The two main problem with electric cars today are:
- The low energy density of existing battery technology
- The slow recharge time of those batteries
For a simple comparison, let’s look at a typical electric car battery vs. a gallon of gasoline. A gallon of gasoline weighs about 2.83 kg and contains about 120 MJ of energy – that’s 42.4 MJ/KG of energy density. A typical 40 amp-hour lithium ion battery weighs about 1.4 kg, so two 40AH batteries would weigh about as much as a gallon of gas. Since the voltage of a lithium ion battery is 3.2 volts and watts = volts x amps, two 40AH batteries will hold 256 watt hours of power. Since 1 watt-hour equals 3,600 joules, two 40AH batteries would hold about 0.9216 MJ of energy. So a gallon of gasoline holds 130 more times energy than the same weight of lithium ion batteries. Or to put it another way, a pile of batteries that weigh as much as a gallon of gasoline would only provide two tablespoons worth of gasoline energy.
Of course electric motors are about 90% efficient while internal combustion engines are about 20% efficient – so you need a lot less initial energy per mile. A car that gets 30 MPG will require 4 MJ per mile of energy while a comparable electric car will only require 1 MJ per mile. Thus, while gasoline is 130 times as energy dense as batteries, on a work-accomplished basis, it is 32.5 times as energy dense. Nevertheless, this is an enormous discrepancy in energy density and still leaves electric cars at a disadvantage. This is why it is so incredibly important to choose the most energy dense battery available on the market.
Electric cars also take far longer to fill up than gasoline cars. The EPA has limited gas pumps to a maximum flow rate of 10 gallons per minute. Most fuel dispensers pump between 5 and 7 gallons per minute. If the fuel filter is clogged on the pump, it will pump far more slowly. So including time spent paying, the typical gas station full-up takes around 5 minutes. Electric cars take far longer to charge and the variability in charging times is enormous depending on the charging source. In the worst-case scenario, charging a large 60-kW Tesla Model S with a standard 110-volt house socket would take 52 hours. Using a 240-volt outlet (like your home dryer uses), you can charge a Nissan Leaf from empty in 4-5 hours. In a best-case scenario, Tesla’s superchargers can provide half a charge to a Model S in as little as 20 minutes. Even in the best-case scenario, however, it still takes far longer to charge an electric car than it takes to fill up a gasoline car. Until battery technology improves, this will simply remain a trade-off of owning an electric car.
There are three main types of batteries you can use in an electric car: lead acid batteries (which are used in golf carts), nickel metal hydride batteries (which are used in the Toyota Prius) or lithium ion batteries (which are used in almost every electric car today). The energy densities of the three technologies are as follows:
- Lead Acid: 33–42 W·h/kg
- Nickel Metal Hydride: 60–120 W·h/kg
- Lithium Ion: 100–265 W·h/kg
In general, the larger batteries (those with more AH) provide more continuous wattage per kilogram of weight and per centiliter of volume:
The larger batteries also generally provide more peak watts per kilogram of weight and centiliter of volume:
So one might be tempted to simply choose the largest battery they can find. However, this ignores the fact that the voltage of most lithium ion batteries is 3.2 volts. While the torque of an electric motor is dependent on amperage, the speed of the motor is dependent on voltage. Putting 3.2 volts of power into a typical electric motor would only allow it to spin at 166 RPM. In order to get the full 5000 RPM out of the motor, you need to input 96 volts. In order to get 96 volts from 3.2 volt batteries, you must connect 30 of them in series. The motor I am planning on using has a peak amperage into the controller of 1,800 amps. So in order to optimize weight, I need to select the battery type that would provide me with 1,800 amps of power at 96 volts with the lowest weight and volume.
Based on this analysis, the best battery is the 90 amp-hour Thunder Sky TS-LFP90AHA. Using the battery, the planned configuration for the battery pack is 2 parallel sets of 30 batteries in series. This pack of 60 batteries would have the following characteristics:
- 96 volts
- 3600 amps of peak current
- 90 amps of continuous current
- 192 kilograms of weight (423 lbs)
- 119 miles of vehicle range at 42 mph with the motor producing 6.0 kW of continuous power
- 5.8 second 0-60 estimate
- $5,805 battery pack price