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I've run into this issue already, where when the battery is in and I'm running Folding@Home or other extremely CPU intensive tasks a temperature monitor reports temperatures up to 200°F/92°C. Obviously running a laptop that high isn't recommended, but the only fix I could come up with is removing the battery. After that temperature dropped to 158°F/70°C. Note that killing the extremely CPU intensive tasks it dropped the temperature to 120°F/49°C.

My question though is why does having the battery in (not used since its plugged in and 100% charged, just in) cause the temperature to jump so high? Its not even the battery that's hot, its just the area where you plug in the power cord. I'm not sure exactly how the power is wired, but if its going through the battery then how is it that only the power plug is hot and not the battery? Why is the power plug so hot? If its not always charging the battery, then why does the mere existence of the battery spike the temperature?

For reference this laptop is a few month old Lenovo Thinkpad X220 Tablet. Clogging fans aren't an issue as I experienced this issue 2 weeks after receiving it. It has a Sandy Bridge CPU so there's no hot GPU which might cause problems. Unsurprisingly the heat issue is there whether I'm in Fedora Linux or Windows 7.

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up vote 12 down vote accepted

You are likely to be using the thinkpad 65W charger which limits the possible speed

Thinkpad does NOT run at the highest possible speed when you are using a 65W charger alone, without battery plugged in. It would run at full speed if you are using the 90W charger, however. Putting a battery in would allow the CPU to run at full speed, irrespective of the charger type, thus the temperature difference.

Consider using TPFancontrol. Good luck.

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I'd only add to this one that the battery releases energy through a chemical reaction that produces heat. Not as much heat as the CPU, but when you're stressing the system like this (which is not a good idea, generally. There should be a throttle option on the application) the heat of the CPU and other internal components is bolstered by the heat the battery is producing, adding to your total thermal output. –  music2myear Oct 20 '11 at 17:50
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Which CPU do you have? 92C is fine for the i5 2520M.

Having the battery connected raises the temperature because it permits the system to run in its maximum performance mode. Without the battery connected, the system has to drop the maximum CPU frequency because the AC power source cannot supply the instantaneous surge of power needed when the CPU comes out of a temporary idle state.

For example, when the CPU is waiting for memory, many of the execution units go idle. Once the blocking instruction completes, the CPU can suddenly go from almost completely idle to almost completely at full burst. The laptop needs the battery connected to accommodate this surge.

Sandy Bridge CPUs have turbo boost. If under maximum load, they will raise their clock frequency until they are just about to overheat so long as they can operate reliably. 92C is just about to overheat for an i5-2520M, so it's functioning as designed. If you'd prefer it run cooler but get a little less performance, you can disable turbo boost.

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The main reason I want to run it cooler is because sweating the battery really shortens its life. Turbo expense at the battery life isn't a fair trade to me –  TheLQ Oct 20 '11 at 19:13
    
Get a laptop stand with a fan.. this will cool the unit and therefore extend it's life while still allowing you to run full out. –  Chris Nava Oct 20 '11 at 19:34
    
You are effectively storing the battery at full charge, not really using it. Storage at 40C with a full charge will approximately halve the life relative to storing it at 25C with a full charge. So this is a reasonable concern. –  David Schwartz Oct 20 '11 at 19:47
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