I've been researching different battery types after finishing reading the "Secrets of El Ninja" motorcycle conversion guide. The guide was published in 2006 and it compares different battery technologies as they stood in 2005. The three technologies I was considering were 1)sealed lead acid (Absorbed Glass Mat), 2)flooded lead acid, and 3)lithium iron phosphate (LiFePO4).
I'm discarding the thought of using flooded lead acid. I don't like the idea of a tipped bike spilling Sulfuric acid all over itself and potentially the rider. The constant need to monitor water levels is also not my idea of maintenance free EV bliss. Sealed lead acid or flooded lead acid were the only economical options for conversions until recently. In 2005 the cost of a 4.1kWh lithium based pack would have been over $8000! Pricing in 2009 is now a little over $1300 for a 4.1kWh lithium pack and is very close in price to a comparable sealed lead acid pack.
It now seems that the logical choice is to go with a LiFePO4 pack. I've decided to start with a relatively low voltage pack, 48Volts, using Thundersky LiFePO4 batteries, http://www.thunder-sky.com/home_en.asp. EV Components is the company that currently has the best pricing on LiFePO4 batteries, http://www.evcomponents.com/SearchResults.asp?Cat=34. Chinese battery makers seem to be the only ones currently selling Lithium Ion based batteries in low volumes to U.S. distributors for the conversion market.
A total of fourteen, 3.3Volt Thundersky batteries are required for approximately 46.2Volts, but what amp-hour capacity to choose? Once the system voltage has been decided upon (46.2V), the amp-hour capacity has to be chosen based on the range needed. I need a minimum of 36 mile roundtrip range in case I cannot plug in at work. The two formulas in "Secrets of El Ninja" for estimating range are 1)best case at 30mph on flat ground with no wind, 2)stop and go range:
- BestCaseRange = kWhrs x 9.8 miles/kWhr Where kWhrs is (pack voltage) x (battery Ah)
- StopAndGoRange = kWhrs x 7.0 miles/kWhr
I analyzed the range for various Thundersky batteries from 40Ah to 150Ah and found that to meet my range target with 48Volts I need to use the 90Ah batteries or greater.
An issue with battery sizing is that I will likely increase the voltage in the future as funds allow. Increasing voltage improves range much as increasing amp-hour capacity can. At higher future voltages I could get away with a greater number of smaller, lower amp-hour batteries for the same range. I will be making some 3-D models to determine what the best battery size is with future expansion in mind.
Using hypermiling techniques, http://www.cleanmpg.com/, and based on my commute route, I estimated my range as the average of the two formulas above. A unique feature I will be adding in my conversion is two 30Watt photovoltaic panels as rear fairings that will be able to tilt up into position at the correct angle while parked. These two panels I estimate will add about 3.5 miles of range based on the following formula:
- AvgSolarMiles=Watts x Derating x AvgSunHrs/1000 x AzimuthOptimizationFactor x miles/kWh
- 60 x 0.85 x 6.53/1000 x 1.064 x 9.8 = 3.5 solar miles
The Azimuth optimization factor adjusts for the fact that being able to adjust the panel angle will help increase power production throughout the year as the position of the sun changes with the seasons. I would also like to add a concentration feature to increase output further and will experiment with concentration once the panels come in.
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