6.1 Alkaline Battery
The alkaline battery consists of a redox reaction in which the anode is Zn (which also usually the casing) with an electrolyte consisting of a paste of NaOH or KOH. The half reactions are:
In this battery, if one mole of zinc is present, and two moles of MnO2, then the Zn is the limiting component for the reaction.
6.2 Nickel-Cadmium Batteries
While lead-acid batteries are undoubtedly the most commonly used batteries in photovoltaic systems, in some photovoltaic applications, nickel-cadmium may be cost effective on a life-cycle/cost basis. Nickel-cadmium batteries consist of a positive electrode of nickel (or hydroxide) and a negative electrode of cadmium hydroxide. They are commonly used in a sealed configuration in small household appliances, but larger vented or sealed batteries are also availible for PV applications. Nickel-cadmium batteries have several advantages as listed below.
Long lifetime and long storage life. In nickel-cadmium batteries, the positive and negative electrodes undergo oxidation and reduction reactions. Material does not enter the electrolyte and then re-plate to the electrodes as it would in lead-acid batteries. This means that the active material does not shed from the plates, and that a process analagous to sulfation of a lead-acid battery does not occur. As these processes reduce the lifetime of lead-acid batteries, nickel-cadmium batteries have a higher lifetime. Furthermore, the electrolyte in nickel-cadmium is less corrosive to battery parts than in a lead-acid battery which also increases lifetime.
Can be fully discharged. Nickel-cadmium batteries can be fully discharged without damage to the battery.
Can be overcharged. Nickel-cadmium batteries are less sensitive to overcharging, thereby reducing the requirements during the charging regime. Due to the ability to completely discharge, the tolerance to overcharging and the charging regimes for these batteries, in some cases the battery regulator may be eliminated.
Reduced sensitivity to temperature. Since the electrolyte composition does not change during charging or discharging, nickel-cadmium batteries are not more susceptible to freezing at low levels of charge, in the same way that lead-acid batteries are. Consequently, nickel-cadmium batteries are less sensitive to colder temperature, tolerating temperatures of -50 C. In addition, the lifetime of
nickel-cadmium batteries is not as strongly affected by high temperature operations as lead-acid.
Low maintenance requirements. As nickel-cadmium batteries emit fewer corrosive elements and have lower gassing, they require less frequent maintenance.
However, they also have a number of disadvantages. Some of the disadvantages include;
Expense. Nickel-cadmium batteries are typically at least twice as expensive than lead-acid batteries. However, some of this cost may be offset by the ability to fully discharge, eliminating the need for oversizing the battery, and by the possible elimination of the regulator. Consequently, in applications which are not critical, nickel-cadmium batteries can be used, assuming that they will be nearly fully discharged each night. If, however during a charging cycle there is a cloudy day, then no power would be available. Nickel-cadmium batteries, therefore can only be used in non-critical loads.
Lower efficiencies. Nickel-cadmium batteries have both lower coulombic efficiencies, between 75% to 85%, and lower overall efficiencies, between 60% to 75%.
Memory effect. Some nickel-cadmium batteries can require full discharge to prevent "memory" development, and subsequent inability (in a normal discharge cycle) to discharge below the level it has been subjected to in the past. Elimination of this effect requires a slow, full discharge/charge cycle.
An additional feature of nickel-cadmium batteries is the relatively constant voltage curve on charging and discharging. While this is an advantage in discharging in that the voltage stays relatively constant between 10% and 80% discharge, it is a disadvantage in charging in that the voltage is a poor indicator of battery state of charge and therefore determining SOC is more difficult.
6.3 Vanadium Redox Flow Battery
Redox flow batteries use a reductio-oxidation between two valence states in solution rather than changing the composition, and hence the valence states of solid material on an electrode. A flow battery consists of two volumes of solution separated by a selective membrane which allows some ions to pass but not others. The two solutions are pumped to the permeable membrane, which allows xxxx.
Flow batteries have several potential advantages over solid batteries. A key advantage, which is particularly important in transport applications, is that the battery may be re-charged simply by pumping out the uncharged solution and replacing the solution with charged solution. This eliminates potentially long recharging times, such as are encountered in electric vehicles. Replacement of the solution allows the electric car to be recharged in the same fashion in which a car is filled with fuel. Another advantage is that the capacity of the battery is determined by the volume of solution, while the power of the battery is determined by the membrane contact area between the two solutions.
The vanadium-Vanadium redox flow battery, developed at the University of New South Wales, is a particularly promising flow battery. It consists of two states of Vanadium. It has high efficiencies, with coulombic efficiencies of 97% and energy efficiencies of 87%. In addition, since both solutions (anode and cathode) in the battery use vanadium, cross contamination between the two solutions may discharge the battery, but will not cause damage the battery.