[Power] Haiti power questions

Timothy Falconer teefal at waveplace.org
Tue Dec 7 12:58:18 EST 2010


Okay, so for my bottom line, is it safe to say that:

Starting with an empty battery and plugged into the grid for 2.5 hours, while using Etoys with ticking clock, it should take 85 kwh per laptop?

And yes, I have a kill-a-watt ... don't know why I didn't think of that before :)


--
Timothy Falconer
Waveplace Foundation
http://waveplace.org
+ 1 610 797 3100 x33




On Dec 2, 2010, at 3:22 AM, Richard A. Smith wrote:

> 
> The following is from a thread that Tim Falconer started with me on one
> of his Haiti trips.  He's trying to establish a template for Haiti
> power.  I've pulled it onto power@ because that's what I created this
> list for.  :)
> 
>>>> Where are the best numbers for Wattage (power consumption) of the
>>>> XO-1: 1) after the battery is full and green?
> 
>>> Assuming AC side of the adapter.
>>> 
>>> According to Google, Haiti is 120Vac 60Hz.  If your generator is
>>> using a different configuration then I'll need to adjust the
>>> numbers.
>>> 
>>> After the battery has been charged the XO draws approx 4-8 Watts
>>> but it can peak up to 9W.  When you reflect that load back to the
>>> AC side of the adapter and compensate for efficiency you get an AC
>>> line draw of 6-11W.  I usually use the XO avg draw as 5W.
>> 
>>> Here's a chart:
>>> 
>>> XO	 AC
>>> --	 --
>>> 4 W	  6   W
>>> 5 W	  7.5 W
>>> 8 W	 11   W
>>> 9 W	 12.4 W
> 
>> Okay, so for the situation where the battery is already charged and
>> it's plugged into a 120Vac 60Hz outlet, I should use 7.5 W as the
>> average draw, correct?
> 
> Thats my average assuming that the XO can go idle (not suspend) since I 
> happen to know that you use etoys extensively I ran some logs with a few 
> machines running etoys.
> 
> if etoys is running the (battery) power draw is between 6 and 7W 
> depending if its in design mode or if its ticking.  That's going to 
> translates to between 9 to 10 W at the Wall.
> 
> Updated Chart.
> 
> XO	 AC
> --	 --
> 4  W	  6   W
> 5  W	  7.5 W
> 6  W      9   W
> 7  W     10   W
> 8  W	 11   W
> 9  W	 12.4 W
> 17 W     23.5 W
> 
>> If the XO is not plugged in, I should use 5 W as the draw, correct?
> 
> I don't think you really care when its not plugged in.  You are running 
> from the battery which is stored charge.  The requirements of that 
> stored charge should be included in the over all AC power draw budget.
> 
> (But just FYI as above in etoys its 6-7. XO sitting there untouched idle 
> is about 4W)
> 
>> What would the average numbers be if the laptop is also charging
>> (assume dead battery at the start of each class, which is basically
>> what happens with kids bringing the laptops in from home)?
> 
> You can't really do an average here.  The reason is that when you are 
> charging the battery the power draw is not constant.  At the moment you 
> plug up the empty battery you are going to peak at 23.5 W but then 
> within the first 3 or 4 minutes that draw will begin to drop.  Thats 
> assuming XO is not powered up,  if its powered up then the the 23.5 draw 
> will stay for a lot longer.  As the power draw decreases the efficiency 
> of the power adapter changes.  So the power draw during battery charging 
> is iterative value that you have to compute across the charging time.
> 
> If you are going to factor in battery charges then the only way to do it 
> without going crazy is to fip over into total energy required to charge 
> a battery from empty to full.  Based on my measurements the amount of 
> juice it takes at the AC plug to charge a battery is about 50-60 
> Watt-hours.
> 
> Lets take the high end number as worst case.  So if you have 50 XOs and 
> you want all of them to get charged then you need a minimum of 60kWh * 
> 50 XO = 3 kilowatt-hours of juice.  How you distribute that juice 
> depends on your setup.
> 
> If the XO is powered up it will be more.  Looking through all my data I 
> see that I don't really have a good measurement of that.  But using the 
> estimated draw of a XO running etoys from above I think its pretty safe 
> to add an extra 20 Wh on the estimate.  So say 80 Wh if the XO is on and 
> 60 Wh if its off.
> 
>> Are these numbers appropriate for both power-grid systems and solar
>> to marine battery systems?
> 
> If you are on a solar power set then there is one more set of efficiency 
> numbers you have to factor in.  Thats the eff% of the AC inverter. 
> Typically those are 90% or more.  That means that you should take all 
> the numbers above and divide them by .9 if you are on solar.  Thats the 
> juice that your batteries will have to provide.
> 
>> What would the comparable average draw be for a generator powered
>> setup?  (Two numbers ... battery starts as charged, battery starts as
>> empty)   Assume your 5 W average.
> 
> There won't be any difference in the power draw on a generator compared 
> to a grid.  You need:
> 
> ~60 Watt-hours to charge the battery
> ~10 Watt-hours for running etoys.
> 
> Full battery is easy. 10W * hours run.
> 
> Empty battery is more difficult to estimate at least 60Wh and if powered 
> on laptop is going to consume 10W/hour and charging will take at least 2 
> hours then you have at least 80 Wh.
> 
> The concern with generators is the power factor.  Generators are 
> specified at a power factor of .8 . That's a good number to use if you 
> are powering a house.
> 
> Switching power supplies however don't behave nicely.  They are 
> non-linear in their operation so they don't map well on to the 
> "standard" power factor estimate.
> 
> AC circuits are not steady state. In a 60Hz the voltage goes from max 
> positive to max negative 60 times a second.  So on the wire there's a 
> lot of things happening.  Without getting in too deep what power factor 
> tells you is how much extra stuff is going on on the wire.  A perfect 
> system has a pf of 1.0 (its a ratio number) .8 is a good all around 
> estimate.  A block of XO power supplies however can present a power 
> factor of .5 or lower.
> 
> Many generators are rated in watts only but there is also a limit to the 
> amount of current they can provide. If you run over that limit for an 
> extended period the windings in the motor get hot and it will burn up. 
>  Power factor is a measure of how large that dynamic current is for a 
> given load.
> 
> So if you take a 3kW generator rated for a pf of .8 and run it at 3kW on 
> a system with a pf of .5 your generator is going to fail pre-maturely.
> 
> You have to oversize the generator to compensate for the lower power 
> factor.  This is the VA rating.
> 
> 3kW / 120 Vac = 25Amps pf = 1. At pf of .8 the maximum dynamic current 
> gets adjusted by the pf.  so that 25A turns into a peak of 31.25A
> at a pf of .5 its 50 Amps.   So for brief periods in the cycle your 
> generator is having to supply 1.6x more current than it was designed to 
> handle.
> 
> So event though your physical real power is 3kW you need a generator 
> that can deal with 6kVA if you want to drive a load with a .5 pf.
> 
> Most of my measurements show that usually the pf of a bank of XOs is in 
> the .6 to .7 range.  But there are cases when it can be down in that .5 
> range.  This usually happens on 240 Vac when the XO power supply is only 
> very lightly loaded, say during suspend.  So in that case even though 
> the power factor is very low the overall power draw is also very low. 
> How a generator deals with low power draw and low power factor is a 
> question for the manufacturer of the generator.
> 
>> AC grid-system, battery starts charged .. AC grid-system, battery
>> starts empty ..
>> 
>> solar+battery, battery starts charged .. solar+battery, battery
>> starts empty ..
>> 
>> generator system, battery starts charged .. generator system, battery
>> starts empty ..
> 
> I think this should all be covered above but here's the summary.
> 
>      Grid:  Batt charged = 10W/hour;
>             Batt empty   = 60Wh + ( 10 * (# hours XO is powered up))
> 
>     Solar:  Take the grid numbers and / .9
> 
> Generator:  Same as grid but peak dynamic power must be considered or 
> generator may fail pre-maturely.  For 50 XOs you need a generator that 
> can handle a reactive load of at least 2kVA.  23.5 W * 50 XOs / .6 pf = 
> 1.95kVA.   You should ask the mfg what happens if its operated with a 
> lower load but a pf of .5 .
> 
>> What number do you use as an average for battery life with XO-1s?
> 
>  2.5 - 3 hours
> 
>> What number do you use for average battery charge time for XO-1s in
>> AC-grid systems, with no laptop use?
> 
> 2 hours.  The LED will often stay lit for 2.5 - 3 hours but thats cell 
> balancing.  After 2 hours of charge very little additional charge is 
> acquired by the battery.
> 
>> What number for average battery charge with AC-grid, with average
>> laptop use simultaneously?  (i'm sure there's some easy math here :)
> 
> When I talk to groups about laptop power I have a quote I use...
> 
> "If you ask a power usage question an you get back a single answer with 
> no qualifiers its wrong."
> 
> Sadly there is no easy math. Its all so situation dependent.
> 
> We don't have any good models for average use.  That's why powerd on the 
> new builds has logging so we can begin to get numbers on that.
> 
> On your next trip you should probably purchase something like a 
> Kill-a-Watt meter or a WattsUP.  That way you can take some in-the-field 
> measurements and close the loop on verifying my measurements and 
> assumptions.
> 
> http://www.p3international.com/products/special/P4400/P4400-CE.html
> https://www.wattsupmeters.com/secure/products.php?pn=0
> 
> I use a WattsUP .net model because it supposed to be much more accurate 
> at measuring the power factor and it has a USB connection to the 
> computer where I log the readings every 10 seconds.  It has memory so 
> you could also sample readings without a compute an then download them 
> later.
> 
> The kill-a-watt is much cheaper but it has no logging and no output to 
> the computer.  You would need to periodically sample it for readings. 
> It does have a kWh meter but its only displays down to .01 kWh so you 
> need a fair bit of power draw to get a good average.
> 
> Both of them are rated to 15A so on a 120Vac setup thats 1800W.  Thats 
> enough if you only have 50 XO's or so on that circuit but more than 65 
> XO's would start to get close to the limit.
> 
> -- 
> Richard A. Smith  <richard at laptop.org>
> One Laptop per Child
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