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I have been reading alot of mixed views on how Lithium Ion batteries work when it comes to laptops. I have the new dell xps 15, with an intel i7 cpu, 16gb ram, and a 512gb ssd.

Is it bad for the laptop to leave it constantly plugged in when not on the move? I keep my laptop very cool all the time as i have a laptop cooler sitting underneath it, i am always mindful about its temperature. My laptop gets about 3 charge cycles per week (Mon, Tue, Wed), this is when I am at university and without a charger. Apart from that, when I am home I just leave it plugged in as I have no reason to use the battery power.

My question is, when laptops reach 100% charge and are left plugged in, does the computer circuitry switch from using the battery power to directly using the power that comes from the charger?

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Keep your battery between 40-80% if possible, unless you plan to need extended battery use, occasionally.

Here's why:

(Note: Circuitry or software inside the battery, your PC, or the software added might already keep your battery below 80%, and report 100%, but I doubt it. As mentioned here by Daniel B, your software may stop charging after, or before it reaches 100%, until it falls to x%, which will extend the life of the battery.)

Both PC and phone manufacturers promote the best usable battery time, per charge, at the expense of your batteries' life expectancy, which could otherwise last almost indefinitely. For instance, the Chevy Volt never allows any of it's lithium cells to be charged over 80%. (There's a great 1 hour YouTube video of a lithium cell manufacturer's FAE, (Field Application Engineer), discussing what and why they advised Chevy during the Volt design.) Here's what I learned:

It isn't the circuitry that is at risk--it's the Lithium battery cell itself, which has different risks than NiMH or NiCad batteries. Lithium cells only work so well and risk-free today, because of layered intelligent circuitry that protects both you and the cell, to some extent. Some intelligence is inside the cell itself, the battery pack, your PC, and some in the reporting software. We can guess or imply how your Dell PC acts, but not definitively, based on design criteria. Here's what it takes to keep cells in such safe and perfect condition, they will safely live almost forever:

1-- Keep as cool as possible: Heat kills cells as the #1 cause, so let them cool before charging, if possible.

2-- Charge as slowly as possible: Charging slower is simply cooler, thus your batteries will live longer.

3-- Never completely drain to near 0% charge or store your battery below 40%: All batteries lose their charge over time, and once it dips below 10%, it will never charge again.

4-- Seldom charge to 100% and never store your battery above 80%. The longer and more often the cells are at 100%, the shorter your cell's life expectancy and available power.

Never allow your battery to reach 0% charge state, or store your battery below 40% Cells are designed to never be allowed to get to 0%, because they would take so much current to start charging. Then there's a high risk of explosion, or at least overheating. So, the cells' internal circuits themselves will generally prevent charging when below 10%.

If you've ever had a cellphone battery, or laptop that won't charge, it is very likely the battery itself, and not charger. Instead, you likely have one or more individual cells, or the battery pack that will prevent ever turning back on, once they are below 10%, without specially designed circuit to safely jump-start them. It's estimated that over 60% of the "dead" cellphone batteries could be revived to a normal life with a specially designed (and available) charging device.

Never store batteries below ~40%. Since all batteries will lose their charge with no use, over time they will all dip below that 10%, and sooner or later will become unchargeable. (Never discharge a power drill battery without recharging very soon after use, or it will never charge again).

Seldom charge your battery to 100% or store a battery at 100% As I learned, batteries at 100% charge will develop little shards of copper, which shorten the life of the battery, or eventually, kill it completely. Almost all devices we use will NOT have protection against this, (as does the Chevy Volt's cells).

There are often times when it is well worth charging to 100% when we know we will be away from a charger for a long time. We often choose which device to buy, based on how long we can use it between charges. So, while we need to occasionally charge to 100%, the cost is slightly reduced battery life. IMO, I'm speculating that how long it stays at 100% determines the extent of harm more than how often.

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    "Keep your battery between 40-80%" and "Seldom charge your battery to 100%": this might be true if a manufacturer provides some management software for that. But if not, then stopping charging below 80 or 100% implies that you are unplugging the charger, hence will be using the battery and are going to discharge it. Companies like Apple define battery life in cycles nowadays, so discharging would add to those cycles. So my 2 cents: leave the thing plugged in whenever convenient and stop worrying about it. Better try to save energy instead. – Arjan May 24 '15 at 16:21
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    Those rules are impractical for a laptop, would make for a part time job to do them. – Moab May 24 '15 at 17:45
  • I agree that trying to manage staying within this range via plugging and unplugging is unrealistic. However, software to manage this for you would extend your battery life almost indefinitely, and mostly negate counting charge/discharge cycles. It would also have to anticipate when to charge to 100%, which is difficult without also burdening the user. The most costly lesson I've learned is to never put a discharged Lithium battery on the shelf, intending to charge it later, which will destroy it. – DaaBoss May 24 '15 at 17:50
  • Hmm very interesting read. As Moab said it would be quite impractical to try and keep it constantly between 40-80%, as i use my laptop as my main device. It does disable charging when 100% is reached. I guess I will focus on keeping it nice and cool and not allow overheating. Thanks alot Boss for all the information. – Sylvoo May 25 '15 at 1:35
  • Do you have a link to that Volt battery advice video? @Moab To do manually, yes. Aside from automatically detecting when to allow the battery to charge more for extended use, they can all be automated. – jpmc26 Feb 7 '16 at 18:46
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When it reaches 100%, it will stop charging until the charge level drops below a certain threshold, usually about 96%. On some devices, this threshold can be configured, with Lenovo’s power management software for example.

If you have a low-power charger powering a high-power laptop, there might be situations where it will draw power from the battery even when the charger is connected.

  • Thanks alot for clearing that up for me :) I have been very confused about the nature of this subject. – Sylvoo May 24 '15 at 15:02
  • I've seen this on newer laptops. My Lenovo is at 96% now, which was likely a good compromise of giving the user a longer charge, and battery life. Lenovo's battery appears its not degrading as fast as my older Dell is. Still, without the ability to control these better, both the heat from charging (and use), and overcharging more than we need prevents us from having batteries that last almost forever. We need a smart app that interacts with us, (but without annoying us), like warning when accidentally unplugged, and which allows easier control of our charging. – DaaBoss Jun 26 '15 at 12:58

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