Quantcast 65W vs. 90W AC Adapter

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  1. #1
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    Default 65W vs. 90W AC Adapter

    What would the difference be between ordering a 65W A/C adapter and a 90W A/C adapter. Is it just a matter of a needing a 90W to power certain things (brightness, watching DVDs, etc.)?

  2. #2
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    Default Re: 65W vs. 90W AC Adapter

    It many cases you may need a bigger adapter with an extended cell upgrade or laptops that allow for video card upgrades.

    I know on Thinkpads that for a T60p and a 9 cell you need a 90Watt adapter or the 65watter won't last very long. Lenovo likes to bundle them for a reason, sometimes the bundles are cheaper than the cell alone.

    If I remember correctly some dells allowed for upgrades from ATI X300s to Nvidia 6800Go and part of that upgrade was you needed the bigger power adapter or it wouldn't charge and power the laptop at the same time.

    Most of the time the 65watter is smaller and lighter and is easier for carry. This depends on the manufacture my 90watt from lenovo is the same size but lighter than my 65 watter from Asus. Maybe the Asus one is better made.
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    Default Re: 65W vs. 90W AC Adapter

    docking stations require more wattage too i think.

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    Default Re: 65W vs. 90W AC Adapter

    I always figured docks had their own power as the IBM ones I saw did.
    Laptop: T60p 2623-D8U
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    Default Re: 65W vs. 90W AC Adapter

    You want to get the AC adapter rated for your laptop. Generally, it'd be better to have the 90watt over the 65watt if you don't know which one you need. Its always safer to have more power available when you need it than not enough.

    Your laptop should have a specific rating. Otherwise, I'd go for more power to be safe, you don't ever want your laptop to start draining the battery while connected to AC power to make up the difference in power draw.

  6. #6
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    Default Re: 65W vs. 90W AC Adapter

    The most important difference b/w a 65W and 90W charger is the charge rate. A 90 Watt charger will charge the battery faster, and while it may sound good, it is actually pretty bad for the battery. To ensure maximum battery life, Li-ion batteries should be charged at a steady, constant and slow rate.
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    Default Re: 65W vs. 90W AC Adapter

    Quote Originally Posted by sesshomaru View Post
    The most important difference b/w a 65W and 90W charger is the charge rate. A 90 Watt charger will charge the battery faster,
    I disagree. The rating with what it's capable of, not what it puts out. Assuming they both output the same voltage they would both put out the same amount of watts to power the laptop. Also there is a charging circuit inside the laptop which controls battery charging, not the power supply. All will be fine as long as the requirements of your laptop do not exceed 65 watts.

    Think of it this way...

    Assume a desktop has a mobo, cpu, onboard graphics and sound, 1 hard drive, and 1 cd and a 350 watt power supply. You buy a $300 superman 650 watt power supply and install it. Assuming the 350 watt power supply was working properly the 650 watt power supply will use the exact same amount of power that the 350 power supply did. The advantage of the 650 watt supply would be if you added 4 hard drives, a couple graphics cards in SLI, etc.

    Think of it like two almost identical cars. One has a 10 gallon gas tank and the other a 20. Both cars will perform exactly alike. The difference is capacity. Or, a 4 cylinder compact and a Corvette. Both can go 55mph, the the Corvette has the capacity to go 150 if needed.

    Hope that didn't confuse anyone!

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    Default Re: 65W vs. 90W AC Adapter

    Except the Corvette blows a lot more gas than your little Prius. But still, I think the 90W will charge your battery faster, as long as the battery is capable of it (which is what I think you're saying, too, right?)

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    Default Re: 65W vs. 90W AC Adapter

    Quote Originally Posted by whistle View Post
    Except the Corvette blows a lot more gas than your little Prius. But still, I think the 90W will charge your battery faster, as long as the battery is capable of it (which is what I think you're saying, too, right?)
    Not exactly. The charging of the battery is controlled by a circuit inside the laptop and has little to do with the power supply. The circuit will control the level and speed of charge.

    Maybe a better anecdote would be a 4cylinder engine and an 8cylinder engine can both power a car, but the 8 cylinder can power a heavier car.

  10. #10
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    Default Re: 65W vs. 90W AC Adapter

    Nope. That kind of throttling is not provided by most mainstream manufacturers.

    To explain how your charging time is decided, let's take a 85WHr Battery. Now, if we have a 90 W adapter, then, assuming an active load of 80%(i.e. a power factor of 0.8), under ideal conditions the 85 WHr battery will be charged in 85/(90*0.8) hours. Or 1.18 hours. Which is 1 hour and 10 minutes . That , of course is when your laptop is off, and the adapter is solely charging the battery. Also, here we have assumed no power dissipation is taking place, and losses are also zero. Which are never true. So a realistic estimate would be about 1 hour and 45 minutes.

    Doing the same with a 65 W adapter would yield figures of about 2 hours and a half(after making allowances for losses, i.e.).

    The circuit that is included in the battery has a different purpose. What it does is to monitor the level of charge. Then, when the charge levels off to about 95~98%, it switches to a trickle charge mode. And when charge level reaches nearly 100%, it stops the charging, and at a fixed interval, sends an impulse to the battery, to maintain the charge(if still connected to ac). It does nothing to control the speed of the charge.

    You could try sticking a wattmeter b/w your 650W supply and the wall socket, and you would see that the power drawn would be somewhat more than a 350 W supply, even if they are supplying the same loads. The reason, again, being that inherent losses in a 650W supply would be more than in the 350W one. And a more fitting analogy would have been a UPS, not the SMPS.

    I am afraid mechanical analogies are wasted on electrical components..
    Last edited by sesshomaru; 23rd July 2007 at 02:01 AM.
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