Re: electricity table (Google Docs)
Bobby Powers wrote: I don't have numbers for these efficiencies. The EC code has comments with magical constants that suggest certain numbers for these values but I've learned that a lot of those comments may be wrong or apply to earlier versions of the batteries. The range suggested is 80 - 90%. would (somehow) updating these magical constants for current batteries (depending on LiFe or NiCad chemistries) improve anything? What I mean is that the comments may not be correct so if it says the charge efficiency in CC mode is 90% I can't be sure that its really 90% without setting up my own testing and verifying. The magic constants may have been adjusted for a battery thats really 87% without updating the comments to match. -- Richard Smith [EMAIL PROTECTED] One Laptop Per Child ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel
Re: electricity table (Google Docs)
On Tue, Jul 22, 2008 at 3:49 PM, Richard A. Smith [EMAIL PROTECTED] wrote: [EMAIL PROTECTED] wrote: I was also wondering if you could give me feedback on this table. The table shows how much kWh is needed a year to power a xo based on different scenarios. If you think I should add or change anything As I often state in my discussions on laptop power, calculations like this are actually pretty complex and the simplistic approach while good for ballpark estimations can have a large amount of error. First as others pointed out you units are wrong. You need to substitute Watts every where you have kW. The next issue is that you are assuming a perfect conversion on the recharge half of your cycle. Which is not correct. The avg power draw of 5-7 watts for the XO is measured internally either via the battery sensor or by our instrumented XO. It does not take into account the efficiency of the DC/DC converter when recharging the XO from external power. It also does not take into consideration the charge efficiency of the battery. It takes more power charge a battery than just the usable capacity of the battery. The DC/DC converter's efficiency is affected by the difference between the input voltage and the output voltage. We don't really have any numbers on the exact range of efficiency for the XO @ 12V but typically your average DC/DC converter is around 85%. The 88% number pops into mind from when I was last looking at such things. Then there is the charge efficiency. Which is more complex because its actually 2 numbers. One for constant current (CC) charge mode and then another for constant voltage (CV) charge. The batteries start off in CC mode and then switch to CV mode after certain criteria are reached the criteria happens around the same time but is a bit different for each battery and much more different between the 2 types of batteries. I don't have numbers for these efficiencies. The EC code has comments with magical constants that suggest certain numbers for these values but I've learned that a lot of those comments may be wrong or apply to earlier versions of the batteries. The range suggested is 80 - 90%. would (somehow) updating these magical constants for current batteries (depending on LiFe or NiCad chemistries) improve anything? bobby The only way to know exactly what a good average for charge efficiency is would be to measure and compare the power in with power out across several batteries of each type (remember we have 2 chemistries). Thats possible in the case where the XO is powered up and you can read the battery sensor, but when the XO is off its not so easy. Guess what? Your %'s will be different in the 2 cases because the charge rate is much faster when the XO is off. Buts lets just say for a quick ball park that DC/DC is 88% and average battery CE is 85%. Now your recharge numbers are off by about 25%. Listing things in cranking hours may also be problematic. If you really were cranking some human power device your output would be so variable that the only way to get meaningful data is to measure it and develop some sort of profile for what the average person can really do. -- Richard Smith [EMAIL PROTECTED] One Laptop Per Child ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel
Re: electricity table (Google Docs)
[EMAIL PROTECTED] wrote: I was also wondering if you could give me feedback on this table. The table shows how much kWh is needed a year to power a xo based on different scenarios. If you think I should add or change anything As I often state in my discussions on laptop power, calculations like this are actually pretty complex and the simplistic approach while good for ballpark estimations can have a large amount of error. First as others pointed out you units are wrong. You need to substitute Watts every where you have kW. The next issue is that you are assuming a perfect conversion on the recharge half of your cycle. Which is not correct. The avg power draw of 5-7 watts for the XO is measured internally either via the battery sensor or by our instrumented XO. It does not take into account the efficiency of the DC/DC converter when recharging the XO from external power. It also does not take into consideration the charge efficiency of the battery. It takes more power charge a battery than just the usable capacity of the battery. The DC/DC converter's efficiency is affected by the difference between the input voltage and the output voltage. We don't really have any numbers on the exact range of efficiency for the XO @ 12V but typically your average DC/DC converter is around 85%. The 88% number pops into mind from when I was last looking at such things. Then there is the charge efficiency. Which is more complex because its actually 2 numbers. One for constant current (CC) charge mode and then another for constant voltage (CV) charge. The batteries start off in CC mode and then switch to CV mode after certain criteria are reached the criteria happens around the same time but is a bit different for each battery and much more different between the 2 types of batteries. I don't have numbers for these efficiencies. The EC code has comments with magical constants that suggest certain numbers for these values but I've learned that a lot of those comments may be wrong or apply to earlier versions of the batteries. The range suggested is 80 - 90%. The only way to know exactly what a good average for charge efficiency is would be to measure and compare the power in with power out across several batteries of each type (remember we have 2 chemistries). Thats possible in the case where the XO is powered up and you can read the battery sensor, but when the XO is off its not so easy. Guess what? Your %'s will be different in the 2 cases because the charge rate is much faster when the XO is off. Buts lets just say for a quick ball park that DC/DC is 88% and average battery CE is 85%. Now your recharge numbers are off by about 25%. Listing things in cranking hours may also be problematic. If you really were cranking some human power device your output would be so variable that the only way to get meaningful data is to measure it and develop some sort of profile for what the average person can really do. -- Richard Smith [EMAIL PROTECTED] One Laptop Per Child ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel
Re: electricity table (Google Docs)
Please don't use information from Potenco, there are too many complications with their design. They assume that the battery being charged can accept all the power their device outputs. If using the crank from Freeplay, It can charge the laptop at 17W (given the user is willing to crank fast). John On Jul 18, 2008, at 9:09 PM, Edward Cherlin wrote: 2008/7/18 [EMAIL PROTECTED]: I've shared a document with you called electricity table: http://spreadsheets.google.com/ccc?key=p6DRYWvC0_wbHL1- bzt-3Mw[EMAIL PROTECTED]t=808765302247483981guest It's not an attachment -- it's stored online at Google Docs. To open this document, just click the link above. Hi guys! I was also wondering if you could give me feedback on this table. The table shows how much kWh is needed a year to power a xo based on different scenarios. If you think I should add or change anything please let me know. Thanks a lot! Mia I don't understand your calculations. Your table labels the power column in kWh, which is the wrong unit. It should be W, so that you get Wh when multiplying by hours. Thus, for example, if we have 6 W ( roughly the power used by a single XO) for 5 hours a day for 365 days annually, it comes to 10950 Wh or 10.9 kWh. You also don't give a formula for power generation time. The Potenco site gives this information: A minute of pulling the PCG generates enough energy for: * 20 minutes of talk time on a mobile phone * 1 hour of ultrabright LED flashlight use * 3 hours play time on an iPod Shuffle (about one pull per song) If one minute of pulling gives one hour of use, this comes to 5 minutes a day in the case I considered above, and similarly for the other possibilities given, from 2 minutes to 15 minutes daily. Your table, unaccountably, gives well over an hour of pulling daily. ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel -- Edward Cherlin End Poverty at a Profit by teaching children business http://www.EarthTreasury.org/ The best way to predict the future is to invent it.--Alan Kay ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel
Re: electricity table (Google Docs)
Try this link: http://spreadsheets.google.com/pub?key=p6DRYWvC0_wbHL1-bzt-3Mw On Fri, Jul 18, 2008 at 7:42 PM, Bobby Powers [EMAIL PROTECTED] wrote: hi Mia this sounds really interesting, how can I find out what the different scenarios are? I don't use google docs a lot, is there just a link I'm forgetting to click? thanks! Bobby 2008/7/18 [EMAIL PROTECTED]: I've shared a document with you called electricity table: http://spreadsheets.google.com/ccc?key=p6DRYWvC0_wbHL1-bzt-3Mw[EMAIL PROTECTED]t=808765302247483981guest It's not an attachment -- it's stored online at Google Docs. To open this document, just click the link above. Hi guys! I was also wondering if you could give me feedback on this table. The table shows how much kWh is needed a year to power a xo based on different scenarios. If you think I should add or change anything please let me know. Thanks a lot! Mia ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel ___ Devel mailing list Devel@lists.laptop.org http://lists.laptop.org/listinfo/devel
