7 3/8 x 9 1/4 T echnical / Build Your Own Electric Vehicle / Leitman / 373-2 / Chapter 8
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Build Your Own Elec tric Vehicle
Resistance
The resistance corresponds to the size of the hole controlling the rate of flow of the water coming out the bottom of the jug. A battery’s voltage is directly related to current flow by resistance via the Ohm’s Law equation you met in Chapter 6: V 5 IR where V is voltage in volts, I is current in amps, and R is resistance in ohms. Actually, there are two resistances: the external resistance of the load (the light bulb in this case) and the internal resistance of the battery. The battery’s internal resistance is important in battery efficiency (heating losses), power transfer, and state-of-charge determinations.
Power
Electrical power is defined as the product of voltage and current: P 5 VI where V is voltage in volts, I is current in amps, and P is the power in watts. To use a 100-watt light bulb instead of a 50-watt light bulb requires twice the amount of power from the battery—twice the current at the same battery voltage. If the Ohm’s Law equation is substituted into the previous equation, P 5 I2R this equation defines the power losses in the resistances in the circuit—either external load or internal battery.
Efficiency
Battery efficiency is Efficiency 5 Power Out/Power In The principal battery losses are due to heat. These come from resistance and chemical sources: internal resistance of the battery determines its heating or PR losses when charging and discharging; chemical reaction between the lead and the sulfuric acid produces heat (called an exothermic reaction) during charging; and chemical reaction absorbs heat (called an endothermic reaction) during recharging. While PR losses are present whether charging or discharging—because they are proportional to the square of current flow—battery heat rise is higher during charging (because PR heating losses add to the internal heat-generating chemical reaction) and lower during discharging (because IR heating losses are balanced by the internal heatabsorbing chemical reaction). Given the PR relationship, charging or discharging at a lower current rate obviously contributes to keeping battery losses lower.
Battery Capacity and Rating Capacity and rating are the two principal battery-specifying factors. Capacity is the measurement of how much energy the battery can contain, analogous to the amount of water in the jug. Capacity depends on many factors, the most important of which are • Area or physical size of plates in contact with the electrolyte • Weight and amount of material in plates
