions take place involving the two electrical terminals (or
electrodes) and a chemical known as the electrolyte
that separate them. These chemical reactions cause electrons (the
tiny particles inside atoms that carry electricity) to pump around
the circuit the battery is connected to, providing power to the
flashlight. But the
cells inside a battery contain only limited supplies of chemicals so
the reactions cannot continue indefinitely. Once the chemicals are
depleted, the reactions stop, the electrons cease flowing through the
outer circuit, the battery is effectively flat—and your lamp goes
That's the bad news. The good news is that if you're using a rechargeable battery, you can
make the chemical reactions run in reverse using a battery charger.
Charging up a battery is the exact opposite of discharging it: where
discharging gives out energy, charging takes energy in and stores it
by resetting the battery chemicals to how they were originally. In
theory, you can charge and discharge a rechargeable battery any
number of times; in practice, even rechargeable batteries degrade
over time and there eventually comes a point where they're no longer
willing to store charge. At that point, you have to recycle them or
throw them away.
How battery chargers work
Photo: This "fast-charge" battery charger is designed to
charge four cylindrical nickel-cadmium (nicad) batteries in five hours or
one square-shaped RX22 battery in 16 hours. It's easy to use, and just as easy to misuse:
it doesn't switch off when the batteries are fully charged and there's nothing to
tell you when charging is complete. With a battery charger like this, charging your batteries
is complete guesswork.
All battery chargers have one thing in common: they work by feeding an
through batteries for a period of time in the hope that the cells inside will
hold on to some of the energy passing through them. That's roughly
where the similarity between chargers begins and ends!
The cheapest, crudest chargers use either a constant voltage or
constant current and apply that to the batteries until you switch them off.
Forget, and you'll overcharge the batteries; take the charger off too soon and you won't charge them
enough, so they'll run flat more quickly. Better chargers use a much lower, gentler "trickle" charge (maybe
3–5 percent of the battery's maximum rated current) for a much longer
period of time.
Batteries are a bit like suitcases: the more you pack in, the harder it is to pack in any more—and the longer it takes. That's easy to understand if you remember that charging a battery essentially involves
reversing the chemical reactions that take place when it discharges. In a laptop battery,
for example, charging and discharging involve shunting lithium ions (atoms missing electrons) back and forth,
from one electrode (where there are many of them) to another electrode (where there are few).
Since the ions all carry a positive charge, it's easier to move them to the "empty" electrode at the start. As
they start to build up there, it gets harder to pack more of them in, making the later stages of charging harder work than the earlier ones.
Graph: Batteries get harder to charge in the later stages. It can take as long to charge the last 25 percent of a battery (orange area) as the first 75 percent (yellow area). It's worth remembering this if you
have limited time to charge a battery and worry that it'll take too long: you might be able to charge it
halfway in much less time than you think.
Overcharging is generally worse than undercharging. If batteries are fully charged and you
don't switch off the charger, they'll have to get rid of the extra
energy you're feeding in to them. They do that by heating up and building
up pressure inside, which can make them rupture, leak chemicals or
gas, and even explode. (Think of overcharging as overcooking a
battery and you might just remember not to do it!)
Photo: The Innovations Battery Manager, popular in the 1990s, was sold as an intelligent battery charger capable of recharging even ordinary zinc-carbon and alkaline batteries. Right: A digital display
showed the voltage of each battery as it charged (in this case, 1.39 volts). After charging, a little bar graph appeared showing how good a condition the battery was in (how many more times you could charge it). Many thousands of these chargers were sold, but there were
on how well they worked.
Slightly more sophisticated timer chargers switch themselves off after a set period,
though that doesn't necessarily prevent overcharging or
undercharging because the ideal charging time varies for all sorts of
reasons (how much charge the battery held to begin with, how hot it
is, how old it is, whether one cell is performing better than others,
and so on). The best chargers work intelligently, using
microchip-based electronic circuits to sense how much charge is
stored in the batteries, figuring out from such things as changes in
the battery voltage (technically called delta V or ΔV) and cell temperature
(delta T or ΔT) when the charging is likely to be "done," and then
switching off the current or changing to a low trickle charge at the
appropriate time; in theory, it's impossible to overcharge with an
Charging different kinds of rechargeable batteries
To complicate matters, different types of rechargeable batteries respond best to different
types of charging, so a charger suitable for one type of battery may
not work well with another.
Photos: An electric toothbrush typically contains either nicad or NiMH batteries and slowly or trickle charges on a stand, which is actually an induction charger.
(also called "nicad" or NiCd), the oldest and perhaps still best
known type of rechargeable batteries, respond best either to fairly
rapid charging (providing it doesn't make them hot) or slow trickle
Nickel metal hydride (NiMH) batteries use newer technology and look exactly
the same as nicads, but they're generally more expensive because they can store
more charge (shown on the battery packaging as a higher rating
in mAH or milliampere-hours). NiMH batteries can be fast charged (on
high current for several hours, at the risk of overheating), slow
charged (for about 12–16 hours using a lower current), or trickle
charged (with a much lower current than nicad), but they should
really be charged only with an NiMH charger: a rapid nicad charger
may overcharge NiMH batteries.
Expert opinions seem to differ on whether nickel batteries experience what's widely known as the memory effect. This is the well-reported phenomenon where failure to discharge a nickel-based battery before charging (when you're "topping up" a partly discharged battery with a
quick recharge) reputedly causes permanent chemical changes that reduce how
much charge the battery will accept in future. Some people swear the
memory effort is real; others are equally insistent that it's a myth.
The real explanation for an apparent memory effect is
voltage depression, where a battery that hasn't been fully discharged before charging temporarily
"thinks" it has a lower voltage and charge-storing capacity than it should have.
Battery experts insist you can cure this problem by charging and discharging
a battery fully a few times more.
It's generally agreed that nickel-based batteries need to be "primed"
(charged fully before they're used for the first time), so be sure to
follow exactly what the manufacturers say when you take your new
batteries out of the packet.
How long should you charge rechargeable batteries?
There are two simple reasons why there are so many different sizes and types of batteries:
a bigger battery has more chemicals inside it so it can store more energy
and release it for longer; bigger batteries also tend to have more cells inside them so they can produce a higher voltage
and current to power bigger things (brighter flashlight bulbs or higher-powered motors).
By the same token, bigger rechargeable batteries need charging for longer.
The more energy you expect to get out of a rechargeable battery
(the longer you expect it to last), the longer you'll need to charge it
(or the higher the charging current you'll need to use). A basic law
of physics called the conservation of energy tells us
you can't get more energy out of a battery than you put into it.
Most people tend to put things on to charge "overnight" without paying too much attention to exactly
what that means—but your batteries will work better and last longer if you charge them for the
right number of hours. How long is that? It can be very confusing, especially if you use batteries that didn't come supplied with your charger.
Never fear! All you have to do is read what it says on your batteries and you should find (often in tiny writing) the recommended charging current and charge times. If you have a basic charger, simply check its current rating and adjust the charge time accordingly. Bear in mind what we've said elsewhere about matching your charger to your batteries, however.
Photo: Battery science is not rocket science—charging rechargeables is easy if you follow the instructions, generally written on the batteries or the package they came in.
For example, these three ordinary, 1.2-volt nickel-based rechargeables have quite different recommendations:
- At the top, the white and green nicad battery recommends a slow charge of 60mA (milliamps) for 14–16 hours or a fast charge of 390mA (over six times higher current) for just two hours (2h). The total charge going into the battery is equal to the current multiplied by the time, so multiply the numbers and you'll get a value of about 800–900 mAh. The battery itself claims its capacity is 0.65Ah (650mAH), but don't forget that the charging process is not 100 percent efficient: the battery won't absorb all the electrical energy passing through it. So the amount of charge you're supplying and the amount the battery will absorb are in the same ball park.
- In the middle, the silver NiMH battery recommends a charge of 200mA (milliamps) for 7 hours, which gives us a charge of about 1400mAh. Again, the battery itself claims its capacity is lower than this (1000mAH).
- At the bottom, the green and orange NiMH battery recommends a charge of 63mA (milliamps) for 18 hours, which gives just over 1000mAh. The battery is rated slightly lower (970mAH).
Lithium-ion rechargeable batteries are usually built into gadgets such as
digital cameras, and laptops. Typically they
come with their own chargers, which automatically sense when charging
is complete and cut off the power supply at the right time.
Lithium-ion batteries can become dangerously unstable when the
battery voltage is either too high or too low, so they're designed
never to operate under those conditions. If the voltage gets
too low (if the battery discharges too much during use), the
appliance should cut out automatically; if the voltage gets too high
(during charging), the charger will cut out instead. Although
lithium-ion batteries don't show a memory effect, they do degrade as
they get older. A typical symptom of aging is gradual discharge for a
period of time (maybe an hour or so) followed by a sudden, dramatic,
and completely unexpected cut-out of the appliance after that.
Read more about how lithium ion batteries work.
Photo: An idiot-proof Canon charger for lithium-ion camera batteries. When the battery needs charging, the camera warns you well in advance. Simply remove the battery (very easy on a digital camera), put it the separate charger unit, and the indicator light shows red, turning green when the battery is fully charged. The whole process is automatic and safe: the camera stops you using the battery before its voltage gets too low; the charger stops you charging it before the voltage gets too high.
The biggest, heaviest, and oldest rechargeable batteries take their name from their
(dilute) sulfuric-acid electrolyte and lead-based electrodes. They're most
familiar to us as car batteries (the initial energy supplies that
get a car engine turning over before the gas starts burning),
though slightly different types of lead-acid batteries are also used in things like golf
buggies and electric wheelchairs.
Photo: Lead-acid car batteries were originally developed in the 19th century, long before nickel- and lithium-based rechargeable technologies came along.
Lead-acid batteries are popular because they're simple, cheap, reliable, and use well-proven technology
that dates back to the middle of the 19th century. Generally they last for several years, though that depends entirely on how well
they are maintained—in other words, charged and discharged. They do take quite a long time to charge (typically up to 16 hours—several times longer than they take to fully discharge), and that can lead to a tendency both to undercharge
(if you don't have time to charge them properly before you next use them) or overcharge (if you put them on charge and forget all about them). Undercharging, charging with the wrong voltage, or leaving batteries unused causes a problem known as sulfation (the formation of hard lead sulfate crystals), while overcharging causes corrosion (permanent degradation of the positive lead plate through oxidation, analogous to rusting in iron and steel). Both will affect the performance and life of a lead-acid battery. Overcharging also tends to degrade the electrolyte, decomposing water (by electrolysis) into hydrogen and oxygen, which are given off as gases and therefore lost to the battery. That makes the acid stronger and more likely to attack the plates, which will reduce the battery's performance. It also means there's less electrolyte available to interact with the plates, also reducing the performance. From time to time, batteries like this have to be topped up with distilled water (not ordinary water) to keep the acid at the optimum strength and at a high enough level to cover the plates.
Matching the batteries to the charger
Different battery chargers are designed to work in different ways at different speeds,
largely to suit different types of batteries. The first rule of
battery charging is that a charger designed for one kind of battery
may not be suitable for charging another: you can't charge a
cellphone with a car battery charger, but neither should you charge
NiMH batteries with a nicad charger. Many modern rechargeable
appliances and gadgets—such things as laptops, MP3 players, and
cellphones—come with their own, special charger when you buy them, so you
don't have to worry about matching the charger to the battery. But if
you buy a packet of generic, rechargeable batteries in a store, it's
important that you buy batteries that suit the charger you have or
replace your charger accordingly. Note the voltage and current that
the batteries require (it will be marked on the battery package or on
the batteries themselves), be sure to choose a charger with the right
voltage and current to go with them, and charge for the correct
amount of time. If you want to buy yourself some rechargeable
batteries but you're not really sure how to go about matching batteries and
charger, go for a combined set—where you buy batteries and charger
in the same package.
Top tips for better battery life
How can you get the best from your batteries? Here are some top tips I've found
by reading through a variety of battery-expert websites:
- Rechargeable batteries work best when used regularly. Don't leave them sitting
around in your shed, fully charged or fully discharged for months.
- Battery experts suggest it's a good idea to "condition" or "recondition"
your batteries. This means you regularly let them discharge
substantially before recharging if you can (though you don't need to completely drain them).
- Match your charger to your batteries. For example, use an NiMH charger for NiMH batteries
and be sure the charger uses appropriate voltage and current.
- Don't overcharge your batteries. You will damage them.
- Don't let your batteries get too hot or too cold, either during charging, storage,
or use (it damages them). They will warm up during charging, but if they get really hot, something's wrong.
- Don't skimp on buying a decent, intelligent charger. Your batteries will last much
longer if the charger treats them right!
- Wherever possible, follow the instructions that come with your appliance. For example, the
instructions that come with the Roomba® robot vacuum cleaner tell you to leave it
"docked" (sitting on its charger), trickle charging, all the time it's not being used. If you don't do this,
you'll find your Roomba loses its charge very quickly (even if you don't use it) and you may well shorten the battery life.
Photo: Batteries come in all shapes and sizes. You can't always tell which are rechargeable
just by looking. Of the ones shown here, only the nickel-cadmium and lithium-ion batteries can be recharged;
the others are all single-use disposables. The large, silver-colored lithium-ion battery on the left comes from a laptop,
while the smaller one (to its right) is from an iPod. The nickel-cadmium batteries are generic rechargeables that fit into a generic
charger, such as the one in our very top photo.
Find out more
On this website
- A Glass Battery That Keeps Getting Better? by Mark Anderson. IEEE Spectrum, May 30, 2019. Do batteries that improve with time violate a basic law of physics?
- It's Big and Long-Lived, and It Won't Catch Fire: The Vanadium Redox-Flow Battery by Z. Gary Yang. IEEE Spectrum, October 26, 2017. Are VRFBs the next big thing in battery technology?
- Potential Hazards at Both Ends of the Lithium-Ion Life Cycle by Mark Anderson. IEEE Spectrum, March 1, 2013. Explores the dangers of manufacturing and recycling lithium-ion batteries.
- Powerful Chemical Cocktail, With a Drawback by Matthew Waldjan. The New York Times, January 17, 2013. Fire risk is an ever-increasing concern as lithium-ion batteries become more commonplace.
- Spray-on Rechargeable Batteries Could Store Energy Anywhere by Liat Clark, Wired, 2 July 2012. If we could turn battery components into liquids, we could spray them onto any flat surface to store electrical energy.
- Virus battery could 'power cars': BBC News, 2 April 2009. Scientists at MIT have built a powerful new battery from viruses.
- Battery that 'charges in seconds': BBC News, 11 March 2009. A new way of making lithium-ion batteries could lead to much-reduced charging times.
More to explore on our website...