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This morning I noticed that my laptop fan was cycling on and off faster than normal.. then I noticed that I had forgotten to plug in the AC charger.

Is is possible that running off a battery makes the laptop run hotter? I guess my question is: are the various PSUs and regulators in the laptop working harder when I run off battery or AC, or should it make no difference?

I'm using an HP Elitebook 8570p.

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I'm going to take some more time to monitor the behaviour. I also suspect that because the laptop had just come out of hibernation, startup tasks were still running, causing more CPU load than normal. By the time I had connected the charger, these tasks may have completed. So it was pure coincidence that the fan cycles slowed down. –  KevinM Mar 12 '13 at 15:59
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2 Answers

Have you checked your power settings?

A messed up configuration could mean that your laptop is running under a higher performance mode on battery than when plugged in, or that your cooling may be set to active on battery and passive on AC power

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Yes, running off a battery can make a laptop run hotter (but it is equally likely that more aggressive power management causing the fan to be "more responsive" to heat buildups for short periods)

It is possible that as you are discharging it, the battery is getting hot (because of the internal resistance of the battery), so more cooling is necessary.

The PSU's and regulators are not working as hard when you are running off battery as when you are running of mains - If you think about it, if a battery is not fully charged the PSU needs to draw MORE power when running on mains as it needs to charge the battery.

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