Vehicle propulsion batteries fall into two general groups. The Deep-Cycle group is used primarily for extended energy storage and requires deep discharge capabilities. This type of battery is most common in pure electric vehicles. The High-Power group is used mainly in hybrid vehicles and are suited for rapid discharge and rapid multi-cycling.
The capacity of most batteries is dependent on the rate of discharge. When the selection of a battery is being made, care must be taken to choose the energy capacity at the required discharge rate. In most electric vehicles the batteries are discharged at a C/1 ("c" over one rate) rate. This means that the entire usable battery capacity is consumed over a 1 hour period. If the batteries last two hours with continues regular use, then the discharge rate would be C/2. In many instances, battery manufacturers state their energy capacity at the C/20 rate. Using a C/20 rate to determine energy capacity can some times lead to the selection of a less than optimum battery for an electric vehicle.
The following chart compares various batteries available for use today and those with a high probability for use in the near future[i]. Rough averages are shown here and should be used for general comparisons. Many of the life cycles numbers are based solely on reasonable estimations.
Below are descriptions of the most common types of secondary (rechargeable) batteries for use in electric and hybrid vehicles.
Anode = Pb (lead) Electrolyte = 30% H2SO4 (dilute sulfuric acid)
Cathode = PbO2 (lead dioxide) Cell Voltage = 2 volts
Positive plate (Cathode)
PbO2 (s) + 4H+ (aq) + SO42- (aq) + 2e- <=> PbSO4 (s) + 2(H2O)
Negative plate (Anode)
Pb (s) + SO42- (aq) <=> PbSO4 (s) + 2e-
PbO2 (s) + Pb (s) + 4H+ (aq) + 2SO42- (aq) <=> 2(PbSO4) (s) + 2(H2O)
Electrolyte densities at 160 C
Lead acid batteries come in several configurations. These include gelled electrolyte, starved electrolyte and flooded electrolyte. The most common and lest expensive, is the flooded electrolyte version. Cleaning and adding water to this battery are the major drawbacks. The gelled electrolyte batteries, or gel-cells, are a more expensive alternative to the flooded batteries. Gel-cell require almost no maintenance under normal use. But, gel-cells are more expensive and have a lower energy density.
At the writing of this handbook, the most sought after battery for high-tech electric vehicle applications is the starved electrolyte battery. These batteries have the higher energy density of the flooded electrolyte batteries, but are maintenance free. The enclosure is completely sealed except for a pressure relief valve used to relieve internal pressures greater than 28 kPa to 40 kPa (4 to 6 pounds per square inch). Most brands can also be operated in any orientation. This allows the designer more flexibility in choosing the location and access to these batteries. As with most all batteries, insurance that the connectors are clean and secure is the only require maintenance for starved electrolyte battery installations.
Research to improve lead-acid batteries has lead to results of increased energy density, power density and cycle life. On average, the energy density at a 3 hour discharge rate is 32 Watthours/kg, and upto 42 Watthours/kg in more advanced batteries. Power density averages 100 Watts/kg for a 30 second period with greater amounts in specialized High-Power batteries. Cycle life at 80% depth of discharge averages near 500 with the advanced batteries claiming to reach 1000. Different rates of discharge will greatly affect these numbers.
Sulphating action on lead acid batteries occurs during the discharge and is reconverted to lead oxide or lead peroxide on recharging. If present in excessive quantities, though, the sulphate adheres to the plates, preventing chemical actions, increasing resistance of the cell and placing an unequal mechanical stress on the plates. The most common causes of excessive sulphating is over discharging, too high acid density, and allow the battery to remain discharged for long period of time. To reverse this condition, the battery should be given a long, slow charge until the battery shows signs of a full charge.
At a 3 hour discharge rate, nickel-iron batteries average 48 watthours/kg of energy density. For a 30 second period the power density rate is 100 watts/kg. Cycle life of 700 to 1000 are average at 80% depth of discharge. These numbers will vary greatly at different rates of discharge.
Anode = Cadmium Cell Voltage = 1.4 volts
Cathode = Nickel dioxide Alkaline (basic) electrolyte
Anode reaction (oxidation)
Cd (s) + 2OH- (aq) => Cd(OH)2 (s) + 2e-
Cathode reaction (reduction)
NiO2 (s) + 2H2O + 2e- => Ni(OH)2 (s) +2OH- (aq)
Cd (s) + NiO2 (s) + 2H2O => Cd(OH)2 (s) + Ni(OH)2 (s)
Sodium-sulphur batteries average  100 watthours/kg at the 2 hour rate of discharge. Some versions of this battery have power densities of 130 and 106 watts/kg at a 50% depth of discharge[ii].
Aluminum is a reactive metal and dissolves when polarized anodically to form a soluble aluminate species. Alkali metal hydroxide is consumed in the reaction.
Al + 3/4 O2 + 3/2 H2O +KOH => K Al (OH)4
As the process proceeds the resistance of the electrolyte increases.
Li - MoS2