How many watts for a laptop adapter?

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How many watts for a laptop adapter?

Postby scruffy » Fri Jan 08, 2010 3:24 am

I recently lost the charging adapter for my HP laptop and I need to get a new one pronto.

My business laptop came with a 120-watt adapter. However, HP offers numerous adapters that are "compatible" with my model, and they range in power from from 65-watts to 230-watts.

The 65-watt adapter is attractive, because it has a smaller form factor that makes it easier for travel.

My question is...how do I know how many watts I "need"? Will any of these be good enough? What is the benefit of having a larger number of watts? I'm not connecting additional hardware to my laptop (e.g., no docking station, etc.). Does having more watts make your battery charge faster?

Should I just get something close to 120 watts because that is what the original adapter was?

And finally, the HP replacement adapters are a bit pricey. Does anyone have a recommendation for a good third-party adapter?

Thanks for any and all help.
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Postby thomas_w_bowman » Fri Jan 08, 2010 5:32 am

First, check power input details located at the bottom of your laptop (No Model specified, and HP makes a lot of different Laptops).

For a small profile, perhaps the Cooler Master RP-095-D19A-A1 SNA 95 Slim Notebook Adapter (about $60):
http://www.newegg.com/Product/Product.a ... -_-Product

This adapter has a reasonably small form factor, yet is about 100W. Reviews are pleased with how it charges batterys.

You can probably get by with the 65 watt version, it's cheaper (if the laptop needs more power it'll take what's needed from the battery) about $36:
http://www.newegg.com/Product/Product.a ... 6834991048
Note that review for this suggests that it may not be able to charge the battery...and has marginal wattage for larger displays. Also at 65W you're trying to operrate at 1/2 of the power that your original adapter had - the more I think about it, the more the 65W model seems like a bad idea - you save nothing if you end up needing a different adapter to charge battery...

Personally I'd get the 95W model, 5.7" x 2.9" x 0.7" is not much larger than the 65W 4.25" x 1.81" x 1.18"
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Postby Big Jake » Fri Jan 08, 2010 8:14 am

You should match the output voltage of the charger to the specs on the laptop. Make sure the tip of the charger fits your laptop. Make sure you get the right type of voltage for the country(s) you travel to. And my experience with "ease of use" and laptop bricks is that cord lenght is just as important as size and weight of the brick.

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Postby Mr T » Fri Jan 08, 2010 8:42 am

I have used loads of universal chargers with varying laptops... Get one that

A) has a compatible tip
B) provides the correct voltage range - most range from 15v to 20v with a max of about 4A - standard laptops charge at 19V with 3.42A..
C) get a 90w or above...
D) Don't buy the manufacturers specified charger unless its a DELL... Normally costs you 3x the price of a universal, and any that are cheaper are usually rip-offs that fail after a short period...
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Postby scruffy » Fri Jan 08, 2010 10:50 am

Thanks for the help!
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Postby Karlsweldt » Fri Jan 08, 2010 1:16 pm

Hopefully, the engineers included charging of the battery while the laptop was in 'normal' use with its lid screen, as shown on the label.
If the power needed is 20 volts, for an example, at 4.0 amps, that would equal 80 watts of power. But do not be concerned if the power pack provides more current than needed, as only what the unit needs will be drawn. The important factor is that the required voltage is met, plus the minimal amperage indicated.
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Postby scruffy » Fri Jan 08, 2010 3:59 pm

Thanks, Karlsweldt. I took a look at the specifics for my laptop (HP Elitebook 8530p) and now I declare myself officially confused.

Here's what I know:

Under the "specifications" in my laptop's maintenance guide it says:
-----
Input power: 19V dc @ 4.74 A -- 90W
-----

If I remove my battery, in the compartment it says:
----
18.5V dc @ 6.5 A
----
(The latter makes about 120 watts if I calculate correctly.)


Under the section that says "Production description" in the manual it shows:
----
AC adapter with localized cable plug support (3-wire plug with ground pin, supports 3-pin DC connector):
● 180-W
● 150-W
● 120-W
------

So.....

Why do the specifications and the physical laptop list different amperage, leading to different watts?

All of these suggest that the laptop needs at last 90 watts. So why does HP claim that my laptop is compatible with a 65-watt adapter they sell?

I know that I can buy a 120W adapter and be fine...but I'd really like to understand all of this.
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Postby Mr T » Sat Jan 09, 2010 12:08 am

The wattage basically is an indication of how long the battery takes to charge... The laptop will run off a 65w charger, but will take ages to charge the battery at the same time... 1 unit I tried with a DELL was 90w, the original DELL charger was 65w and the thing would not charge... Turns out it is a compatability issue with the DELL and the charger ( I then tried different units that said they were compatible and no dice).. As I said a 90w will do you, but get one that is compatible and do not get a cheap one... (The main advanatge of a universal I see dau after day is next to a bad DC socket chargers are the things that go... Why? The main reason is either people tripping over trailing cable and ripping the socket out which A) damages the DC socket and B) damages the thin cable by stretching it to the DC socket... Or wrapping the cables in a nice neat ball which also stretches the thin cable, which is armoured and the armour foil cuts the cable inside its protecting... Now if you have a universal, it can go with you to the next laptop as a spare)...
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Postby Karlsweldt » Sat Jan 09, 2010 3:31 pm

AC adapters (power supplies) and batteries are rated for their maximum output, in voltage and amperes, possibly volt-amperes. AC adapters do have limiters in their workings to prevent over-current and over-voltage from happening, as well as shutting down if excess current draw or heat is sensed.
With electronics batteries, there is a safety fuse (thermal as well as short-circuit) within the case, to help avert a disaster. But not so with automotive type batteries.
The note about 180/150/120 watt adapters refers to alternate units.
Any AC adapter or battery charger will put out a bit more than what the battery capacity notes, to ensure that the battery charges. With most auto systems having a 12 volt system, the charging voltage is typically 13.8~14.2 volts. Too high a voltage with too much current can cause a battery to overheat and explode.
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Postby Copper » Sun Jan 10, 2010 10:41 am

when i have had to get replacement laptop PSU i found 90watt dose the job in all cases
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