The term, “traction battery”, until recently had a relatively specific application. This was the type of “deep cycle” battery used in floor scrubbers / sweepers and electric golf carts. Although hobbyists for many years had experimented with electric cars, these were for the most part curiosities. Sometime in the mid to late 1990s things began to change. Automotive engineers became serious about electric drivetrains and they wanted traction batteries with better characteristics.
The decision as to which battery type to use is central to designing and building an electric vehicle. While budget is a foremost concern, there are other battery characteristics besides cost that should be of concern. The following excellent summary of battery characteristics comes from Thermoanalytics web site:
The battery capacity is a measure of how much energy the battery can store. Batteries do not simply serve as a bucket into which one dumps electricity and later extracts it. The amount of energy that can be extracted from a fully charged battery, for instance, depends on temperature, the rate of discharge, battery age, and battery type. Consequently it is difficult to specify a battery’s capacity with a single number. There are primarily three ratings that are used to specify the capacity of a battery:
Ampere-hour: The Amphere-hour (Ah) denotes the current at which a battery can discharge at a constant rate over a specified length of time. For SLI (starting-lighting-ignition) batteries that are commonly used in cars, the standard is to specify Ampere-hours for a 20 hours discharge. This standard is denoted by the nomenclature of C/20. A 60 Ah C/20 battery will produce 60 Ah for a 20 hour discharge. This means that the new and fully charged battery will produce 3 Amps for 20 hours – it does not mean that the battery can produce 6 Amps for 10 hours (that would be signified by a C/10 60 Ah rating).
Reserve Capacity: The reserve capacity denotes the length of time, in minutes, that a battery can produce a specified level of discharge. A value of 35 minutes at 25 Amps for the reserve capacity for a battery means that the fully charged battery can produce 25 Amps for 35 minutes.
kWh Capacity: The kWh capacity metric is a measure of the energy (Volt * Amps * Time) required to fully charge a depleted battery. A depleted battery is usually not a fully discharged battery; a 12 V car battery is considered depleted when its voltage drops to 10.5 V. Similarly, a 6V battery is usually considered depleted when its voltage drops to 5.25 V.
None of these capacity ratings completely describe the capacity of a battery. Each one is a measure of the capacity under specific conditions. The performance of a battery in an actual application may vary substantially due to different discharge / recharge rates, battery age, cycle history, and / or temperature.
By definition a battery consists of two or more cells wired together. A lead-acid type cell produces approximately 2.1 Volts. A three cell lead-acid battery thus produces 6.3 V (6.3 = 2.1 * 3) and a six cell lead-acid battery produces 12.6 V. For a battery with fill caps, the number of cells can be determined by counting the number of fill caps. The voltage rating is that of a fully charged battery; its voltage will decrease as the battery is discharged.
- CYCLE DEPTH:
Fully discharging a battery often destroys the battery or, at a minimum, dramatically shortens its life. Deep-cycle lead-acid batteries can be routinely discharged down to 15-20% of its capacity – this represents a depth of discharge (DOD) of 85 to 80%. These deep-cycle batteries are constructed with thick plates for the cathodes and anodes in order to resist warping whereas in a conventional lead-acid batteries the plates are paper-thin. Regardless of whether or not the battery is deep-cycle or not, deep discharges shorten the life of a battery. A deep-cycle battery that can last 300 discharge-recharge cycles of 80% DOD (depth of discharge) may last 600 cycles at 50% DOD.
- WEIGHT / VOLUME:
The designer must consider the weight and volume of the battery pack during the vehicle design process. Different battery types will provide the designer with different energy and power capacities per a given weight or volume. The key ratings to consider are the Specific Power/Energy and the Power/Energy densities. These ratings reveal how much power or energy the battery will provide per given weight or volume.
- ENERGY DENSITY / SPECIFIC ENERGY:
Energy density is a measure of how much energy can be extracted from a battery per unit of battery weight or volume. By default, deep-cycle batteries provide the potential for higher energy densities than non-deep-cycle varieties since more of the energy in the battery can be extracted (e.g. larger acceptable DOD).
- POWER DENSITY / SPECIFIC POWER:
Power density is a measure of how much power can be extracted from a battery per unit of battery weight or volume. In an analogy to a car’s fuel system, the energy density is analogous to the size of the fuel tank and the power density is analogous to the octane of the fuel.
- OPERATING TEMPERATURE:
Batteries work best within a limited temperature range. Most wet-cell lead-acid batteries perform best around 85 to 95 F. At temperatures above 125 F, lead-acid batteries will be damaged and, consequently, their life shortened. Performance of lead-acid batteries suffers at temperatures below 72 F; the colder it is the greater the degradation in performance. As the temperature falls below freezing (32 F), lead-acid batteries will act sluggish – the battery has not lost its energy; its chemistry restrains it from delivering the energy. Batteries can also freeze. A fully charged lead-acid battery can survive 40 to 50 degrees below freezing, but a battery with a low state of charge (SOC) can freeze at temperatures as high as 30 F. When the water in a battery freezes it expands and can cause unrepairable damage to the cells.
A low state of charge (SOC) in a lead acid battery can lead to sulfation that can seriously damage the battery. In a low SOC state, lead crystals that are formed during discharge can become so large that they resist being dissolved during the recharge process. This prevents the battery from being recharged. Sulfation can occur when the battery is left at a low SOC for a long period of time.
A battery that is left alone will eventually discharge itself. This is particularly true of secondary (rechargeable) batteries as opposed to primary (non-rechargeable) batteries.
Another important characteristic (unmentioned in the above list) is thermal behavior. How much need is there for thermal management? Is there a chance of thermal “runaway”?
It is an arguable point as to which came first, the realizable potential of a fuel cell or the demand for a power source to complement a practical electric drive, in any case, carmakers began to investigate electric vehicle seriously with promises of something available sometime between 2010 and 2020.
Then, Toyota took a risk with an intermediate step. Perhaps, they simply wanted to take the “high road” and offer a more eco-friendly car sooner and / or they knew that other manufacturers might be ahead of them in offering a fuel cell vehicle. As Fortune recently noted what they call “hybrid envy”:
Competitors used to pooh-pooh the hybrid gasoline-electric motor as an expensive gimmick appealing mainly to tree-huggers. But now that Toyota is on track to sell 400,000 hybrids next year, they've changed their tune.
Since Toyota’s success in offering a full hybrid electric vehicle, DaimlerChrysler, Ford and Nissan have FCVs going through on-the-road tests and Honda has begun leasing its FCX. No longer is the question of when will fuel cell vehicles be available, the question is when they will be affordable. In any case, people now want hybrid electric vehicles.
Meanwhile, another product recently has come to the fore: lithium batteries. Nickel metal hydride (NiMH) batteries are what you will find in the Toyota Prius or the electric RAV4, or other commercial and military HEVs and everyone else was using lead acid batteries.
At the high end, there has been a significant shift from NiMH to Li ion. SAFT, a major military supplier, switched emphasis and Johnson Controls / Varta recently announced that they have begun development of lithium batteries.
Indeed, manufacturers of lead acid batteries have tried to remain competitive by offering better batteries, gel or Absorbed Glass Mat. Unfortunately, they are unable to compete in the electric vehicle market when Li ion batteries are one third the size and one-fifth the weight of valve-regulated, lead acid batteries, especially as there have been recent improvements made to lithium batteries. Furthermore, Li ion batteries seem to be in an enviable position with regard to responsible environmental assessment of battery systems, as part of total, Life Cycle Management.
Lithium batteries may herald another significant shift in the global market. While much of the technology for the development of these newer, energy storage devices has come from North American and European labs, at least according to the scuttlebutt heard by one retired NASA engineer who discusses NEVs on EVTrader, China has most of the raw material.
You are right about your observation that lighter batteries will yield a harsher ride. As a footnote, though, it doesn’t appear that Li-Ion and Li-Poly are in our future. A tidbit I heard was that the only commercially-viable source of the raw material for this technology is in China, so there is already a supply disincentive.
On the other hand, a typical electric car uses around 150 to 250 Watt-hours per mile depending on the terrain and the driving style, so the power density of Li ion batteries make them the preferred choice until fuel cells prove to be a satisfactory power source. These developments in traction batteries and fuels cells have implications for various electric vehicles, to include personal mobility and robots.
Carmakers are living in very interesting times.
For more information about development of traction batteries, refer to mPower.