HEV Vehicle Battery Types

BATTERIES

The function of the battery in a HEV may be varied. The battery may be a major power source, or may be used in conjunction with the primary power source(s) to level out the supply of power to the drivetrain. As a consequence, the amount of battery power aboard a HEV may vary between a single battery to a pack of many batteries connected together. When using batteries as a primary source of power, the HEV designer becomes concerned with the mass and volume of the battery pack required to meet the power and energy needs of the vehicle. The drive to achieve high power and energy densities have led the HEV community to investigate many types of batteries. These new battery types also promise greater cycle depth, power and energy capacity.

BATTERY RATINGS AND CHARACTERISTICS

The decision as to which battery type to use in a HEV application depends on how well the characteristics of that battery match the needs of the HEV design. The battery characteristics of most concern to the HEV designer are:

  • CAPACITY: 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.

 

  • VOLTAGE: 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.
  • SULFATION: 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.
  • SELF-DISCHARGE: 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.

BATTERY TYPES

There are many types of batteries that are currently being used - or being developed for use - in HEVs. The following table lists these types along with their common characteristics. The types are listed in descending order of popularity for use in HEVs, with the most popular choices at the top of the table. Typically the Energy Density, sometimes called Specific Energy, is rated at the C/3 rate (i.e. 3 hour discharge). Typical conditions for the Power Density or Specific Power rating is a 20 second discharge to 80% DOD. Cycle life is usually measured at 80% DOD.

 Battery Type  Energy Density [Wh/kg]  Power Density [W/kg]  Cycle Life  Operating Temp. [C]  Storage Temp. [C]  Self Discharge Rate 
[% per month]
 Maturity  Current Cost [$/kWh]  Future Cost {$/kWh]  Principal Manuf. Vehicles Used_In   Other Notes
 Lead-Acid 25 to 35  75 to 130  200 to 400 -18 to +70  ambient  2 to 3  production 100 to 125  75 Trojan, Hawker, Exide, Interstate   CARTA bus, Solectria E10 (sealed)  
 Advanced Lead Acid 35 to 42  240 to 412 500 to 800        production      Delphi, Horizon, Electrosource Audi Duo, GM EV1 (VLRA), Solectria Force Potential: 55 Wh/kg, 450 W/kg, and 2000 cycle life 
Nickel-Metal Hydride  50 to 80 150 to 250 600 to 1500       prototype  525 to 540  115 to 300 Panasonic, Ovonic, SAFT   Toyota RAV4-EV, Toyota Prius, Chrysler Epic minivan, Honda EV, Chevy S-10 Potential: 120 Wh/kg, and 2200 cycle life 
Nickel-Cadmium  35 to 57 50 to 200 1000 to 2000 -40 to +60  -60 to +60 10 to 20  mature  300 to 600   110 SAFT  >WWU Viking 23  Potential: 2200 cycle life  
 Lithium-Ion 100 to 150 300 400 to 1200       laboratory       SONY, SAFT Nissan Altra EV  Potential: 1000 Wh/kg
Zinc-Bromide   56 to 70 100   500          300        
 Lithium Polymer  100 to 155 100 to 315   400 to 600 60 to 100      laboratory     100      
NaNiCl   90 100   270 to 350 (300 optimal)    400 prototype     AEG Anglo     
Zinc-Air  110 to 200 100  240 to 450       prototype 300  100  Liquid Fuel Ltd     
 Vanadium Redox 50 110 400          300        

A brief description of each battery type follows:

  • Lead Acid: Low cost and available now vs low energy density and only fair cycle life. The lead acid battery is composed of lead plates of grids suspended in an electrolyte solution of sulfuric acid and water. These batteries can be ruined by completely discharging them.
  • Advanced Lead Acid: Available now. Longer cycle life than conventional lead acid. Valve regulated lead/acid (VLRA) batteries are showing great promise.
  • Nickel-Cadmium: Higher energy density than lead acid and available now vs cost. memory effect and toxicity. The nickel-cadmium battery is composed of a nickel hydroxide cathode and a cadmium anode in an alkaline electrolyte solution. If these batteries are discharged only partially before recharging, the cells have a tendency to act as if they have a lower storage capacity than they actually designed for; this is the memory effect. Nickel-cadmium batteries can often be restored to full potential (i.e. "full memory") with a few cycles of discharge and recharge. These batteries are often used to power small appliances, garden tools, and cellular telephones. Batteries made from Ni-Cd cells offer high currents at relatively constant voltage and they are tolerant of physical abuse.
  • Nickel-Metal Hydride: High efficiency and environmentally friendly. The nickel-metal hydride battery is composed of a hydrogen storage metal alloy, a nickel oxide cathode, and a potassium hydroxide electrolyte. These batteries can be quickly recharged. They have been used for a long time to power flashlights, lap-top computers, and cellular telephones.
  • Lithium-Ion: Lithium seems an ideal material for a battery: it is the lightest metal in addition to having the highest electric potential of all metals. Unfortunately, lithium is an unstable metal, so batteries that use lithium must be made using lithium ions (such as lithium-thionyl chloride). Even so, dangers persist with lithium-ion batteries. Many of the inorganic components of the battery and its casing are destroyed by the lithium ions and, on contact with water, lithium will react to create hydrogen which can ignite or can create excess pressure in the cell. If the lithium melts (melting point is 180 C), it may come into direct contact with the cathode, causing violent chemical reactions. As a consequence, lithium batteries are often limited to small sizes. Portable devices, such as notebook computers, smart cards, and cellular telephones, are often powered by lithium ion batteries. These batteries have no memory effect and do not use poisonous metals, such as lead, mercury or cadmium.
  • Zinc-Bromide: High energy density and long cycle life vs complex and toxicity. Zinc-bromine batteries pass two oppositely charged liquids through an ion-exchange membrane to produce electricity. The electrolyte is usually a zinc bromide-potassium chloride solution. Bromine, in both liquid and vapor form, is toxic and a strong irritant. The required pumping system makes the system complexity.
  • Lithium Polymer: Lithium-polymer cells have shown great promise, at the laboratory scale, for fulfilling the need for a battery of high specific power and energy in electric vehicle applications. A major uncertainty is whether heat generated in Li-polymer batteries during discharge at high power can be transported to the outside without excessive internal temperatures occurring. A second concern is whether lithium-polymer batteries can be brought up to operating temperatures in times acceptable to consumers.
  • Sodium Nickel Chloride: In the charged state, the cell consists of a negative liquid sodium electrode and a solid positive electrode containing nickel chloride and nickel. The electrodes and electrolyte are encapsulated in a steel cell case which simultaneously functions as the negative pole of the cell.
  • Zinc-air: High energy density vs short cycle life, low power density and low efficiency. The cathode of this battery is made of porous carbon which absorbs oxygen from the air. The zinc-air battery uses a zinc anode and the electrolyte is a base (rather than an acid), typically potassium hydroxide. Zinc-air batteries have been used in hearing aids for many years.
  • Vanadium Redox: High efficiency and can be completely discharged without damage vs high cost. The term redox is an abbreviation of "reduction oxidation". This battery, along with the Iron Redox battery, obtain their power when one the chemicals is reduced (i.e. gains electrons) while the other is oxidized (i.e. loses electrons). This battery is still very much in the development stages but shows great promise for EV use.

There are several other battery types that researchers have considered for HEVs, but their use are not common. These include, listed with their major strengths and weaknesses:

  • Aluminum-air : long shelf-life and high energy density vs complex and low efficiency. Aluminum-air batteries obtain their energy from the interaction of aluminum with air. The incoming air must be filtered, scrubbed of CO2, and dehumidified; the water and electrolyte must be pumped and maintained within a narrow temperature range - hence the complexity of the battery. The batteries are not electrically recharged but are "refueled" by replacing the aluminum anodes and the water supply.
  • Iron-air : high energy density vs complex, short cycle life, and high self-discharge rate. The iron-air battery uses electrodes made of iron and carbon. The carbon electrode provides oxygen for the electrochemical reaction. These batteries can be electrically recharged. Iron-air batteries are significantly effected by temperature; they perform poorly below 0 C.
  • Lithium-iron sulfide: high energy density vs high operating temperature. The lithium-iron sulfide battery is composed of a lithium alloy anode and an iron sulfide cathode suspended in an electrolyte molten salt solution. A variation of this battery system uses a cathode made of lithium-iron sulfide.
  • Nickel-iron: high energy density and long life vs high cost and high self-discharge rate. Nickel-iron batteries employ cathodes of nickel-oxide and anodes of iron in a potassium hydroxide solution. Nickel-iron batteries have long been used in European mining operations because of their ability to withstand vibrations, high temperatures and other physical stress. Also known as the Edison battery (invented by Thomas Edison in 1901).
  • Nickel-zinc: high power density vs short cycle life. The nickel-zinc battery is composed of a nickel oxide cathode and a zinc anode in a small amount of potassium hydroxide electrolyte. Recharging can be tricky in that zinc can be redeposited in areas where it is not desired, leading to the physical weakening and eventual failure of the electrode..
  • Silver-zinc: high energy density vs high cost and short cycle life. The cathode in a silver-zinc battery is a silver screen pasted with silver oxide. The anode is a porous plate of zinc, and the electrolyte is a solution of potassium hydroxide saturated with zinc hydroxide. Their high cost results from the amount of silver needed for the construction of these batteries.
  • Sodium-sulfur: high energy density and high efficiency vs high operating temperature. Used in Ford Ecostar van. The Ford Motor Company patented the sodium-sulfur battery in 1965. The battery, unlike most other batteries, uses a solid electrolyte (beta aluminum) and liquid electrodes (molten sulfur and sodium). These batteries require to be heated to around 325 C in order to operate because it is at these temperatures that sulfur and sodium will melt (i.e. be liquid).
  • Zinc-chlorine: high energy density and long cycle life vs complex, requires refrigeration, and toxicity. Similar to the zinc-bromide battery (bromine and chlorine are both halogens), the zinc-chlorine battery is even more complex since it requires refrigeration during the recharging process to remove heat.Chlorine gas is highly lethal.
  • Zinc-Manganese: Low peak power output and short cycle life. Zinc-Manganese Dioxide Alkaline Cells: When an alkaline electrolyte--instead of the mildly acidic electrolyte--is used in a regular zinc-carbon battery, it is called an "alkaline" battery.

FACTORS AFFECTING BATTERY PERFORMANCE

TEMPERATURE

Battery performance is highly dependent on temperature. Each type of battery works best within a limited range of temperatures. Concerns related to battery temperature include:

  • Poor energy and power extraction performance for temperatures outside of operating temperature range
  • Thermal runaway - during high power extraction the temperature of the battery increases which makes further power extraction more difficult which causes subsequent increases in temperature, and so on
  • Long heat up times before battery reaches operating temperature - this is a concern for ambient temperature batteries such as lead-acid in cold environments and also for batteries such as lithium/polymer-electrolyte which requires an operating temperature that is elevated above ambient

The battery temperature can change with changing current flowing through the internal resistance of the battery. The internal resistance can vary with the changing state of charge (SOC) of the battery. The temperature of battery can also be quite different between different cells since the cells in the center are more insulated from outside convective cooling than the cells at the ends/edges. Consequently, the cells in the center may see a higher temperature rise than the ones near the outer boundaries of the battery package.

The impact that temperature exerts on battery capacity can be explained with a simple model of the battery electrochemistry. As the temperature increase towards the peak-performance-operating temperature the electrolyte viscosity decreases, thus allowing for increased diffusion of ions and hence increased battery performance. As the temperature increases past this peak point, the battery electrodes begin to corrode - thus leading to a reduced "active" electrode area and thus to fewer electrode reactions and reduced battery capacity.

BATTERY AGE/SHELF LIFE

Corrosion is the main component behind decreased performance in lead acid type batteries by age.

DEPTH OF DISCHARGE

Batteries are able to maintain their performance longer when they are not deeply discharged regularly.


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