[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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