On Sun, 20 Sep 2009 12:02:13 +0200, you wrote: >John, > >OK so you want actual numbers regarding battery savings. Let's study >the case of the latest version (3G-S, July 2009) of the popular iPhone >smartphone: > >http://www.apple.com/batteries/iphone.html : >"A properly maintained iPhone battery is designed to retain up to 80% >of its original capacity at 400 full charge and discharge cycles" > >http://www.apple.com/support/iphone/service/battery/ : >"iPhone Out-of-Warranty Battery Replacement... >The program cost is $85.95 per unit." > >http://en.wikipedia.org/wiki/IPhone : >"3GS: 3.7 V 1219 mAh[5] >Internal rechargeable non-removable lithium-ion polymer battery[6]" > >So total delivered energy is 3.7V*1.219Ah*0.4kcycles = 1.8kWh, that's >a cost per delivered kWh of 85.95/1.8 ~= USD 48 per kWh > >So that's "only" ~ 500 times the cost of grid power (which is ~ USD >0.10 per kWh, right?), the several thousands factor I had in mind must >have been for non-rechargeables. > >A factor 500 is still huge, it means that all the time you're on 50% >efficient witricity power instead of battery power, consuming what >would cost you x USD/s if you were operating off the grid with 100% >efficiency, instead of spending 500x you spend 2x, that's a saving of >99.6%. > >Note that even if the factor was only 50 (10 times lower than >calculated above) and the witricity efficiency was only 10%, instead >of spending 50x you spend 10x, that's still a saving of 80%, for each >second you spend on witricity instead of battery power. > >So witricity is not just practical, it also saves money. If it also >complies with health regulations regarding radiation levels which they >claim it does, it is definitely a good thing.
--- Actually, the replacement cost of the battery is immaterial. Let's look at this thing from a different viewpoint. Let's say we have two identical cell phones outfitted with identically discharged batteries, that one cell phone is being charged by a "wireless" charger running at the 50% efficiency you quoted, while the other is being charged with a properly designed conventional switchmode charger running at 90%, and that it takes one hour to fully charge both batteries. Now, disconnecting both chargers from the mains during the times they're not charging and cycling both systems until battery failure occurs leads to the conclusion that since the cost of electricity per kilowatt-hour is the same for either system and the wired system will waste less electricity in charging its battery than the wireless system, the wired system will be cheaper to run from day one. How much will it save over 400 cycles? Since the wireless system is 50% efficient it'll eat 1.8kWh while delivering 1.8kWh, while the wired system, being 90% efficient, will eat only 0.2kWh. At USD 0.1 per kWh, that's $1.80 for the wireless system, while only $0.2 for the wired system. Notice that the cost per battery is the same for either system, so that part drops out of the equation, leaving us with a ninefold increase in energy to charge the battery. Looking at _that_ globally should give one pause.
