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# UGENT! watts and hours...

Bert Bates
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Sheriff
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Hi Guys,

My basic Physics E&M is a little rusty

If my laptop is listed as 90 watts, I'm assuming that if I run it for an hour I've used about 90 watt hours? (or .09 kilowatt hours)

So if a battery backup system says it has a "900 watt" capacity, would that mean I can run my laptop for roughly 10 hours (i know there will be inefficiencies, I'm just trying to get the basics down here.)

I guess the question is this, if a battery backup system says it's capacity is X watts, do they more precisely mean X watt hours?

Mike Simmons
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My guess is no, it's not 900 watt-hours, but 900 watts. The 900 is not intended to say anything about how long it will run. But it does tell you how much power you can put out at once. If you have a device that consumes 400 W, you can power it with this backup. And you could even power two of them at the same time, for 800 W. But if you have a third and try to hook that up too, there's a reasonable chance that all 3 devices will fail for insufficient power.

Bert Bates
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Thanks Mike,

So is it possible to determine - by reading the specs - how long it would run? (even roughly)

Mike Simmons
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Not from the 900 W number, no. If they provide some other info, in Watt-hours for example, or some spec about how long they could power a 50 W device, we could work it out. But while the 900 W number is useful info for other purposes, it does nothing to answer your question.

Mike Simmons
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" The light that burns twice as bright burns for half as long - and you have burned so very, very brightly, Roy."

Bert Bates
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Hey Mike,

Thanks - you've confirmed my suspicion that I need to see some form of "Watt hour" specification to answer my question.

It's interesting that the folks who make these backup devices don't put that kind of spec front and center

Bear Bibeault
Author and ninkuma
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You're quite a prize!

Martin Vajsar
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Bert Bates wrote:... I need to see some form of "Watt hour" specification to answer my question.

Battery capacity is usually specified in amperhours. You'd have to multiply this by volts to get watt-hours, I'd say.

However, my advice would be to simply plug the laptop into the (fully charged) backup system, pull the plug and measure how long will it last. The consumption listed on your laptop is peak consumption, not average consumption anyway. And it will wary depending on your use of the laptop.

Make sure your laptop internal battery is not recharged from the backup power system - it would be very inefficient use of that backup. And, out of curiosity, why do you need a backup power for a laptop at all?

Bert Bates
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It seems like most of these UPS type systems aren't intended to run a laptop for a long time.

I read that some DIY guys get a car battery and put some filter on it, and run there laptops for a long time that way, for cheap.

So I wonder how to find a non-DIY solution along those lines...

So, that's the bottom line:

when the power goes out - what's a cheap way to run a laptop for a long time (say 10 hours)?

Bear Bibeault
Author and ninkuma
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Squirrel power?

Bert Bates
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Sheriff
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Hi Marten,

I'm trying to finish K&B 7 and my house might lose power for a day or two.

I need to power my laptop so I can keep working

Since our posts got crossed, the bottom line question is:

When the power goes out - what's a cheap way to run a laptop for a long time (say 10 hours)?

Martin Vajsar
Sheriff
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Just thinking out loud: some laptops have adapters that can be plugged in in cars (don't know how this is called in English ). And these car plugs can be powered by battery - the power goes there when you turn the ignition on without starting the car. I'm not sure whether there is any electronics between the battery and the car plug, but I'd say there shouldn't be. So, if you can get a car adapter for your laptop, this would seem like quite an easy route for me.

Jayesh A Lalwani
Rancher
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Well, since car batterries are 12V DC, and your laptop needs a DC current you need to convert the DC voltages. I just checked my laptop needs 19.5V. You can either use a step up converter or put 2 baterries in series and a step down converter. I think you will have enough wattage from the car batteries to provide enough power for a laptop.

I forgot all my electronics courses (and probably by now the technology is improved from what I had learnt) that I won't comment any further

Mike Simmons
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You could also get a car power adapter that converts a cigarette lighter into a 120 V AC outlet. Here's one: [link]. Plug in your power supply to that, and there you go. It's a bit odd to convert DC to AC and back to DC, but what the heck, it works, and lets you charge all sorts of things from your car while traveling. I've powered a laptop with one of these (well, one somewhat like it) years ago, didn't have any problems.

Paul Anilprem
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Jayesh A Lalwani wrote:Well, since car batterries are 12V DC, and your laptop needs a DC current you need to convert the DC voltages. I just checked my laptop needs 19.5V. You can either use a step up converter or put 2 baterries in series and a step down converter. I think you will have enough wattage from the car batteries to provide enough power for a laptop.

I forgot all my electronics courses (and probably by now the technology is improved from what I had learnt) that I won't comment any further

DC can't be easily stepped up or down. That is one of the reasons it fell out of favor.
Step down is a little bit easier if you know the resistance of your desired device. For unknown or variable resistance, step down is also very difficult. Hence the need to convert DC into AC first.

Paul Anilprem
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Mike Simmons wrote:You could also get a car power adapter that converts a cigarette lighter into a 120 V AC outlet. Here's one: [link]. Plug in your power supply to that, and there you go. It's a bit odd to convert DC to AC and back to DC, but what the heck, it works, and lets you charge all sorts of things from your car while traveling. I've powered a laptop with one of these (well, one somewhat like it) years ago, didn't have any problems.

I have such an adapter. It came with pins for different sockets and voltages. I use it very often. Works very well.

Bert Bates
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I think I want to give this route a try, kind of a riff off everyone's feedback:

I already have a standalone battery with jumper cables built in, for jumping car batteries.

This thing also has a "cigarette lighter" style outlet to which you could plug the car-adapters people have been discussing.

So I'm thinking of hooking the adapter to the jumper battery, then plugging in the laptop cord to the adapter.

Two issues:

- could this damage my laptop?

- how long will it run - if it doesn't damage the laptop, it's a cheap experiment to find out

Paul Anilprem
Enthuware Software Support
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Bert Bates wrote:I think I want to give this route a try, kind of a riff off everyone's feedback:

I already have a standalone battery with jumper cables built in, for jumping car batteries.

This thing also has a "cigarette lighter" style outlet to which you could plug the car-adapters people have been discussing.

So I'm thinking of hooking the adapter to the jumper battery, then plugging in the laptop cord to the adapter.

Two issues:

- could this damage my laptop?

I have been using it for over an year for HP as well as Lenovo laptops. No issues so far.
Bert Bates wrote:
- how long will it run - if it doesn't damage the laptop, it's a cheap experiment to find out

That really depends on the size of the battery. The Wattage rating doesn't necessarily tell you the time it will run because it may also refer to the peak wattage. For example, a 900 Watt rating may mean that it will allow an appliance to draw power at the rate of 900W per hour but not necessarily for 1 hour. It could be just for a few seconds.

Sorry I didn't realize you are talking about using it standalone (ie. without the car running). I wouldn't not recommend this set up because all batteries drop voltage as they deplete, specially the lead acid ones. So unless it is connected to a car's alternator (which charges the battery as it runs), it may damage the adapter and/or your laptop.

Bert Bates
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So Paul,

This thing mentions auto-shutdown, would you trust it?

http://www.amazon.com/gp/product/B003SGJIRO/ref=s9_simh_gw_p422_d0_i4?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-2&pf_rd_r=09MR0P612ZYS5MWB69JW&pf_rd_t=101&pf_rd_p=1389517282&pf_rd_i=507846

Paul Anilprem
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Looks like it explicitly mentions over and under voltage protection so I guess, yes, it should be ok. BTW, the laptop adapters are also quite sturdy. It amazes me that the same adapter works in US (110 V) and in India (220V) (I have tried that out numerous times, no issues). 110V is a big range so there is good level of tolerance built in the adapters as well. So if the above mentioned device shuts down gracefully, I think the laptop will be ok.

Pat Farrell
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Bert Bates wrote: when the power goes out - what's a cheap way to run a laptop for a long time (say 10 hours)?

Plug it into an inexpensive inverter in your car. Will run until you run out of gas/diesel in your car. Just remember to open the garage door.

Ernest Friedman-Hill
author and iconoclast
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I remember reading this when it came out on HackADay. It took me a while to find it because the original site it gone but I did. Enjoy:

Homemade laptop battery pack using 20 D-Cell Batteries

The guy says this powered his G4 Powerbook on a 13-hour airplane flight!

Bear Bibeault
Author and ninkuma
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I can imagine the hilarity that would ensue should someone try to take that on a plane these days.

Andy Jack
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Bert Bates wrote:
When the power goes out - what's a cheap way to run a laptop for a long time (say 10 hours)?

Go to the neighbors house :P

But seriously speaking, consider investing in a cheap long power extension cord which can be hooked to your neighbors house.

Campbell Ritchie
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Surely the neighbours’ power will go off too.

What is this thread doing in MD rather than general computing?

Mike Simmons
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Campbell Ritchie wrote:What is this thread doing in MD rather than general computing?

Bert Bates
author
Sheriff
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Alright. How about - to be extra, double-dog safe - getting an external battery charger (like below) plugging that into the car-thingy, and charging the laptop battery externally?

http://www.amazon.com/gp/product/B001O3VN62/ref=ox_sc_act_title_1?ie=UTF8&smid=A151D61AL6T7YU

Andy Jack
Ranch Hand
Posts: 257
Bert Bates wrote:Alright. How about - to be extra, double-dog safe - getting an external battery charger (like below) plugging that into the car-thingy, and charging the laptop battery externally?

http://www.amazon.com/gp/product/B001O3VN62/ref=ox_sc_act_title_1?ie=UTF8&smid=A151D61AL6T7YU

Rent one of these -

Campbell Ritchie
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Mike Simmons wrote: . . . People read MD.
Touché

Martha Simmons
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Bear: Squirrel power?

fred rosenberger
lowercase baba
Bartender
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Install solar panels or a windmill?

Bear Bibeault
Author and ninkuma
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Martha Simmons wrote:Bear: Squirrel power?

OK, if you have ethical issues putting rodents to work, here's the variant for you.

Martha Simmons
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Posts: 130
Now we're getting somewhere!

How many people in the US exercise? And all this energy is just wasted!

But seriously, is it possible to power a small laptop by a pedal generator? The end of the world is close, after all.

Paul Clapham
Sheriff
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Hand-cranked radios are a common thing these days. But laptops use a lot more power than a radio -- you might need the guy in the hamster wheel to power one of those.

Pat Farrell
Rancher
Posts: 4678
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Martha Simmons wrote:But seriously, is it possible to power a small laptop by a pedal generator?

What do you mean by "power"?
Can a generator be driven by pedals or a treadmill? Sure.
Can said voltage charge a battery? Sure.

Both true, but mostly useless information.

A more realistic question would be: if I run on the treadmill for a hour, how many hours will the laptop run?

My guess is that it will take more hours of running than laptop usage. Guess, three hours running per hour of running Open Office.

Paul Clapham
Sheriff
Posts: 20719
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As the Wikipedia artice Bicycle performance says:

Amateur bicycle racers can typically produce 3 watts/kg for more than an hour (e.g., around 210 watts for a 70 kg rider), with top amateurs producing 5 W/kg and elite athletes achieving 6 W/kg for similar lengths of time.

Martha Simmons
Ranch Hand
Posts: 130
Pat: What do you mean by "power"?

Charge a battery, I suppose. So now we need to determine how much kWh my laptop uses.

Look at the power supply or back of any semi-sophisticated electronics device you want to know the kilowatt usage of. It should have one number on it with an "A" next to the number. This is the amperage. Multiply that by 120 (Alternating Current) and you have the wattage. Divide by 1000. After that, as you may be able to figure yourself already, if you use that device for one hour then you will be said to be using it for that many kilowatts.
How many kilowatts does it take to run a laptop?

Multiply by 120 = 410.4 watts
Divide by 1000 = 0.4kWh

Is this correct?

Martha Simmons
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Posts: 130
Paul citing Wikipedia: Amateur bicycle racers can typically produce 3 watts/kg for more than an hour (e.g., around 210 watts for a 70 kg rider), with top amateurs producing 5 W/kg and elite athletes achieving 6 W/kg for similar lengths of time.

This is life-changing information. Only one question: what is "amateur bicycle racers" amperage?

Martin Vajsar
Sheriff
Posts: 3751
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Martha Simmons wrote:
Multiply by 120 = 410.4 watts
Divide by 1000 = 0.4kWh

Is this correct?

I don't think so.

Firstly, on second line you have watts, which is a unit of power. On third line you have (kilo)watthours, which is a unit of energy. You need to multiply power by time to obtain energy. (Ya know, watt x hour = watthour )

Secondly, 120 is the voltage of the power grid. You need to multiply the adapter amperage by adapter voltage (probably somewhere between 18-20 V) to get the amount of power the adapter outputs. So we're somewhere around 68 W. However, some power is necessarily lost in the adapter as it converts and stabilizes the voltage, so the adapter's power consumption will be a bit higher.

Thirdly, the 3.42A is the peak output. On average, the laptop's consumption will probably be significantly lower. But generally you must be able to provide peak power whenever the laptop wants it, so the squirrel in the wheel needs to be able to produce more than 70 W of electrical power at whim. And considerable amount of energy will be lost in the generator. Poor squirrel.

Martha Simmons
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Ok, my adapter voltage is listed as 19V, which gives us approx. 65W

On the other hand, my kill-a-watt device constantly reads around 25W for my laptop, which probably account for your "3.42A is the peak output" statement.
And don't misunderstand me, I am all for rodents exploitation, I just wonder if it's worth it. I need some reliable source of power when the end of the world comes to run my Civilization Starter Kit DVD.

Now, am I right that Paul's post means that a 70 kg rider can output almost 10 times more power than his laptop consumes?