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A laptop power supply can supply a limited amount of power. Conceivably, then, when the computer is running, some of the power must be diverted to the processor and other components, leaving less to charge the battery. (Power usage is roughly 10 to 30 watts, or maybe more if you have a graphics card. In my case, my charger is rated for 65 watts.) Why doesn't my laptop battery charge while the laptop is in use? is an example of an extreme case. So it's plausible that this would affect charging speed.

So do laptops in fact charge significantly faster when turned off or asleep (while plugged into a sufficient power supply)?

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There is not a single problem with you answering your own question, but please keep question and answer separate (i.e. post what is currently in the question area in the answer box, then you are free to accept your own answer) –  Duijf Sep 19 '11 at 20:48
    
@Duijf: I'm not answering it; I just intended to give enough background information to establish its plausibility. Edited to clarify. –  Mechanical snail Sep 19 '11 at 20:54
    
Apologies, it seemed like it. –  Duijf Sep 19 '11 at 21:09

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It depends on the power supply. We have a few computers which can be run off a lower-wattage power supply while running - it will not charge the battery at all. If you attach a larger power supply, then it can charge and run.

Check with your laptop manufacturer. It seems somewhat common to see power supplies with different wattage being offered. The power supply with a higher wattage may be what you need to charge quickly and power the system while on. Check to see if a larger power supply is available if you need to charge quickly while using the system.

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No one answer fits all, it really depends on the laptop!

Some have built in features (vendor specific) that speed up the battery charging, others (including most of mine) charge a lot slower when in use.

Again, no one answer fits all.

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is it ever slower when off? –  barlop Sep 19 '11 at 21:31
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@barlop - never seen that ever. –  William Hilsum Sep 19 '11 at 21:33

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