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I purchased a Lenovo T420S laptop with a 90W/20V AC adapter P/N 42t4426 through my IT department. Subsequently, I ordered a replacement adapter and received a 135W/20V AC adapter P/N 45N0054.

The 135W version is about twice as big, twice as heavy, and twice as expensive as the 90W version.

  • Is there an advantage to the 135W version like faster battery recharging?
  • Are there any negative effects (other than weight), like reduced battery life?
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3 Answers 3

up vote 3 down vote accepted

In general, there is little point to getting a larger power-supply for laptops.

Using a larger power supply makes sense for desktop computers since it allows you to install more components like drives and such that draw a lot of power.

With a laptop however, there is little room for adding components. A wired keyboard and mouse will consume a little bit of power, but certainly nothing that will require a larger power-supply.

The most power-consuming components that you can add to a laptop will be bus-powered USB devices. Obviously using a larger power-supply will allow you to chain more of them (e.g., through a non-external–powered hub), but unless the laptop is from a cheap manufacturer or a super-cheap model, then the power-supply that is provided with it will be rated to be more than the laptop’s maximum draw by at least a small margin. This allows you to add several power-drawing components to each of the ports and usually still have some unused capacity.

You can determine your maximum power draw with a meter like the Kill-a-Watt (your local library may have them for loan). Plug in all of your components and run a 3D game and see how high you can push it. Then you’ll know for sure if you need a larger power supply or not.

Kill-a-Watt

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90W/20V gives you a max current of 4.5A where the 130W version gives you a max current of 6.5A.1

A higher max current allows the laptop to charge faster (the laptop could still be a limiting factor), or charge while under a heavy load (some cheep power adapters can not charge a laptop if it has the graphics card active).

1: Power (Watts) is Voltage (V) times Current (Amps) so Watts = Volts*Amps or in your case Watts/Volts = Amps.

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It won't charge the battery faster since charge current is limited by the battery charge controller. –  Keith Sep 19 '12 at 20:37
3  
Yes, but some vendors ship power supplies that output a current that is below the max current of the battery charge controller. (glares at HP) –  Scott Chamberlain Sep 19 '12 at 20:38
    
+1 "glares at HP" lol –  Moab Sep 19 '12 at 21:03

No advantage on the charging time and no negative effect. From the laptop side, that would be essentially the same, i.e. a 20 V power supply. The wattage is just the maximum power the adapter can supply but the actual one only depends on the laptop consumption.

The new AC adapter might last longer as it will be used farther than its limits, and it might also generate less heat.

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Do you suppose lifetime justifies the fact that the 130W version costs twice as much? Or am I just paying for weight? –  David Sep 19 '12 at 20:21
    
@david If the PS with the smaller capacity fails twice as often due to being used near its limits then the large one pays for itself. But you'll only know with testing and looking at failure rates. –  Keith Sep 19 '12 at 20:35

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