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I have a Dell Studios XPS 1640, and it requires a 90W charger for use. I lost my power cable, so I'm now using a 65W charger.

My computer has slowed down noticeably whenever it is charging. The computer speeds up directly after I unplug the cord.

Could this be because of the power cable itself?

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3 Answers 3

up vote 24 down vote accepted

Many Dell laptops are able to use 65, 90, and 130w power supplies, but they will adjust performance accordingly. Dell Support article 12174 (KB 168345) notes:

The Dell Universal Auto/Air Laptop Adapter is a 65-watt power adapter. Dell recommends that you use a 90-watt adapter with your portable system. Using a 65-watt power adapter will not harm your system, but will cause slower performance.

Specific performance throttling will vary depending on your CPU, chipset, and GPU, but overall every component will be slowed down to afford enough power to charge the battery and operate simultaneously. Laptops that require more than 65w minimum (Precision workstation class laptops, for example) will simply refuse to charge when a 65w adapter is plugged in.

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Stated differently, the hardware is reducing its power state in order to operate with less than full power available to it. Many devices have a specific requirement for operation. Plug a 9V adapter into a 12V electronic keyboard, it won't power on at all! Laptops, however, are designed to accept varying levels of power, and operate at various power levels to provide a longer battery life. However, if consumption exceeds the battery charge rate, the hardware will immediate lose power when the battery's level is critical, because it couldn't maintain operation. – phyrfox Mar 1 '14 at 21:16
I would love to know how to override this behavior and force the hardware to not throttle. The system is unusable with 65W plugged in, yet I wouldn't mind if it charged slower or even lost power slowly, as long as I could do my work. – Artem Russakovskii Jan 6 at 21:20
I figured it out. See my answer below. – Artem Russakovskii Jan 6 at 22:45

If you can't help it and need to use this specific underpowered power adapter but don't want to pretty much end up with a useless brick due to a severely throttled CPU, I just figured out the way to make it go to the full speed.

Pretty much what you need to do is reboot into the BIOS and in Performance settings disable SpeedStep and C states (I'm not sure which one did it, maybe SpeedStep alone is enough, maybe both).

After that, my CPU stopped locking to 800MHz and performance is as good as ever. Funny enough, the battery is actually charging and not depleting slowly, as I was afraid it might.

As for the concern that the power adapter will overheat and potentially cause fire because it'll try to overexert itself, I don't see it as an issue here myself. It's warm, but not warmer than usual, and not remotely hot.

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Thanks for your response! Can you provide your computer model and OS please? I've since moved onto an upgraded computer, but other users may find this information useful, so it would be great if you had the time to explain what steps you went through (keys pressed, etc.) to boot into BIOS and disable SpeedStep and C states. – user3172050 Jan 7 at 1:36
My Dell is a Latitude E6520. I think the rest of the response is self explanatory, to be honest. If you plug in an underpowered charger and reboot, it'll prompt you to boot into BIOS automatically if you press F2. – Artem Russakovskii Jan 7 at 23:40

Sounds like a possible 'safe mode' operation or throttling because of improper hardware/power source.

It is likely.

I have even seen it go as far as to not charge the battery at all if the proper adapter isn't used.

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