Question

How to convert Watts per Hour on a laptop (external battery)

Asked by: smyers051972

Hi all,

I travel a lot and find that a lot of planes I am on dont have the 12V plugs on them and figured I would buy a universal external battery for my laptop.  It runs 19V @ 4.74A.  The battery I am eyeballing is 73 watts/hour.  If I calculated this correctly I would need 90-91 watts/hour to run the laptop, it claims to have 3-4 hours run time on it which is about what I will need, that would be about 7-8 hours run time in total.  Usually I can plug in some where in about 8 hours to recharge.

Could someone perhaps spell out for me what 73 watts/hour should be able to do for me.  I imagine that if the laptop is off it can recharge better or would this simply just slow down the time it takes to discharge the battery?

Thanks!

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Asked On
2009-03-20 at 13:03:58ID24250723
Tags

external laptop battery recharger

Topics

Business Travel

,

PC Laptops

,

Power Supplies / UPS

Participating Experts
3
Points
500
Comments
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Answers

 

by: nobusPosted on 2009-03-21 at 01:53:27ID: 23946436

19 V x 4.74A = 90.06 W is what your laptop will use; as you calculated.
do you know the Total capacity of the battery ?  


not sure what you mean with "it claims to have 3-4 hours run time on it which is about what I will need, that would be about 7-8 hours run time in total"

maybe this helps too :  http://www.laptoptravel.com/Article.aspx?ID=211

 

by: PCBONEZPosted on 2009-03-21 at 04:54:42ID: 23946886

19V x 4.74A = 90.06W is the rating for your adapter. It's not a normal, it's a limit.
That's -maximum- watts your laptop can ever 'pull' under any condition.
That's like CPU loaded down to 100%, all USB ports sending out 500 mA each, hard drive running full blast, wireless or LAN in use, and playing a CD/Sound/video intensive game all at the same time.
Typical (average) power use would probably be around 50-75% of that when it's not in standby.

Are you sure you got the rating right?
73 watts/hour and 73 watt-hours are completely different things.
watts/hour is a -rate-, like gallons/hour. [watts divided by hours]
watt-hours [for a battery] is a storage -capacity- or -volume-, like gallons. [watts times hours]

73 watts/hour x 2 hours = 146 watt-hours
73 gallons/hour x 2 hours = 146 gallons

Batteries are actually rated in volts and amp-hours [someone played with the numbers, which is okay]
Assuming a 20v battery,, with a 10 amp-hours capacity,, it would be 200 watt-hours and can.
- Send out 1 amp for 10 hours. -> 2 watts/hour
- Send out 10 amps for 1 hour. -> 200 watts/hour

So: 73 watts/hour doesn't tell you anything useful about capacity.
.

 

by: smyers051972Posted on 2009-03-21 at 06:55:15ID: 23947364

Here is the link to the item I purchased maybe this will help.

http://www.global-batteries.com/product_info.php/products_id/5877

When I say 7-8 hours of max use, I was thinking between the battery on the laptop and this battery in combination.

 

by: nobusPosted on 2009-03-21 at 09:24:10ID: 23947856

can you post your laptop model ? then we have more info

 

by: smyers051972Posted on 2009-03-21 at 09:29:51ID: 23947882

Gateway CX2755 convertible notebook

 

by: smyers051972Posted on 2009-03-21 at 09:31:48ID: 23947888

Max power supply notes 4.72A 19V, low number shows 3.42A 19V input.

 

by: PCBONEZPosted on 2009-03-21 at 10:25:39ID: 23948066

Those are numbers for different but compatible power supplies.
Gateway says the 'stock' unit is 3.42A 19V so that means that's the max the unit could even pull.
- That's only 65 watts.

The info in the ad makes no sense unless there's a typo.

Here's an idea.
You know how long it lasts on batteries so look at the batteries.
Generally the amp-hour (usually in mA-hr) is written right on them.
You can use that number and the time they last and figure out how much power is used up when YOU use the laptop for what you do.

.

 

by: nobusPosted on 2009-03-21 at 10:34:13ID: 23948091

strange - it looks quite different :  http://www.mundocorp.com/shopexd.asp?id=10720
and :  Replacement Laptop 5200mAh Lithium-ion Battery For Gateway 104891

did i miss something?

 

by: PCBONEZPosted on 2009-03-21 at 10:37:48ID: 23948104

There are three different batteries that fit it.
http://support.gateway.com/s/Mobile/2007/ViperSR/2905955R/2905955Rcl3.shtml

There are 7 different adapters that fit it.
http://support.gateway.com/s/Mobile/Q106/Oasis/2527829R/2527829Rtc.shtml

Obviously the lightest duty of the bunch is adequate power.

 

by: nobusPosted on 2009-03-21 at 10:42:33ID: 23948118

as usual PCBONEZ; tx

 

by: PCBONEZPosted on 2009-03-21 at 10:45:40ID: 23948128

The stock batteries (most of them) say 4800 mAhr.

 

by: smyers051972Posted on 2009-03-21 at 13:47:00ID: 23948797

Guys this is an external battery to help with possibly recharging my laptop say im on a 12 hour flight or something, want to have power for most of the flight :)

 

by: ridPosted on 2009-03-21 at 14:37:58ID: 23949013

The idea is to have an energy reservoir to tap from. A laptop will take like 10 - 15 W at "normal" use, depending a little on model and such, that is of course like 10-15 Wh per hour (as "watt-hour" is watts times hours). If you have a 100 Wh battery, you'll have energy for 8-10 hours in that battery.

Normally, a battery isn't labeled in Wh, but in Volts and Ampere-hours (Ah) or milliampere-hours (mAh). The total energy must be calculated from standard formulae, like "Power is Volts times Amperes" and then you use the Ah/mAh figure out the total stored energy of the battery. The voltage may be assumed to be stable during discharge.

The power brick is designed to be able to charge the battery while the laptop is doing all the things PCBONEZ mentioned, so its total power output is way more than the laptop will take on its own (from the battery).
/RID

 

by: PCBONEZPosted on 2009-03-21 at 21:38:10ID: 23950245

smyers051972
Yes I know exactly what you're looking at buying.

My suggestion was that as you know how long the existing battery lasts and the mAh of that battery you can calculate the actual watt-hours that YOU use for what you do.
[Everyone's use is a little different.]

You can then use that to estimate how long the external battery pack will last for YOU.

.

 

by: smyers051972Posted on 2009-03-22 at 10:04:07ID: 23952276

Hey guys thanks for the quick replies.

Ok so I took out the battery its "RATING" is 11Volts 4800 M.Amps.  It will normally last about 3-3 1/2 hours if the screen is dimmed down, if its not ill get about 2 1/2 - 3 hours MAX usually closer to 2 1/2 hours.

And taking into account the previous messages, looks like this battery can do 19V (more then what the battery rating is by 8 volts).  So I should be able to charge the battery fully for another 4 hours use MAX then giving me about 7 - 7 1/2 hours use?

If I could get the final opinion on this cool let me know who I should award the points to as well.

Thanks for the help guys!

 

by: nobusPosted on 2009-03-22 at 10:23:16ID: 23952372

i vote for PCBonez
your battery holds 52.8 W - if this supplies you for 3 hrs, then you need 17W per hour.
so your new one of 90 W should give you 5.2 extar hours

 

by: ridPosted on 2009-03-22 at 10:28:34ID: 23952393

The voltage of the external battery must match the input requirements of the laptop, that's why it's 19 V. The 8 extra volts don't give more working time in themselves. What you need to look at is the mAh (or Ah) rating of the external battery. What matters in the end is the number of Watt-hours that can be had out of it. Your estimate (4 h extra from the external battery) may be dead on, actually, but without knowing the battery's capacity it is difficult to say anything more precise.

A very rough check gives that your internal battery will hold about 50 Wh (4800 mAh at 11 V), so if it gives you 3 h of working time, the laptop has a mean power demand of about 15-17 W. If the external battery can hold 60-65 Wh, you have 4 more hours of useful working time for your laptop from that battery.
/RID

 

by: smyers051972Posted on 2009-03-22 at 18:19:44ID: 31560734

Thanks!

20120131-EE-VQP-002

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