How battery chargers work

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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.


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 waterproof battery 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.


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!)


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 waterproof car 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.