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.)?

 

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.

 

docking stations require more wattage too i think.

 

I always figured docks had their own power as the IBM ones I saw did.

 

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.

 

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.

 

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!

 

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.

 

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..

 

Usually, but not always, notebooks with a dedicated GPU need a power supply bigger than 65W.

However, some designers may choose to make compromises, such as allowing a longer battery recharge time if the CPU / GPU are under heavy load.

John

 

While I agree with your formula I still disagree. Surely (especially with the volatility of lithium ion technology) the current is limited to the maximum safe current for that battery. If not the maximum, then an acceptable level considering the power requirements of the other components in the laptop. If so, then a larger power supply could not apply more current than what is limited.

I agree of that. I was trying to illustrate (poorly) my point in a simple manner.

It's the best I could do, I'm tired!

Either way... We'll have to agree to disagree

 

Nope, the power rating has absolutely nothing to do with charging rates.

The rate that a battery will charge is purely a funtion of the chargers supplied voltage and the batterys impedence. If a 1000Whr charger supplies 15V and a 65Whr supplies 15V then they will charge at the same rate (assuming the device does not exceed the ratings of the smaller charger). The Whr will determine the amount of current the charger can supply to maintain the 15V.

A different charger will matter if the voltage ratings are different. Most notebooks are 15 to 19 volt supply. Use the wrong one and you could fry something. The voltage output by the charger has to match what the notebook wants and the power rating should be at least as high as the original charger.

As far as manufacturer not having charging control circuits, I have no idea. I would think that most would include them because they are a very cheap thing to implement and improve battery life/performance, but I have never really had cause to find out if they use them.

 

a 65w adapter for the same manufacturer with the same plug is the same and can be used to replace 90w and vise versa. you can run into a problem when replacing a 90w with a 65w. the problem is usage. if the laptop was sold with a 90w that means there are certain times when a 65w was not enough (gaming and any activity that pushes the laptop). in this even the 65w adapter would overheat and shut down or die. It is ok to replace a 65w adapter with a 90w as long as the plugs are IDENTICAL and the adapter was specifically made for a similar laptop. It is NOT ok to use an adapte for your printer if the plug is identical. We use 90w adapter for all asus models for testing. There are different adaoters however. some have longer or shorter plugs. the newer longe4r plugs are downward compatible while the older shorter l shaped plugs are not up compatible

 

I am afraid that you have it the wrong way. The current is not a function of the impedance. Rather, it is the cuttent that charges the battery. Q=I*T. Basic school physics... The charging voltage determines whether the battery will be charged or not. If the Applied Potential difference across the battery terminals is lesser than the Open circuit voltage of the battery, the battery won't be charged at all. If it is greater, it will be. Of course, too high a potential difference would mean frying something.

I study this for a career, so I would know before arguing on the wrong side...

 

The voltage determines the charge rate. The higher the voltage the more current (which does the charging, yes) and the faster the charge.

A charger will supply a constant voltage. The current will vary as the battery charges and the batteries impedence varies not based on the Whr rating of the charger. The Whr rating of the charger determines the MAX current it CAN supply.
If the batteries impedence is low then the voltage drop will decrease so the charger (as long as it can do so) will increase the voltage (with current going up also) back up to its setpoint. This will give a higher rate of charge. thi will happen in larger body batteries or even when a battery is drained.

I studi ed and use this for a career.

 

V=IZ. NOT I=V/Z. I=V/Z is a valid relation. But it does NOT automatically imply it's physically correct. To give a real world example, take the case of power transmission lines. AC power is transmitted at high voltage to reduce dissipation losses, since P=I^2*R. If I=V/R would still hold, then the dissipation in transmission lines would be P=V^2/R, which would lead to huge losses, since obviously the impedance of the transmission lines does not change. Which obviously would have negated the benefits of HVAC transmission. And which does not happen....

EDIT: I was wrong.. to an extent. The transmission line analogy would be invalid. The battery's impedance does change with its level of charge.. Should have taken my chemistry classes more seriously .

 

Aw, I shouldn't have come across this post . . . I'm having my circuits exams on August 2nd and I'm pretty sure my goose is cooked, broiled, and fried . . .. .


p.s. It's really getting on my nerves . . . T-T

 

Circuits was one of my easier in major subjects, it always just made sense to me.
Now Feedback was a whole different story. I still think amplidynes are PFM