Plug Bug: Battery Research

Electric Bug

How far can I go? That depends on how fast you want to go, and how many hills you hit. Here’s some great info on the diy forums that got me started. I want to go 100 miles at 65mph (excluding hills and acceleration, for simplicity). Realistically, that probably won’t happen because it will cost too much and take up too much space (and too much weight!)

201005021312.jpgI’m going with a 154V DC system and 48 Thundersky Lithium Iron Phosphate batteries. The nominal volts per lithium battery is 3.2 V. What Ah do I need per cell? At 65mph on flat ground will consume about 23kW of power (31Hp). 100 miles at 65 mph = 1.54 hours, or 1 hr 32 minutes. 23kw*1.54 = 35.4kw needed. 35,400 watts / 154 v = 230Ah. We want to only run the pack to 80% “Depth of Discharge” (DoD) to avoid killing the pack too early. So, at 230Ah I need 230Ah* 1.2 = ~275 Ah cells to make it work (roughly). I’m actually planning on using 200Ah cells (due to size and cost), so I won’t be able to get that far. Also note that batteries have cycle life associated with them– you get more cycles and more life depending on how far you discharge the pack, and how fast you discharge a pack (the C rating). From my research, it appears that 1/2 C is the best rating for the maximum cycle life, but are usually rated continuous at 2C. I’ll be pulling less than that, so I should be okay.

With the 48 3.2V 200Ah cells I will have 154V, and 154V*200Ah*80% = ~25kW of available energy in my pack. That means I should be able to do about 71 miles based on 35kW to maintain 65mph (ignoring acceleration and any other factors). A little less than I wanted, and I’m really hoping it is enough to do 50 miles (round trip), including my “hill” so I can commute to work / gym / home and charge at work only. Otherwise, I’ll be dependent on charging at work and home, which won’t be a big deal.

Real life “Watts per mile” is another good estimate of range. Travis (who has a nice conversion I mentioned before) said he gets about 250Watts/mile when driving conservatively. Based on that, and 80% DoD, I could get up to a 98 mile range. That likely won’t happen, but would be cool. A 300W/mile estimate would give me an 82 mile range. My calculations are probably conservative, as I used higher values to not get my hopes too high. I’m excited to actually get my project together and find out what real data I actually get.

I’ve basically gone from zero EV knowledge a month ago to knowing “a little bit”. I’m still learning as I go…

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Bronson
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Very cool. I am now a dealer for the “human-electric hybrid” cargo bike system called the Stokemonkey. It should give efficiency of about 10-20 Wh/mile or around 1,000-2,000 MPG and be able to climb 30% grades loaded. I will have a demo setup in the next week or so. http://clevercycles.com/products/stokemonkey/ Here is a pretty good example of what the system is capable of: http://clevercycles.com/2008/08/10/family-bike-trip-portland-to-breitenbush-and-back-again/ ““In all, we used 1642 watt hours of assist each, so 3284 total over 193 miles, or 17 watt hours per mile for the 3 of us together. (Our capacity was half that; we recharged from… Read more »

Joshua Tulberg
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Sweet deal dude. I’m so sick of the EV’s that only hit 40 Miles an hour.
If you can get this thing to hit 70 and it has a range of 50 then you should be really happy with it. “theory” is often shot out the window *especially with having to drive up a “hill” and also whether or not you plan on using the vw transmission. Also, did you factor in wind resistance? How much are these batteries costing you?

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