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Forum » Eco Car Forums » Tech Log

Performance/Range of electric cars...need help

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Written on: 15. 03. 08 [19:09]
ecoadmin Administrator Topic creator registered since: 20.07.2007 Posts: 367	Hello, I am planning to buy an electric car and have in mind to buy a Twike. Yes, I know, its not really a car, but I do like the fact that I can move my legs while driving and on top of it its a very efficient and eco-friendly way of getting around. And its fun! But I do have a few questions about power consumption and performance of EV's. Your help is appreciated. When reading thru advertisements and announcements of EV manufacturers, it is often listed how much energy in kwh is consumed per 100km(62miles). E.g. The new Think City battery has a capacity of 27kwh and can do about 100miles (180km) with it. But how does this compare to a Li-Ion Twike with a 20kwh battery pack? The Twike is less than 500kg with a full load and its 3kW motor (5kW peak) gives a range of 100-200km (62-144miles). How is it possible then that the 1400kg Think City can do 100miles with 27kwh pack carrying all that weight around? Is the figure of 180km/100miles just a best-case-scenario? ecocarforum.com - Green Car Network Community for sustainable transport
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Written on: 16. 03. 08 [11:32]
Franko30 Moderator registered since: 08.09.2007 Posts: 62	Hi ecoadmin, That's not an easy one to explain. You wrote But how does this compare to a Li-Ion Twike with a 20kwh battery pack? The Twike is less than 500kg with a full load and its 3kW motor (5kW peak) gives a range of 100-200km (62-144miles). How is it possible then that the 1400kg Think City can do 100miles with 27kwh pack carrying all that weight around? Is the figure of 180km/100miles just a best-case-scenario? Yes - these are always best case scenarios - just like it is with fossile fuel cars: You never can drive as economically as they do on their test stands. Only with electric cars like a Twike a possible difference of 10 % is a really annoying portion of its total range - whereas with a fossile fuel car, it doesn't really matter whether you can go 600 or 660 km... With the Th!nk, it is very problematic to gain access to detailed information. But on elweb I found out that the think uses about 15 kWh/100 km - as opposed to the Twike which uses 4-8 kWh/100 km. The other problem ist that you can't compare 10 Ah of a 12 V (lead) battery-pack to 10 Ah of a 360 V (LiIon) battery-pack. The voltage has also to be taken into consideration. And I don't know about the voltage of the Th!nk's version of the Zebra battery. So, if you know the standard voltage of a battery pack, you can calculate how many kWh it "contains". And after converting to kWh, you can compare the efficiency of electric vehicles. On the other hand, the voltage of a battery never remains the same during the whole trip. Therefore you can only measure the energy you use correctly while charging. Another important factor is that manufacturers tend to specify the energy needed to go 100 km with "battery only" values - but what's important for the user is how many energy he has to use to charge the battery. And, to make it even more complicated: how the charging cycle works is important, too. Plus: How efficient/inefficient is the charging technology/equipment? An example: With 10 Ah/336 V NiCd batteries I had to use 5,2 kWh of electricity to fully charge the 10 Ah. I could drive about 65 km with these 5,2 kWh. With my new 20 Ah/353 V LiIon batteries I only need 7,6 kWh to fully charge the 20 Ah. With these 7,6 kWh I can drive about 116 km. Why is that so? The charging equipment is still the same. Martin Feneberg of Fine Mobile explained it to me like this: The NiCd-batteries have a special charging scheme and a higher internal resistance, both leading to quite a portion of the charging energy to be lost in form of a temperature rise in the batteries. The higher internal resistance is an obvious efficiency killer. On to the charging scheme: The Twike's NiCd batteries need something that has to be translated to "symmetric charge" on a regular basis. This is used to get all batteries to the same charging level. As some batteries are already full while some still need charging, the full ones transform the "surplus" energy into heat (lost energy). The LiIon batteries have a much lower internal resistance and don't need a complicated charging scheme: charge for a certain time and just stop - no "symmetric charge" needed. Much more efficient. This (together with my style of driving, see below) leads to an energy consumption of 8 kWh/100km (about 0,8 l gas/100 km, meaning 294 US-mpg) with NiCd batteries and 6,55 kWh/100 km (about 0,66 l gas/100 km, meaning 356 US-mpg) with LiIon batteries. Conversion mpg <-> l/100 km according to: http://www.pege.org/fuel/convert.htm , conversion kWh <-> liter gasoline according to http://www.twikeklub.ch/challenge98/Energie.htm For a comparison of different "low-energy vehicles" see (German): http://de.wikipedia.org/wiki/Niedrigenergiefahrzeug#Serienmodelle My style of driving: With my Twike, I drive at 55 km/h (cruise control) within city limits and 67 km/h (cruise cntrol) out of town. I use cruise control, because it is much more efficient than controlling the speed manually. These figures classify me as a "speedy" Twike pilot, according to Fine Mobile (manufacturer of the Twike) standards. But the possible ranges you can achieve with a Twike ar always specified for the "economical" driver, going about 35-40 km/h maximum and only in a very flat landscape with no stop-and-go city traffic. So, I hope I could help you a little bit with your questions. Cheers Franko30 [This article was edited 9 times, at last 16.03.2008 at 12:39.] TW 808 / 20 AH LiIon-Akkus seit 03/2008 / Höchste Reichweite bisher: 140 km. Verbrauch 6,3 kWh/100 km (372 US-miles per Gallon)
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Written on: 16. 03. 08 [13:35]
ecoadmin Administrator Topic creator registered since: 20.07.2007 Posts: 367	Hi Franko30, Many thanks for taking the time to sort out my questions...It's been a long time since I've studied this in school. Sorry for mixing up kWh and Ah in my previous post. Of course the Twike has a 20Ah battery pack and not 20kwh. [quote=]With my new 20 Ah/353 V LiIon batteries I only need 7,6 kWh to fully charge the 20 Ah. With these 7,6 kWh I can drive about 116 km Since U(Volt)x I(Ampere) = P(Watt) is it correct to say that: 353V x 20Ah = 7060 Wh (7,06kwh) ? Can we then say that the Twike has a 7.1 kwh battery pack if we want to express it in kwh? The Th!nk City Zebra Battery uses a MES DEA – Zebra, 28.3kWh sodium battery. If the car uses about 15kWh/100km, I now can understand the range of 180km. It also explains why a Twike can be recharged much quicker than a Th!nk... Hope these calculations also help others to understand the technology of electrical cars better. In my case, I want to know how it works before I go ahead... Cheers, Stefan ecocarforum.com - Green Car Network Community for sustainable transport
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