Range, Batteries and Some Musings

One of the design goals of our conversion is to be able to get 70 to 80 miles on a charge. This will let us make most of the local trips we'd ever need to make in the CR-V. With a range of 70 miles, trips like picking someone up from the airport, or going to a client site in Washington PA will be possible, even without charging at the far end of the trip. We'd only need a gas car when taking longer trips like visiting our parents, where the drives are 80+ miles.

To achieve the 70 mile range, we'll be using between 35 and 38 large format Lithium Iron Phosphate (LiFePO4) batteries. These cells will be rated at 200 Ah, or amp-hours. The cells will operate nominally at 3.2 volts, between 3.4 volts at full charge and 2 volts when fully depleted. This configuration will provide between 22.4 and 24.3 Kilowatt-hours, or kWh of power. The math used here is simple. You take the nominal voltage x Ah x the number of cells and divide that number by 1000 to get kWh, so (3.2v x 200Ah x 38 cells)/1000 = 24.32 kWh.

These batteries are gaining a lot of popularity in the DIY community.  Their rated cycle life to 80% depth of discharge (DoD) is an order of magnitude (at least 10 times) higher than deep cycle or marine batteries.  Lead acid batteries may get 200 cycles before they degrade to the point where they have less than 80% of their rated capacity.  Once they reach this point, my own experiences show they degrade even faster.  The LiFePO4's coming out now also have Yttrium as part of the chemistry (sometimes seen as LiFeYPO4) which has improved the already great cycle life of LiFePO4.  With the newer LiFeYPO4's by companies like Winston Battery (aka Thundersky), now you can expect to get 3000 cycles to 80% DoD before the battery degrades to 80% of it's rated capacity.  If you build your car with enough reserve capacity, these batteries should provide ten years of daily driving before wear starts to be dramatically apparent.

Lithium also has a much higher specific energy than lead acid.  Specific energy is the amount of power that can be stored per unit of mass, usually measured in Wh/kg.  Think of it as 'power to weight ratio', although that term technically means something else.  A lead acid battery capable of storing 24 kWh, which is the capacity of our target battery would weigh over 1800 pounds.  Our lithium battery will weigh only about 610 pounds.  The lighter battery helps to stay within the GVWR of the vehicle.  GVWR or gross vehicle weight rating is basically the weight that the vehicle was designed to carry added to the weight of the vehicle itself.  Adding one ton of weight, as would be the case of using a lead acid battery, would not be a safe modification to a vehicle.  Plus, the added weight would disrupt the ride quality, reduce the range and impede the acceleration ability of the car.  Lithium batteries help make medium range battery electric vehicles (BEVs) possible.

LiFePO4 is also a very safe chemistry.  If you read any of the tech sheets that happen to be published in English, such as http://www.batteryspace.com/prod-specs/5493.pdf, you can see that they are quite safe to use.  Even if overcharged, physically destroyed or even shorted out, they will not catch fire or explode.   Now, Tesla is using either Lithium-Nickel or Lithium-Cobalt cells in their Roadster.  The specific energy of batteries like these can be much higher, but the downside is they are much more dangerous and require sophisticated engineering to use safely.  This totally rules them out for DIY-class projects.

Now, back to range.  When we drive a gas car, we think in terms of miles per gallon. When driving an electric vehicle, we use the metric of miles per kWh. A common figure achieved in an AC driven car conversions is around 4 miles per kWh. Using this information we can estimate the range of our CR-V. Even if we use 3 mi/kWh as a pessimistic figure, the CR-V will still get almost 73 miles on a charge. If we can approach the magical figure of 4 mi/kWh, it would get an amazing 97 miles on a charge.

Using the comparison of MPG to mi/kWh, we can also calculate the cost to drive per mile. If the gas car is getting 27 MPG on average, and gas is $3.50 USD per gallon, it will cost 12.96 cents per mile of driving. This is calculated simply by dividing the price per gallon by the miles per gallon. In the electric CR-V, it will go between 3 and 4 miles per kWh. In my region, a kWh costs about 12 cents. To find the cents per mile figure, we'll divide $0.12 by 3 (and 4) to find that we'll be paying between 3 and 4 cents per mile. This means using gas to get around would cost between 3.2 and 4.3 times as much to drive a gas car. Put another way, for 10000 miles at 27 MPG at $3.50 gas, it will cost $1296 in gas, but it will only cost between $300 and $400 in electricity. Over the 10 year expected life of the car (and battery) this saves no less than $8900 assuming current prices.

You may say that gas and electricity will both go up.  That is very likely.  You might be interested to look at the information at http://www.eia.doe.gov/aer/txt/ptb0810.html , http://www1.eere.energy.gov/vehiclesandfuels/facts/2005/fcvt_fotw364.html , and http://www.randomuseless.info/gasprice/gasprice.html .  You will learn that while gas is fairly high right now, it has actually been on a downward slope in cost over the last 90 years.  Prices now are going up due to increases in global demand, political unrest, etc.  These factors simply don't affect electricity production since it is generally produced domestically, using whichever fuels are economically available domestically.  You'll also notice that inflation adjusted gasoline prices have been much more volatile during any time period than electricity has been.  Gasoline is currently trading at three times the inflation adjusted price of it's historical lows between 1960 to today.  Meanwhile, electricity has never varied more than 60% of its own inflation adjusted price in the same period, and now it is trading on the low side of the historical price range.

The Motor: HPGC AC-50

A quick look at EVAlbum will show that lot of people use less expensive series wound DC motors made by companies like ADC or NetGain. A long time ago I decided on using AC drive, but it was just too expensive.

A DC system is cheaper because it's simpler, but it also has certain limitations that are hard to ignore. There are a lot of technical challenges to getting a DC system to do regenerative braking effectively without destroying the brushes. Plus, eventually, the brushes in the DC system will wear out anyway and need replaced.

In contrast, a 3 phase AC motor has no problem doing regen. I prefer a simple and reliable system for regen. Let the motor do the braking. I'm not changing brake pads all the time. Plus, an AC motor uses induction to transfer power to/from the rotor, so it doesn't even have brushes that can wear out. Part of the idea with an electric car is less maintenance. Let's really try and minimize it.

There also exists a type of DC motor that doesn't use brushes. Some electric bikes, like mine, use these. I am talking about brushless permanent magnet motors that even can do regen well with the help of a good controller. You just don't see any PM DC motors that are big enough to push a car to highway speeds.

This is sidebar but, on their Scion EV, ChargeCar was creative by mechanically tying 4 PM DC motors together, but that vehicle can only go about 40 mph. In their defense, this car is used for data collection and research into intelligent vehicle control algorithms, and their goals with that car are very different than the goals of building a daily use commuter EV.

Back on topic, an AC system is still more expensive, although it's getting cheaper. AC systems used to cost 2-3 times or more of the cost of a comparable DC system. Now, you may only pay 50% more for the AC system. So, unless you're really budget constrained, you should consider AC, as it will a be a more efficient conversion.

I've seen the AC-35 motor used in a Honda Civic conversion done by ChargeCar at CMU. The car can accelerate from 0-20 mph in about 2 seconds and has been driven at over 70mph on the highway. Taking that the Civic's stock curb weight is 2600 lbs and the CR-V weighs around 3200 stock, plus the fact that I'm installing a bigger battery than the Civic has, I think it's better to go with the slightly bigger motor that has more torque, so I'm thinking the AC-50 motor will work best. I ran it by ChargeCar's mailing list and Ben, who worked on the Civic, agreed.

[ power curves for the AC-50 at 550 amps. I plan to use a 650 amp controller -more on that later-, so I'll get more power than this is showing.]

In my next post, I'll talk about the controller.