Battery watt-hours and laptop wattage? - General Hardware - Laptop - Dell Community

# Battery watt-hours and laptop wattage?

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#### Battery watt-hours and laptop wattage?

• (1) How many watt-hours is the battery capacity?
(2) What is the wattage drawn by the laptop?
(3)  The reasons I want to know this is because I am going to purchase a
PCMCIA card that draws 400mA at 3.3V.  This is approximately 1.3 watts.
If this is small compared to the wattage of the laptop, it will not
affect the battery life much.  If it significant compared to the laptop,
it will the battery life noticably.
I wonder if the watt-hours and wattage figures are available somewhere
on the web.  I can't seem to find it at the Dell website.

Thanks.

Fred

P.S. Is there a way to copy my posting (the initial posting that starts off
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• What battery for what model notebook. This info would be need to even try to answer your question.

The battery will last until if goes dead. As to how long that will be it all depends on how you use your notebook.

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• Sorry, I thought that this general hardware group was
general hardware for Inspiron -- I shows up when I follow

I have an Inspiron 8000, with 8-cell lithium ion.  Suppose
to last 3 hours on a full charge (when new).  Voltage is
14.8V, takes 1 hour to charge if laptop is off, 2.5 hours if
laptop is on.

The processor is a Pentium III, running approximately
500-700MHz.

Here's my math to get a ball-park figure.  The AC adapter
output is 3.5A.  Let's make a major assumption, and say that
all of it gets packed into the battery.  Over 2.5 hours
charging, that packs Q=31500 coulombs of charge into the
battery.  Since the battery supplies power at V=14.8V (say
15V), the energy is E = Q x V = 472500 joules, or E = 131
watt-hours.  This energy is fed out over t = 3 hours, so the
power is P = E / t = 131 / 3 = 44 watts.  So the laptop
draws around 44 watts.  The number seems high, but for all I
know, this could be normal for laptops.  The PCMCIA card's
1.3 watts would be neglegible.

However, I've heard people mutter that the battery might
be around 60 watt-hours, in which case the laptop draws
20 watts over 3 hours.  The card's 1.3W would still
represent a neglegible load, but it would be nice to be
able to get a sanity check (or even better yet, and
a figure based on vast experience).

P.S.  Is there a way to copy these postings to myself?
The PC that I'm using to read the forum doesn't have access
to my mail server, which I access from a solaris environment
running mozilla.  When I try to read this forum using mozilla,
the popup for emailing a forum posting doesn't work.

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• Oh, it's worthwhile adding that the estimated 1.3 watts of the PCMCIA card
is based on an assumption.  It assumes that the battery's 14.8V can be
converged to 3.3V for the card, and that this conversions is done is a way
that is lossless with respect to power e.g. a transformer rather than dropping
it across a resistor.  There are many possibilities, and I'm not that familiar
with area of DC voltage shifting for power supplies (either in general, or
for portable devices).

If the 3.3V is not losslessly obtained from 14.8V, then in worst case, the
card's 400mA is somehow drawn from the 14.8V battery, which would
give it a power impact of 6 watts on the battery.  Ouch.  If the laptop
does indeed draw 20 watts, that's quite an extra burden.

Fred
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• according to here

http://www.computerbatteries.co.uk/gb3/m/dell/inspiron-8000.html

a replacement battery for an Inspiron 8000 is rated at 4460mAh 14.8Volt

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• A couple different batteries are listed for the I8000. One is rated at around 3300mAh and another at 66WHr/44000mAh. This latter battery is also rated for 2-3 hours in "normal" use, but the spec fails to differentiate between several notebooks for which it is listed.

My guess is that "normal" use runs anywhere from 20-30 watts on the I8000. A 60 watt-hour battery would last two or three hours, and that roughly correlates to the specifications.

Your charger is rated to deliver up to around 50 watts, depending on the load applied to it's constant voltage output. A 60 watt-hour battery is not likely to be entirely exhausted, so the rating of a one hour charge time with the notebook off is not too far off, though a bit longer charge may be required.

Given the rating of a two and a half hour charge with the notebook on, 60 watt-hours could be delivered by 24 watts per hour, or about half of the charger capacity (3.5A/2=1.75A), leaving the assumption that the notebook will average 1.75Ax15V, or around 25 watts in use, though the notebook load is given priority and the current available beyond it's need is available to charge the battery. Again, these figures indicate the notebook is presumed to average 20-30 watts.

Another consideration is the duty cycle of the load applied to the TI CardBus Controller. The 400mA figure is the maximum capacity required from the controller, but may not be constant or continuous. Again, the voltage is constant but the current delivered is dependant upon the load applied. The PC Card manufacturer may list additional detail.

GM
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• 4460mAh, I /assume/ means 4.46 Amp-Hours, which would be 14760 coulombs.  About half of what I calculated, which corroborates with the 60 watt-hour figure that I said people were muttering.  Looks like they were right, and the power adapter's output current doesn't all go into charging the battery.  Thanks.

Fred

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• Doh!  My mistake.  The 2.5 hours charging time I used was for when the laptop is turned on.  Duh.  No wonder it gives a Watt-hour figure for the batter that is twice what people were muttering.

A card wattage is specified here http://www.smc.com/files/AN%5C2835WV3.pdf as a current drawn from 3.3V.  Because of the context, I expect it is meant to be interpretted as the constant-current equivalent (equivalent in terms of power).  /If/ the laptop's 14.8V is converted relatively losslessly to 3.3V, then the wattage is trivial, around 1.3W.  If the 400mA is drawn from the 14.8V somehow, then it is nontrivial, about 6W.  It all depends on how the Inspiron was designed to get 3.3V from from 14.8V.

Strange that the 1.3W doensn't seem to jive with
http://www.synack.net/wireless/consumption.html in terms of order-of-magnitutde, which shows hundred of mircrowatts.

Fred

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• Yup, I would have expected a more significant reduction in the idle state, but I'm at a loss to correlate the data from the test report with the mfr. spec sheet, except the test report was on 802.11b only, and the extra speed of the 802.11g would drive up consumption. That SMC card is rated up to 1,150 feet, quite a range for a built-in antenna. Still ... How do those power consumption and RF Power output specs compare to similar cards?

Maybe those power consumption figures are mis-labeled? 480mW + 380mW?

I'll bet that's it! Well, I'll bet a nickel anyway.

GM
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• I think the savings in idle state may depend alot on the air interface standard, and how much overhead exists in staying known to the access point when there is no data to transmit.

How do you know that the test report were on 802.11b?  It says the test was done in Infrastructure Station mode.  I was under the impression that 802.11g has 2 modes, one for talking with access points, and one to talk peer-to-peer.

A browse around shows that hundreds of milliamps is typical for a card, so regardless of whether it works off 3.3V or 5VC, the power should be in the order of 1W (give or take 50% -- it's only an order of magnitude estimate).  This would be neglegible for a 20W laptop.  The typo is probably in the test report, which shows consumption in the order of 100 microwatts.

Despite this, wireless cards have been known to be power hogs.  I suspect that the hundreds of milliamps is drawn directly from the 14.8V battery, probably with voltage dropped across something in a power-wasting way in order to get 3.3V.  Only a guess.

What are the experiences of wireless card users on this?  Does it seem to impact the duration of a battery charge noticably?  If the card boosts power from 20 watts to 26 watts, that would decrease battery duration by more than 25%.  Assuming 2 hours on a relatively new batter, that would drop the charge so that it lasts 1.5 hours.

Fred
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• Yes, I see similar specifications on other wireless cards. As you point out, the savings depend largely on the idle state consumption and the overhead required to remain awake, and the published specifications are less revealing in this regard.

I noted a reference in the Synack test report, immediately preceding the summary, stating the cards were all 11Mbps (802.11b) devices. It's disappointing that so many web "documents" are undated, and curious that this data deviates so significantly from all other sources.

This article might be interesting, and there are clues to indicate it's relatively current.
http://www.crhc.uiuc.edu/~mahesri/classes/project_report_cs497yyz.pdf

There seem to be several abstracts on power saving possibilities, but it's hard to determine which specific methods are implemented in wireless cards currently on the market and what the practical impact is. Even those marketed as "low power consumption" simply state the Tx and Rx load figures without mention of power control or mode algorithms.

GM
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• Regarding overhead in keeping known to the access point during idle periods, the power consumption probably depends as much on the card design as it does the spec.

Thanks for pointing out the 11Mbps, it was confusing so I must have glossed over it.  (It says 11 Mb).

Fred

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