There are several metrics to how batteries are measured. And those metrics play various roles in determining how well the batteries would (or wouldn’t) perform in an electric car. From standard hybrids to plug-ins and pure electric vehicles, they all have different battery needs.
We’ll start with the basic measurement of capacity. Your vehicle’s fuel tank might hold 19 gallons of fuel, and the battery equivalent is energy, measured in kilowatt-hours, or kWh. Sometimes the figures are offered in Amp-hours or Ah, to get kWh multiply that figure by the cell’s nominal voltage.
A laptop battery might have around 0.05kWh of energy, while the Chevy Volt has 16kWh, so you can image how many laptop batteries you would need to put in a car to get 16kWh. The biggest battery in a vehicle is the Telsa Roadster’s with 53kWh.
It is estimated that a small sedan would use about 200Wh per mile of driving, and a large SUV would use around 400Wh per mile. Those are average figures, with regenerative braking decreasing city driving figures and highway driving increasing those figures. As the vehicle weight go up, and as your driving speed increases, the amount of energy needed per mile goes up as well. Obviously, increased weight requires a greater force, and aerodynamic resistance increases as the square of speed, so the force required to cut through the air at 70MPH is twice of the force at 50MPH.
So how does all that energy get from the battery to the electric motor? It goes through an inverter to convert the DC (direct current) energy into AC (alternating current) energy to power the motor. This raises the issue of how much power can those batteries deliver at any given moment.
Reading from a battery’s spec sheet, you’ll usually find a pulse power rate. This is measured in W (for each battery) or W/kg, and is the short-term power the battery can put out. Combining this with the total number of cells or total cell weight will determine the maximum power the batteries can deliver to the motor. The motor in the Tesla Roadster is 185kW, and the Volt’s motor is about 110kW.
One of the most important battery characteristics is the energy to weight ratio, usually called energy density, measured in Wh/kg. This is the battery’s usable energy divided by the weight of the battery. This is the most critical factors when it comes to examining batteries – the energy density determines how heavy the battery is, which is a very important factor when it comes to automobiles. Closely correlated to energy density is the energy to volume ratio, measured in Wh/L, this determines how large the batteries are.
Today, automotive lithium ion batteries are approximately 70-100Wh/kg. This means for each kWh of energy storage, the battery weighs around 10kg, or 22lbs, plus other necessary equipment to connect the batteries together, to cool them, protect them in case of an accident, etc. It is hoped that energy density will increase approximately 10% per year for the next 10 years, more than doubling by 2020 and providing for cutting battery size and weight in half.
The next most critical attribute for an automotive battery is the cycle count. This is measured in the number of complete battery cycles until the battery can only hold 80% of its original capacity. A complete battery cycle is when the battery has been discharged and charged at its full capacity. So discharging a battery 50% twice or 33% three times, both equal one complete cycle. Batteries also have an expected calendar life, but this isn’t related to the cycle count.
To extend the the cycle count of the battery, you can use a smaller depth of discharge. The Chevy Volt is the perfect example. It has a 16kWh battery but only will use 8kWh, charging to 85% total capacity and discharging down to 35% for daily driving on electricity. Using only 50% of the battery capacity doubles the number of recharges the battery can withstand until it can only store 80% of original capacity.
The lithium ion battery in your laptop has a life of 18-24 months, and a cycle count of around 300. This isn’t anywhere near suitable for use in electric vehicles, so different battery formulations that last longer and have higher cycle counts are being developed. No one will really know what the Volt’s batteries are capable of until they start selling units, but the battery would need to be capable of around 3,750 recharges (a cycle count of 1,875).
California regulations require electric cars to have batteries that are warrantied for 10 years and 150,000 miles, so automakers are targeting those figures. Granted you have to live in CA or have bought it there to be covered, but because the market is so large automakers really have no choice but to match those warranty numbers.
Finally in this battery tutorial is the charge/discharge, or sometime just called the charge rate, noted as nC where n is an integer (1C, 2C, 6C). This can have an affect on the cycle count – the faster you charge and discharge the battery, the fewer cycles it is capable of. 1C is charging the battery so it will recharge in 1 hour – so an 11Ah battery charged at 11A (at the cell nominal voltage) is being charged at 1C, 22A would be 2C, and 66A would be 6C. Adding up all the cells in a battery pack would tell you what the total current the motor would be capable of receiving.
So thats it as far as this battery tutorial goes.