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While using laptop I usually plug in my Laptop charger (i.e., from Mains power).

What is more effective - to keep Laptop plugged in at all times when possible, or some other approach (e.g., running on battery till it fully discharges, then plugging in the charger, and so on)?

"Effectiveness" here could relate to battery lifetime as well as what is good for the laptop internals' life.

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    \$\begingroup\$ This a consumer level electronics question, not about electrical engineering, so is off topic here. However, if everything is designed properly you should be able to keep the charger connected and on anytime you want. \$\endgroup\$ – Olin Lathrop Sep 3 '12 at 13:26
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    \$\begingroup\$ This actually would be a good question for this site as it is basically about the impact of a design issue, except that it depends on knowledge of how the particular, unspecified charger / battery manager circuit is implemented, which isn't really available information, beyond doing a survey of ICs which might commonly be used. On any of the other SO sites you are likely to get quite uninformed speculation or anecdote != data errors. Additionally, beyond the power circuit the laptop may operate more aggressively (cpu clock, display brightness) when on external power. \$\endgroup\$ – Chris Stratton Sep 3 '12 at 13:28
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    \$\begingroup\$ My old Dell laptop has retired now, but I've had the charger plugged in 24/365 for 6+ years. Never had a problem, neither with the charger, nor with the battery. \$\endgroup\$ – stevenvh Sep 3 '12 at 13:39
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It is certainly better to have your laptop plugged in all the time. During that time, the battery level will oscillate between ~97% and 100%. This will have a minimal strain on the battery.

Every full charging cycle significantly reduces battery life. A typical li-ion battery will survive up to a thousand charging cycles.

High temperature also affects battery life. Bad heat management of your laptop will take its toll on battery life. Keeping your laptop directly on your lap will shorten battery life.

Basically everything you do shortens battery life! I have a friend who decided to maintain his battery capacity at a maximum by not using it. He keeps his battery in a shelf and plugs it in once a week. This way, the battery will have its maximum capacity in times of need. I guess it works for some people.

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  • \$\begingroup\$ If the laptop gets very hot - this will potentially shorten battery life too. \$\endgroup\$ – Ujjwal Singh Sep 3 '12 at 19:43
  • \$\begingroup\$ Laptop batteries have a hard life. The two factors that most shorten the life are being kept fully charged and being at a high temperature 30 deg C or higher. In a plugged in laptop the battery will be fully charged and next to a powerful heater (the CPU). This combination is more harmful than going through charge/discharge cycles. The other problem is that the battery capacity will degrade over time whether the battery is used or not. There will be a noticeable reduction in capacity after a year so the best solution is keep the battery in your fridge, 40% charged. \$\endgroup\$ – uɐɪ Sep 4 '12 at 7:00
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    \$\begingroup\$ Wish I could do this. And carry a spare. But of course Apple decided to spoil the otherwise almost-perfect 13" MacBook Pro by making it impossible to remove the battery. :) \$\endgroup\$ – 5arx Sep 15 '12 at 8:53
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Depends on a lot of variables, but I will answer for a specific (but very common) setup:

Assuming the laptop is powered by a Lithium-ion battery pack (which is true for most laptops), I would say it is better (for battery lifetime) to keep the charger plugged in as much as possible, as opposed to running off batteries.

The reasoning:

  • Li-ion charger designs usually have load sharing such that circuit power is ensured alonside any battery charging that might be required, but the key point here being, at all times, the charging power source itself is providing the power to the circuit once the battery charging is finished (a one-time thing).

  • Whereas, on the other hand, if you were running off the battery then charging, and so on, you would drain the battery (adding to its discharge history), and then it would have to use the power from the charger to recharge the battery again. This discharging-and-charging is not desirable because Li-ion batteries have a limited number of charge-discharge cycles: e.g., after typically 300 cycles, the battery pack's capacity goes down about 25%.

  • In particular, allowing a Li-ion to undergo smaller depth of discharge (i.e., no discharge or even only 50% discharge, as opposed to 100%) will allow the battery to last longer by increasing the number of possible discharge cycles for a Li-ion battery.

  • Note that overcharging, overdischarging, etc., while the charger is left plugged in, are not issues at all because battery packs and the charger designs are smart, i.e., they already have overvoltage and undervoltage protection built-in.

Something else that is of interest for certain situations is to even consider pulling the battery out of its socket while running off the charger alone. This might be of particular relevance to laptops that run very hot. This is because Lithium-ion batteries, among others, suffer from stress under heat, with even 30 degrees celsius considered as "elevated temperature", and recoverable capacity declining markedly at higher temperatures.

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  • \$\begingroup\$ Looking at Li-Ion characteristics, the best life would be to take out the battery at 75% charge and store it in the freezer. \$\endgroup\$ – markrages Nov 28 '12 at 17:05
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My main job in life is mending laptops, and the main fault is a dead battery or faulty charging circuit.

From my own personal experience (owning several laptops of the years) I would never constantly run the laptop from the mains supply with the battery attached. I have found that similar to mobile phone batteries, the battery of the laptop gets to a point where it will not properly charge, and its active usable life is reduced to only minutes or an hour or so. Some say the battery gets a "Memory" point (or it has hit it max number if recharges), where it will not fully charge past?

With my Sony Viao, I only run from mains when the battery is almost flat, or when I have fully charged the battery, and the battery is removed from the laptop.

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In fact, recent laptops nowadays use Lithium-Ion batteries, which means that the charger will stop after the battery reaches the full charge. So you don't have to worry about the battery and its lifetime if you keep working with the battery in and the power is from AC.

Nevertheless, you can take out the battery and keep running the laptop from AC power, it's up to you.

Now talking about charging non-fully charged batteries, it doesn't matter if the battery is fully discharged or partly discharged; what really matters is the cycles of use... each battery has a limited number of using cycles and differs from one to another. So, every time the battery is fully discharged its lifetime is decreased by a cycle.

From this point I recommend you to use the laptop on AC power as long as you don't have to be away from it.

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