How do battery chargers work? (2024)

by Chris Woodford. Last updated: February 11, 2023.

Power to go—aren't batteries brilliant?The trouble is, they store only a fixed amountof electric charge before running flat, usually at the mostinconvenient of times. If you use rechargeable batteries, that's lessof a problem: click your batteries in the charger, plug in, and in afew hours they're as good as new and ready to use again. A typicalrechargeable battery can be charged up hundreds of times, may lastyou anything from three or four years to a decade or more, and willprobably save you hundreds of dollars in buying disposables (so it'sbrilliant for the environment too). But exactly how well your batteriesperform depends on how you use them and how carefully you chargethem. That's why a decent battery chargeris as important as the batteries you put into it. What is a battery charger and how doesit work? Let's take a closer look!

Photo: Solar-powered battery chargers, like this one made by BEAM, are sure to become increasingly common as more of us switch to electric cars. The overhead canopy contains a 4.3kW, photovoltaic, sun-tracking solar paneland feeds onboard batteries so it even works at night. It can charge up to six electric vehiclesat a time. Photo by Erin Rohn courtesy of US Marine Corps and DVIDS.

What are batteries and how do they work?

If you've read our main article on batteries,you'll know all about these portable powerplants. An example of what scientists refer to as electrochemistry,they use the power of chemistry to release stored electricity verygradually.

What happens inside a typical battery—like the one in a flashlight?When you click the power switch, you'regiving 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 butentirely invisible ways. The chemicals from which their componentsare made begin to rearrange themselves. Inside each cell, chemicalreactions take place involving the two electrical terminals (orelectrodes) and a chemical known as the electrolytethat separate them. These chemical reactions cause electrons (thetiny particles inside atoms that carry electricity) to pump aroundthe circuit the battery is connected to, providing power to theflashlight. But the cells inside a battery contain only limited supplies of chemicals sothe reactions cannot continue indefinitely. Once the chemicals aredepleted, the reactions stop, the electrons cease flowing through theouter circuit, the battery is effectively flat—and your lamp goesout.

Photo: Ordinary batteries (like this everyday zinc-carbon battery)are only designed to be used once—so don't attempt to recharge them. If youdon't like zinc carbon batteries, don't start trying to recharge them: buy rechargeable ones to begin with.

That's the bad news. The good news is that if you're using a rechargeable battery, you canmake the chemical reactions run in reverse using a battery charger.Charging up a battery is the exact opposite of discharging it: wheredischarging gives out energy, charging takes energy in and stores itby resetting the battery chemicals to how they were originally. Intheory, you can charge and discharge a rechargeable battery anynumber of times; in practice, even rechargeable batteries degradeover time and there eventually comes a point where they're no longerwilling to store charge. At that point, you have to recycle them orthrow them away.

How battery chargers work

All battery chargers have one thing in common: they work by feeding aDC electric current through batteries for a period of time in the hope that the cells inside willhold on to some of the energy passing through them. That's roughlywhere the similarity between chargers begins and ends!

Charging methods

There are, broadly speaking, two different ways to charge a battery: quickly or slowly.Fast charging essentially means using a higher charging current for a shorter time,whereas slow charging uses a lower current for longer.That doesn't mean the charging process is just a simple matter of passing a steadycurrent through the battery until it's charged.There are several common methods of charging (plus a few more we won't go into here).[1]

Photo: Battery chargers look simple, but they're surprisingly complex inside.Different types of rechargeable batteries need charging in different ways, for different times,sometimes using several different methods in turn, which make up what's called the charging algorithm. A charger like this is constantly sensing what the batteries inside it are doingand adjusting the charging process accordingly.

Pulse charging involves sending intermittent pulses of high current through the battery,with rest periods in between to allow the battery chemicals to absorb the charge. In crude terms, the pulses are a little bit like the thumping charges to the chest you see an emergency responder giving to someone who's suffered a cardiac arrest, except that they continue until the battery's voltage climbs toward its rated, peak value and the battery is fully charged. (Pulse charging can also be useful for reviving older, degraded batteries, such as lead-acid or nickel-cadmium, in which crystals have grown and impeded the batteries' ability to keep on working; the pulses of electricity break the crystals down so the battery works normally again.)

In taper-current charging, the charger starts off using a high, constant current,which progressively lowers to a trickle as the battery fills with charge and reaches its peak voltage. Inexpensive chargers often work this way.[8]

Two alternative ways of charging are constant current (CC) and constant voltage (CV). As their names suggest, constant current applies a steady current (usually the battery's peak current), while constant voltage applies a steady voltage (usually the battery's peak voltage), and the two are often used together, one after another, in constant current constant voltage (CCCV) chargers.Typically, they start off applying a constant current until the battery voltagepasses a certain threshold; then they apply a constant voltage until thecurrent passes another threshold.Another variation is two-step constant-current charging that begins with a fast high-currentcharge and switches to a slower, lower-current charge part way through the process.[9]

Photo: This "fast-charge" battery charger is designed tocharge four cylindrical nickel-cadmium (nicad) batteries in five hours orone square-shaped RX22 battery in 16 hours. I think it's an example of a constant-currentor maybe taper-current charger, though I've not tested it to find out.It's easy to use, and just as easy to misuse: there's nothing to tell you when charging is complete. With a battery charger like this, charging batteries is complete guesswork.

The final method is called trickle charging, and is similar to constant current charging but uses a much smaller current (perhaps 5–10 percent) for much longer. Some appliances (like cordless phones and electric toothbrushes) are designed to sit on trickle chargers indefinitely.

However you charge, it's worth remembering that, in a very crude sense, 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 and supplies useful current. 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. Asthey 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 (red area) as the first 75 percent (orange area). [2]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. If the battery in thisexample takes an hour to charge, you can see that it would reach 50 percent charge (dotted lines)in just 6.5 minutes.

Charging algorithms

Different charging methods are suited to different types of batteries. Simple pulse charging works well for nickel cadmium and nickel metal-hydride batteries, which are also widely charged by the constantcurrent (CC) method, but pulse charging is quite crude and unsuitable for lithium-ion batteries, which are generally charged by CCCV instead.

Better chargers work more intelligently, combining different types of charging in sequence according to how the battery performs as it's being charged. So, for example, a battery may be slowly pre-charged (by trickle charging) for a short time to test how well it's accepting charge, then fast-charged fully by CC and CV, which may be alternated multiple times. [3]The combination of charging methods used by a particular charger is known as its charging algorithm.

Graph: A simple charging algorithm might involve three stages: brief trickle charging to test the battery followed by periods of fast constant-current and constant-voltage charging.[7]

Charging time

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). How does a charger know when to stop?Different methods are used for different types of batteries, and for slow charge or fast charge.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.

There's usually a primary method of figuring out that the charge is complete(such as measuring the voltage) and one or more backup methods(temperature changes or a preset timer).[4]NiCd chargers, for example, often use a primary method called −ΔV (also written negative delta V or NDV, which refers to the slight voltage drop that a NiCd battery shows just after it's fully charged),with a backup timer or temperature-change detector.NiMH chargers are more likely to rely on temperature changes as their primary method with a backup timer cut-off circuit. In theory, it's impossible to overcharge or undercharge with an intelligent charger.

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 differing opinions on how well they worked.

If you're charging batteries, you probably think fast charge is automatically better—youwant to use your laptop or phone as soon as you can. But it comes with major drawbacks.The chemicals in batteries take time to absorb charge and faster charging can shortenthe life of a battery (a big problem for things like expensive electric car batteries), or risk safety problems such as overheating and fires.[5]

Multiple batteries

Most chargers are designed to charge two, three, or four batteries at the same time,which adds a few extra complications. If you simply connect them in series and try to charge them, how do you know whichbatteries are in a good condition and charging well and which ones are poorer andaccepting less charge? One battery is almost certain to reach full charge before the others,so it's almost inevitable that some will be overcharged (and potentially damaged) whileothers remain undercharged. Decent battery chargers get around this with circuits that monitor each battery individually, switching off or reducing its charging current to a trickle, independently, when it's fully charged.[6]

AC and DC

Batteries are direct current (DC) devices: current flows in one way (during charging)and out the other (during discharging).But most of us live in homes with alternating current (AC) supplies, so plug-in battery chargershave to convert AC electricity to DC before they can charge the batteries youwant to put into them. Exactly how they do this affects the quality of the DC charging current and how cleanly and effectively they charge. Typically, AC-powered chargers use some combination of step-down transformers (to converthigh voltages, typically 110–240 volts, to lower ones more like 1.5–20 volts); rectifiers (diode-type circuits) and thyristors (silicon controlled rectifiers), to convert AC to DC; and integrated circuits to filter and smooth their output.

Charging different kinds of rechargeable batteries

To complicate matters, different types of rechargeable batteries respond best to differenttypes of charging, so a charger suitable for one type of battery maynot work well with another.

Nickel-based batteries

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.

Nickel cadmium(also called "nicad" or NiCd), the oldest and perhaps still bestknown types of everyday rechargeable batteries, respond best either to fairlyrapid charging (providing it doesn't make them hot) or slow tricklecharging.[10]

Nickel metal hydride (NiMH) batteries use newer technology and look exactlythe same as nicads, but they're generally more expensive because they can store more charge (shown on the battery packaging as a higher ratingin mAH or milliampere-hours). NiMH batteries can be fast charged (onhigh current for several hours, at the risk of overheating), slowcharged (for about 12–16 hours using a lower current), or briefly tricklecharged (with a much lower current than nicad), but they shouldreally be charged only with an NiMH charger: a rapid nicad chargermay 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 aquick recharge) reputedly causes permanent chemical changes that reduce howmuch charge the battery will accept in future. Some people swear thememory effort is real; others are equally insistent that it's a myth. The real explanation for an apparent memory effect isvoltage 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 discharginga 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 tofollow exactly what the manufacturers say when you take your newbatteries out of the packet.

Lithium-ion batteries

Lithium-ion rechargeable batteries are usually built into gadgets such ascellphones,MP3 players,digital cameras, and laptops. Typically theycome with their own chargers, which automatically sense when chargingis complete and cut off the power supply at the right time.Lithium-ion batteries can become dangerously unstable when thebattery voltage is either too high or too low, so they're designednever to operate under those conditions. If the voltage getstoo low (if the battery discharges too much during use), theappliance should cut out automatically; if the voltage gets too high(during charging), the charger will cut out instead. Althoughlithium-ion batteries don't show a memory effect, they do degrade asthey get older. A typical symptom of aging is gradual discharge for aperiod 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.

Lead-acid batteries

The biggest, heaviest, and oldest rechargeable batteries take their name from their(dilute) sulfuric-acid electrolyte and lead-based electrodes. They're mostfamiliar to us as car batteries (the initial energy supplies thatget a car engine turning over before the gas starts burning),though slightly different types of lead-acid batteries are also used in things like golfbuggies 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 technologythat dates back to the middle of the 19th century. Generally they last for several years, though that depends entirely on how wellthey 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 ofbattery charging is that a charger designed for one kind of batterymay not be suitable for charging another: you can't charge acellphone with a car battery charger, but neither should you chargeNiMH batteries with a nicad charger. Many modern rechargeableappliances and gadgets—such things as laptops, MP3 players, andcellphones—come with their own, special charger when you buy them, so youdon't have to worry about matching the charger to the battery. But ifyou buy a packet of generic, rechargeable batteries in a store, it'simportant that you buy batteries that suit the charger you have orreplace your charger accordingly. Note the voltage and current thatthe batteries require (it will be marked on the battery package or onthe batteries themselves), be sure to choose a charger with the rightvoltage and current to go with them, and charge for the correctamount of time. If you want to buy yourself some rechargeablebatteries but you're not really sure how to go about matching batteries andcharger, go for a combined set—where you buy batteries and chargerin the same package.

Photo: Matching the battery to the charger. As the world shifts to more environmentally friendly battery-powered electric cars, we'll need a lot more properly equipped, conveniently sited charging stations. This one uses photovoltaic solar cells (in the canopy) to charge the vehicles parked below. Photo by Dennis Schroeder courtesy of NREL.

How long do rechargeable batteries last?

Not surprisingly, it depends on how you treat them, store them, and use them.Small rechargeables (like NiCd, NiMH, and lithium ion) typically last hundreds of "cycles"(you can charge and discharge them that many times), which can mean anything from several yearsof decent life in a laptop to a decade of use in a portable radio. Treated well,lead-acid car batteries are usually good for thousands of cycles and can easily last5–10 years in a car that's driven each day. But if you leave rechargeable batteriesin a product you barely ever use, never charge or discharge them, overcharge them,let them overheat, or store them in poor conditions, don't expect them to last long.

How do you know when it's time to replace batteries? In something like a laptop, you mightnotice a lithium-ion battery discharges normally for a time, then suddenly loses all its remainingcharge very quickly. If you're using rechargeable NiCd or NIMH batteries in things like flashlights,you'll see very gradually reducing capacity and the need to recharge much more often.

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Find out more

On this website

• Batteries
• Electricity
• Induction chargers
• Lithium-ion batteries

On other sites

• Battery University: Isidor Buchmann's wonderfully detailed website covers everything you could possibly want to know about virtually every type of battery you can think of.
• The Electropaedia: Barrie Lawson has a similarly detailed site with an alternative explanation of batteries, energy storage, and related physics.

Books

• Lead-Acid Batteries: Science and Technology byD. Pavlov. Elsevier, 2017.
• The Battery: How Portable Power Sparked a Technological Revolution by Henry Schlesinger. Smithsonian Books, 2010. A "popular-science" book running through the history of batteries from the Voltaic pile to the latest lithium-ion rechargeables.
• Understanding Batteries by Ronald Dell and David Rand. Royal Society of Chemistry, 2001. A very comprehensive guide covering the history of batteries, the various types, how to choose a battery for a certain application, and the electrochemistry of charging and discharging.

Articles

• 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 co*cktail, 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.

1. ↑An Overview of the Fundamentals of Battery Chargers by Bora Tar and Ayman Fayed, 2016 IEEE 59th International Midwest Symposium on Circuits and Systems (MWSCAS), 16–19 October 2016, Abu Dhabi, UAE, 2016, pp. 1–4, doi: 10.1109/MWSCAS.2016.7870048.
2. ↑"... it takes ~ 100 minutes to fully charge a QC 2.0 supported Galaxy S6 Edge smartphone , although about 75% of the charging can be completed in 50 minutes" according to Method to charge lithium-ion batteries with user, cell and temperature awareness by Kang G. Shin and Liang He, University of Michigan, 2022.
3. ↑See for example Soft transition from constant-current to a constant-voltage mode in a battery charger by Bilal Manai and Xavier Rabeyrin, Atmel Corp, 2007. See also reference (1) above.
4. ↑NiCd Fast Charger Circuit is an example of a NiCd charger that works in exactly this way.
5. ↑For a brief discussion of the drawbacks of fast charging, see "Background" section in Method to charge lithium-ion batteries with user, cell and temperature awareness by Kang G. Shin and Liang He, University of Michigan, 2022.
6. ↑See "V. Multi-cell battery chargers" in An Overview of the Fundamentals of Battery Chargers by Bora Tar and Ayman Fayed, 2016 IEEE 59th International Midwest Symposium on Circuits and Systems (MWSCAS), 16–19 October 2016, Abu Dhabi, UAE, 2016, p.4, doi: 10.1109/MWSCAS.2016.7870048.
7. ↑This figue is loosely inspired by Figure 3 from An Overview of the Fundamentals of Battery Chargers by Bora Tar and Ayman Fayed, 2016 IEEE 59th International Midwest Symposium on Circuits and Systems (MWSCAS), 16–19 October 2016, Abu Dhabi, UAE, 2016, p.4, doi: 10.1109/MWSCAS.2016.7870048, Figure 2 from Soft transition from constant-current to a constant-voltage mode in a battery charger by Bilal Manai and Xavier Rabeyrin, Atmel Corp, 2007, Figure 4.1e on p.37 of Understanding Batteries by Ronald Dell and David Rand. Royal Society of Chemistry, 2001, and numerous similar artworks.
8. ↑Taper-current charging is briefly described in Understanding Batteries by Ronald Dell and David Rand. Royal Society of Chemistry, 2001, p.38.
9. ↑Variations on constant-current and constant-voltage charging are described in Understanding Batteries by Ronald Dell and David Rand. Royal Society of Chemistry, 2001, pp.36–38.
10. ↑Lead-acid batteries were the first rechargeables, invented in 1859 by Gaston Planté, but I'm thinking here of the kind of compact, everyday rechargeables we use around the home. Nicads were invented c.1898 by Swedish electrical engineer Waldemar Jungner,who earned a number of patents for battery innovations, includingElectrode for reversible galvanic batteries, filed in September 1904 and granted in April 1908.