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Does a fully-charged battery supply power to a laptop/notebook if the mains is connected?
Does it behave like a mobile-phone (which needs a battery)?

Will the battery-life be improved if it is used lesser? (Li ion)

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possible duplicate of Removing Battery and running the laptop in pluged in mode? –  RedGrittyBrick Apr 12 '12 at 9:06
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Welcome to Superuser. The search facility is at least as useful as the ask-question facility. –  RedGrittyBrick Apr 12 '12 at 9:07
    
I am not asking about the health of laptop. Battery is concern in this question. After a battery replacement, I use battery only during power-cuts. –  pop stack Apr 12 '12 at 9:38
    
Worth reading to improve battery life...h10025.www1.hp.com/ewfrf/wc/… –  Moab Apr 12 '12 at 15:17
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3 Answers

up vote 2 down vote accepted

To answer your specific questions (as opposed to just chiming in and disputing other answers)...

Does a fully-charged battery supply power to a laptop/notebook if the mains is connected?

Only if the power coming from the DC adapter is interrupted or reduced past a certain point. If the DC adapter supplies close to the requirements of the laptop, the battery will not discharge while the unit is running on current being supplied by the mains. You can easily test this yourself by replacing your adapter with another of a similar wattage. For a more concrete example, a Toshiba Satellite A75 requires a 120 watt adapter (19v 6.3a) but you can use the laptop with a 90 watt adapter. You must take certain steps to lower the power drain (reduce screen brightness, lower processor speed, etc) or when the laptop has to work hard, it will kick off the 90 watt adapter, and start using just the battery. Otherwise, if the proper DC adapter is being used for a laptop, and there is no break in the flow of electricity, and the DC adapter does not fail, then the laptop will not use the battery. You could remove the battery if you so decided, and still use the laptop. This is not disputable. This is a proven fact.

Does it behave like a mobile-phone (which needs a battery)?

Again, providing that you are using the proper DC adapter, and there is no interruption of electricity to the DC adapter, and the DC adapter does not fail, a laptop does NOT behave like a mobile phone. It behaves like a desktop computer.

Think of it this way. Do you have a UPS for your desktop? You know.. an uninterruptible power source? The battery in a laptop functions pretty much exactly the same way. With a UPS, you plug it into the wall, and you plug your computer into it. As long as there is power from the wall, you never use the battery part. You only use the battery part if there is no power from the wall. But... if you remove the battery part, and you kill power from the wall, you don't use the computer.

Will the battery-life be improved if it is used lesser? (Li ion)

Search the web and you will find experts of all calibers giving varied advice on this. NiCad batteries had a "memory" which worked in such a way that the battery had to be fully discharged before being recharged. LiOn batteries do not suffer from this. That said, you should expect about the same lifespan length as an average hard drive... 2 to 4 years. This is not to say you will get optimal use after 2 or 3 years. This is to say it will still hold a charge for some actual measurable amount of time (even if it is only 20% of original capacity). Will using the battery less extend it's lifespan? Of course. LiOn batteries have a limited number of available charge cycles. They will hold less charge after 250 cycles. They will retain less charge after another 250 cycles, etc. The fewer number of times that you charge/discharge the battery, the longer it will last. This article specifically tests smartphone batteries... but you get the gist. Read up. It's interesting.

How do I know what I know? I've taken apart and repaired more laptops than most people have ever seen... and that includes people who sell them at the big stores (seeing them, I mean). One common diagnostic step is to power the "laptop" without the battery. This also includes removing the motherboard and powering it. In some cases when monitoring heat bleed on older boards, this would mean powering a motherboard for hours or even days without the battery.

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Your proven fact is simply false. As just one example (and there are many), read this Apple kb article. Your answer is almost entirely based on obsolete information and misinformation. The wall power supply is connected to the laptop through a long wire with high inductance, it cannot provide the surges of power a modern CPU needs as it constantly turns portions of itself on and off to track load to its cores. Modern laptops are not at all like the laptops of a decade ago, but old information seems to never die. –  David Schwartz Apr 12 '12 at 13:23
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I'm typing this on a laptop that is two years old, and has not had a battery in it since for just about a year. So, my well proven fact is not only correct, but current. Now... Apple designs might be different, but they are not the norm... as they are about 10% of the market. Sorry. Old information might never die, but ignorance also seems to last forever. –  Bon Gart Apr 12 '12 at 19:13
    
That's complete nonsense. The question was about what happens when the battery is inserted, not what happens when the battery is removed. Nothing about how your laptop works with no battery in it can tell us anything about what it does when it has a battery. And the fact is, when the battery is inserted and the laptop is on mains, the battery manager can and will draw power from the battery if that's what it thinks it best. (It may not have to do this, but it can do it, and it will do it if that's best for battery life.) –  David Schwartz Apr 15 '12 at 6:54
    
Regarding the UPS remark: there are both "online" and "offline" UPS solutions (and others). Offline works as you describe, but online varieties draw power from the battery while using incoming power to recharge it. Today, many cheap UPS units are "line-interactive" which is a middle ground, so the analogy with a "common" UPS is halting; it needs to be specified to "offline" if you want to make the point that the battery in a laptop is not used while plugged in. –  Daniel Andersson Apr 19 '12 at 12:43
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All HP laptops and IBM I have used work without the battery. And when connected to the mains the battery charges but is not used to power the laptop.

As for the battery-life, if a li-ion is unused for a long time it will lose capacity. Optimal storage charge is around 40%. more info here

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You are incorrect, or you have used a very unusual set of laptops. Many modern laptops draw off the battery, even when running on the mains, to handle the surges of power needed as the CPU comes out of low-power modes. The long cable from the power brick to the laptop has too much inductance to allow a sudden increase in power draw without a serious voltage drop. –  David Schwartz Apr 12 '12 at 9:58
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I have run several laptops without the battery inserted for long periods of time and had no problem so it is certainly possible to run laptops on mains power alone –  Sibster Apr 12 '12 at 10:30
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Whether you can run laptops on the mains power alone and whether they in fact run on mains power alone when the battery is inserted are two completely different things. –  David Schwartz Apr 12 '12 at 10:32
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God, will these twenty year old battery myths ever die?!

Of course not!

Look, with the battery in, the battery manager can do whatever it thinks is best for the battery. It can float the battery, it can charge the battery, it can discharge the battery.

With the battery out, the battery manager can't do a thing. The battery gets what it gets.

How could it possibly help to remove it?

The only thing I could possibly imagine is if the battery got hot in the laptop. But that would indicate a pretty crappy laptop design.

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I think we all know that... ...Nevertheless, we need an answer. Battery manager shows, "plugged-in/fully-charged". What gives the processing-power, the battery, or the mains? –  pop stack Apr 12 '12 at 9:50
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It's up to the battery manager. If it thinks draining the battery will extend its life, it will run off the battery. If it thinks using the battery to stabilize ripple from the supply will be best, that's what it will do. If it thinks float charging the battery is best, that's what it will do. These things are designed by experts who know the battery chemistry and physics. –  David Schwartz Apr 12 '12 at 9:52
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So, what does it do? How to KNOW that (in Windows 7)? –  pop stack Apr 12 '12 at 9:57
    
There's no way to know. The manager only reports whether or not it has AC power, whether it's charging the battery at its maximum rate or not, the estimated battery charge level, and whether the battery is healthy. It doesn't provide any greater level of detail, nor is there really any reason you should care. –  David Schwartz Apr 12 '12 at 9:59
    
Li-ion batteries lives the longest at ~20°C operating temperature. At 45°C, which is closer to the temperature in a laptop under load, its life time is ~halved [ref]. Temperature is the main reason to not keep the battery plugged in while using the laptop if power is available. Also, Li-ion batteries actually last longer if kept at a lower than full charge. Batteries are stored and shipped at ~40% charge for this very reason [ref]. –  Daniel Andersson Apr 19 '12 at 12:57
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