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Artwork showing a bucket tipping electric charge into a battery.

Battery chargers

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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?

An ordinary zinc-carbon battery

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 gradually.

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 out.

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.

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How battery chargers work

Photo: A typical nickel-cadmium nicad battery charger.

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 electric current 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.

Line graph showing that it can take as long to charge the last 25% of a battery as the first 75%.

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!)

Innovations Battery Manager and a closeup of its LCD voltage display

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.

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 intelligent charger.

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.

Nickel-based batteries

An electric toothbrush standing on its induction charger

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 best known type of rechargeable batteries, respond best either to fairly rapid charging (providing it doesn't make them hot) or slow trickle charging.

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 showing the charging instructions and times written on the side of three rechargeable batteries

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:

  1. 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.
  2. 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).
  3. 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 batteries

Lithium-ion rechargeable batteries are usually built into gadgets such as cellphones, MP3 players, 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.

A Canon battery charger for Ixus digital cameras.

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 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.

A typical car battery

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:

  1. Rechargeable batteries work best when used regularly. Don't leave them sitting around in your shed, fully charged or fully discharged for months.
  2. 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).
  3. 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.
  4. Don't overcharge your batteries. You will damage them.
  5. 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.
  6. Don't skimp on buying a decent, intelligent charger. Your batteries will last much longer if the charger treats them right!
  7. 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.

The more popular types of batteries compared.

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.

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Woodford, Chris. (2009/2020) Battery chargers. Retrieved from [Accessed (Insert date here)]

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