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The Truth About Notebook Batteries

Everything you ever wanted to know about this crucial notebook component—and more.


by Avram Piltch, Online Editorial Director on August 22, 2009

batteries_sh.jpgIt’s one of the most important and least understood parts of your notebook—and it provokes so many unanswered questions. Why do batteries gradually lose their ability to hold a charge? Why does it takes longer for a charging notebook to go from 80 to 100 percent charge than it does to go from zero to 80 percent? What’s the difference between a three-cell and a six-cell battery? Rather than shrug our shoulders for another day, we demanded answers from two experts, Matt Kohut, worldwide competitive analyst at Lenovo; and John Wozniak, Ph.D. distinguished technologist at HP.

LAPTOP: At what point during its life cycle does a typical notebook battery last longest?

Kohut: The industry average is about 30 percent degradation per year. You’ll probably start to notice after about six months.

John Wozniak: Batteries tend to fade pretty quickly over the first 10 or 20 cycles, and then capacity levels off.


How long does a typical notebook battery last?

Kohut: Roughly, 350 charge cycles. A cycle is cumulative use that reaches 70 percent degradation, or seven times when 90 percent of the battery capacity was used.

Note: A new type of battery, the Boston Power Sonata, is supposed to last for 1,000 cycles. For more details, see our article on the Boston Power Sonata.


What causes a battery to lose capacity over time?

Wozniak: Think of a battery as having pores, where the electrons flow back and forth between the cathode and the anode. When those pores get clogged, you can’t move the electrons as efficiently anymore. Then the lithium gets bound up on one side or the other, and it no longer contributes to the movement of electrons. It gets backed up.


Do you need to build up a battery memory, or condition it, to get the best performance?

Kohut: This was true in the lithium-metal hydride days, however, with a standard lithium-ion battery, you just need to make sure you charge it fully. Most batteries ship from the manufacturers with an 80 percent charge.

Conditioning, meaning draining the battery to zero and then charging it fully while Windows monitors it, resets the curve. It doesn’t prolong the battery’s life, but it improves accuracy [of the battery monitor]. Over time, the accuracy drifts in the Windows program that measures battery life. That is why it can say that you suddenly went from 80 to 20 percent in five minutes.


How much can you trust the charge status that Windows gives you?

Kohut: If the battery reports full, Windows says it is full. Windows periodically checks the battery, but it is more of a thumbs-up/thumbs-down than a real assessment. Windows is crude; trust it if it’s full, trust it if it’s empty. But know that it doesn’t have the communication to know that the battery degrades over time. We recommend the Lenovo Power Manager, because it has a lot more smarts. It knows the status of each individual cell, whether it’s balanced, imbalanced, charging, not charging, etc.

John Wozniak: If the BIOS is written correctly, Windows is really accurate; within 1 to 2 percent. The problem is how often it updates.


When charging a notebook, why does the last 5 to 10 Percent take longer than the first 85 to 90 percent?

Kohut: If the battery is low, we will charge it at a faster rate. But when the battery fills up, we back off, because it’s the last bit of charging that causes the issues to form. Other vendors that do rapid charge still back off around the last 10 percent. Dell, for example, does fast charge, pedal to the metal, from zero to 80 percent. Then backs off and coasts into the finish line. Lenovo starts fast, but backs off more and more through the entire charge. This allows for a more even rate of charge. It takes longer overall, but it helps the battery’s longevity.


Some notebooks won’t start charging unless the battery is below a certain level, SAY, 95 percent. Why is that?

Kohut: Charging it to 100 percent every time adds cycles that cause the battery to wear out faster. In the typical office situation, you’re plugged in 100 percent all day. Then you take it home, and plug it in again, after losing maybe only 2 percent. If you charge to 100 percent again, you start adding cycles that wear out the battery because you’re initiating a full charge cycle. The thought behind our process is that unless the battery gets below a certain percentage, don’t put it through the stress of charging.


How important is the number of cells in determining battery capacity?

Kohut: The three-, six, nine-celled battery is shorthand for explaining how much battery life a machine has, but watt-hours hours determine capacity. The average consumer has no clue that a nine-cell battery has around 84-watt-hours of power, they just know it’s a nine-cell. One of Lenovo’s competitor’s has two six-cell batteries—a 53-watt-hour and a 47-watt-hour. The second doesn’t deliver the same capacity. To really understand how much power a battery will hold, you need to look at the watt-hours.


Some vendors are advertising two- or three-cell batteries that claim a longer life than six-cell models. Is that possible?

Kohut: There is a fine line. Battery capacity has gone up in the past few years by making the cells more energy-dense. There used to be 2 amp hour (Ah) cells, but now cells go up to 2.9 amps. If you can control the higher concentration of energy in the smaller space, it’s not a problem. But sometimes this results in violent reactions such as overheating and over-charging. [If a vendor claims its battery lasts a long time with fewer cells,] you want to ask how many watt-hours the pack holds, and about the kind of cells (2.6, 2.9 Ah, etc.).


Should consumers trust third-party batteries?

Kohut: We’ll warranty our own batteries, but if you put a third-party battery in, you void the warranty. That’s because there are good battery manufacturers and there are bad ones. Some of the good ones are Panasonic, Sanyo, and Sony. A less reputable company will buy whatever is cheapest, and just drop it in there. The reason a lot of the battery incidents occur in this industry is because of quality control.

We always say that you are taking [the life of your notebook] into your own hands when you buy a third-party battery pack. Yes, you are saving some money up-front but you don’t have the backing of the company behind it.

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