The chances are good that you have at least one mobile device: smartphone, media player, digital camera, Bluetooth headset, laptop computer, tablet, ebook reader, or any of a dozen other devices. And the chances are equally good that this device relies on rechargeable batteries. You don’t have to carry spare batteries and you keep all those depleted alkaline batteries out of the landfills.
But how much do you know about those batteries that you depend on for your phone calls and music, entertainment, and information? You likely have a limited understanding of how the batteries work and what goes on when you plug them in to recharge. Here’s a quick overview of some key concepts, and suggestions for ways to get the most out of your battery-powered devices.
The Charge Cycle
Almost all mobile devices today rely on lithium-ion technology. This type of battery has many advantages. They can hold a lot of energy in a small volume, they hold the charge well when the power is off, and they can deliver the power reliably over a long period of time. They are relatively safe and tend to tolerate all sorts of situations fairly well. Unlike nicad batteries, lithium-ion batteries don’t lose capacity when you recharge them before they are fully discharged.
All the same, lithium-ion technology has some characteristics that it helps to understand. And a good place to start is the charging cycle.
[Credit: Alfred Poor]
The graph shows a generic charge profile for a lithium-ion battery. The red line indicates the cell voltage, the orange line shows the amount of electrical current used to charge the battery, and the green line represents the stored charge.
One detail that you may not realize is that almost all lithium-ion devices include microprocessors in their charging circuitry. Some are more sophisticated than others, but in general you need to manage the charging process by more than just applying a current to the battery and waiting for it to fill up.
The first step is sometimes called a “pre-charge” phase, but its main function is to simply test the battery to confirm that it can take a charge. (This is represented by the blue area in the graph.) This is accomplished by applying a lower voltage with a low current, which avoids overheating the battery if it is fully discharged.
Once the cell voltage reaches a pre-defined level – generally about 3 volts for a typical 4.2 volt lithium-ion battery – the charger switches to a “fast charge” mode. (The green area in the graph indicates the fast charge section.) The amount of current applied to the battery is limited by the charger’s controller, again to avoid overheating the battery. Once the battery reaches its full voltage, the amount of charge current that the battery can accept declines as it gets closer to its full capacity.
Once the battery is “full,” the charger can shut off and an indicator light can signal the user that the device is ready. (This is shown with the orange section of the graph.)
But here’s the first fly in the ointment: How do you define full?
As it turns out, the idea of “fully charged” is a slippery concept that is subject to interpretation. Design engineers have difficult choices to make when they design the charging system for a mobile device. And as they build more sophistication into the charger controller’s logic, the more expensive the manufacturer’s bill of materials becomes.
Power Play
Herein lies the source of the problem. As the battery gets closer to full capacity, the charge current gets smaller; this means that the battery will charge more slowly from that point forward. The task of the designer is to decide just what percentage of capacity is “full enough.” This is why we represent the “full” level with a dotted line; there is no fixed definition of where it belongs.
For example, one designer might decide that the user should be able to grab the device and go as soon as possible. In this case, the controller might be programmed to accept as little as 80% of total capacity as representing the “full” state. This keeps “recharging” times to a minimum.
On the other hand, another designer might choose to make long operating times a priority, and thus define the “full” state as a higher percentage of the capacity, even though it could take another hour to gain just single-digits of percentage capacity.
The Light Fantastic
Ah, but that’s just the start. What about that indicator light? If the definition of “fully charged” is fuzzy, just what exactly is it telling us? The answer is, “We can’t be sure.”
Most devices have a three state power indicator. It’s off when the device is powered down (or sleeping, or some other less-than-fully-operational state). It glows one color when the battery is charging – often amber – and another color when the battery is full – often green. (Green is also the typical color when the device is disconnect from a power source and turned on.) But what triggers those colors?
The amber/charging light is easy; it generally means that current is flowing to the battery and that charging is not complete. The green/charged light is a bit trickier. On most devices, it is turned on when the battery reaches the pre-programmed percentage of the total charge capacity. What happens next, however, is anybody’s guess (except for the original designer).
In some cases – especially for low-cost devices with relatively simple charger controllers – the charger shuts down at that point. It monitors the capacity of the battery to watch for it dropping below a pre-defined threshold, at which point the recharging process begins again.
In other cases, however, the designer may have wanted to let the user know that the battery was charged “enough,” but if the device is left connected to the charger, then current continues to flow to the battery. Some designs then turn off the charger (with no change to the indicator light) when the battery reaches another programmed capacity level. Others rely on a simpler method, and just start a timer that turns off the charger in 30 minutes (or an hour or some other arbitrary period) to provide some extra time to “top off” the battery.
As an end user, there is no easy way to know just what your charger is doing. The best way to figure out what “fully charged” means is to disconnect the device just as soon as the “charged” light goes on, then run down the battery under controlled conditions. Then recharge the device, and let it continue to charge for an hour after the “charged” light goes on. If the device then runs longer under the same conditions, then you know that it is “topping off” the charge even though the light is on.
The Care and Feeding of Your Battery
So what’s the best way to treat your rechargeable batteries? You might think that the best practice is to simply leave the device plugged in all the time when you’re near a power source, just to make sure that it stays topped off. That’s probably not a good idea.
First, every time you recharge a lithium-ion battery, you do a little damage. Some of the electrolyte crystallizes near the electrodes, and it can’t store power in that state. As a result, the battery’s useful life is limited by the number of times that it can be recharged. There’s not a hard number (for reasons that you find out in a moment) but it’s in the thousands.
The reason you can’t put a fixed count on battery recharges is that the charging capacity steadily declines, but the battery doesn’t actually fail. So how much does the battery run time have to be shortened before you feel that you have to replace the battery?
Also, the damage done by recharging is not constant. Heat accelerates the process, so overcharging the battery with too high a voltage can cause it to overheat and be damaged. Some sources indicate that less damage is done when you recharge a battery that is already near capacity than when you charge a battery that is almost depleted, but it gets damaged nonetheless.
If your device is turned off, you should be able to leave it plugged in; when the current draw falls below a programmed level, the charger should turn off. And if the device is really off – stone dead cold “not listening to the world at all” off – then there should be no current drain to lower the battery’s charge.
If your device is not turned off, however, it’s a whole different ballgame. It’s like filling your car’s gas tank with the engine running; after you top it off, the level eventually drops to where you’ll be able to pour in more gasoline. The same thing happens when your phone is sort of “sleeping” but still wakes up to poll for email or answer calls. This draws the power down, and even though the charger may have turned off, the battery will eventually drop below a predefined percentage of capacity, and the charger will turn back on and start charging again. And that costs your battery one of its “lives.”
Now, some of the more expensive chargers can work around this. They can detect whether the device is drawing power from the battery, and bypass the battery to provide power directly to the device. Meanwhile, the battery continues to recharge as normal and the charger turns off the current to the battery once it reaches its “fully charged” level. The device continues to get the bypass power from the charger, so there is no draw down on the battery, and you should see a fully-charged battery whenever you unplug it from the power source.
Best Practice
What does all this mean in practical terms? Unless you have specific instructions from the manufacturer to the contrary, you probably can leave a device plugged in if it is truly off. Otherwise, you probably should not leave it plugged in for much more than an hour after the “fully charged” indicator comes on. This gives you the greatest chance of giving your battery the longest life possible. This may not matter for a device that you are going to replace every year or two anyway – such as a cell phone – but for devices that you intend to keep for several years, such as a laptop or tablet, you may want to be more conservative in your recharging practices.
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