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.
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.
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 waterproof marine 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.
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 electric transportation battery 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.
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.
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