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General Comparisons. A secondary battery is equivalent to a motor-generator set that has a fuel tank of limited capacity. A battery can be recharged with electricity, but this may take several hours if the power supply has limited capabilities. In contrast, a generator can be refuelled in a few seconds, with a typical rate of energy transfer of many MW. The battery weight depends on the amount of energy it can store, whereas the generator weight depends on the power that it can deliver.
Once discharged, a primary battery is discarded and its cost and weight can therefore be compared with those of the equivalent amount of fuel. For example, a D-size alkaline cell costs about $2.00, weighs 130 g and can deliver 21 Wh of electrical energy. The equivalent quantity of petrol needed to produce this energy, even allowing for only a 10% conversion efficiency, would weigh 17 g and cost $0.10, whereas the same energy taken from a mains supply would cost a mere 0.4 ¢. Clearly, there must be some other justification for using batteries. The example of a missile power source will be used to illustrate the point.
Missile power source example
Weight and/or Volume: For a given missile duty, weight and/or volume are important considerations when choosing between batteries and generators. The weight of the battery in this example is approximately proportional to the required energy, and thus to the flight time. For short flight times, the slope of the curve changes because the SE of small batteries is always smaller than that of large batteries, due to the cell hardware. The weight of the motor-generator system is in two parts - the machine (including fuel container, valves, regulators, etc.) and the fuel. The fuel weight is proportional to the flight time but the machine weight (generator and turbine) is fixed by the power requirement (in theory it could run forever, given a continuous supply of fuel).
The battery is an obvious choice for short flight times but, in practice, batteries exhibit reduced performance at high rates of discharge and cannot safely be used above certain specific power levels. Allowing for this, the choice lies between a limited number of battery types, i.e. thermal batteries, followed by silver-zinc and lithium reserve batteries.
For very long flight times, e.g. for cruise missiles, a generator is the obvious choice, especially if ram-air driven or, more efficient still, driven directly by the main engine and thus not needing a separate fuel supply. A Ragone plot can be used to assess the relative merits of the competing alternatives. These suggest that a generator needing its own fuel supply could never compete with a thermal battery, due to the added weight of the fuel container and management system. For flight times of l0 min or more, however, engine-driven or ram-air powered generators would be lighter than a battery system.
