Below this level, the regulator's output drops off sharply, effectively ending the usefulness of the circuit (even though the battery might still have sufficient capacity to feed the load by itself for a somewhat longer period). In fact, most regulators stop functioning when the voltage difference between the battery and load is reduced below a specified "dropout" level. As the battery is used, its output voltage gradually drops to a level where the regulator is no longer needed for voltage reduction. One reason for using a voltage regulator is to maintain a constant voltage across the load at a level substantially less than a fresh, fully charged battery's terminal voltage, in order to minimize power dissipation in the load. The inset shows the basic feedback principle. The error amplifier should have enough output current to drive the base of the pass transistor (an OP50 might be a good choice for high-current applications). Q1 should be selected to have the appropriate power-dissipation capability a Darlington transistor could be used for higher power, but be sure to allow for V sat of the Darlington. The inset shows the basic scheme, in which the voltage output from the multiplier, representing power, (1 V corresponds to 1 W) is compared with a setpoint, and manipulates the discharge current to maintain power constant at the preset level. Figure 2 shows a circuit for discharging a battery at a controlled power level.
If you desire to measure the battery's terminal performance as it is being discharged at constant power, a power-measuring circuit like Figure 1 can be used in a feedback loop to enforce the constant power constraint. Note that R L could be an arbitrary linear or nonlinear grounded load circuit. The battery voltage, V B, is applied to the Y input.The AD534's output is proportional to the battery's true instantaneous output power. The voltage drop across R SENSE, applied to the X input, measures the current through load R L. In the example of Figure 1, using an AD534 multiplier, with impedance differential inputs, the total load on the battery is R L + R SENSE. Using an analog multiplier to measure battery discharge power. Both discharge power and total energy can be displayed vs. To measure the total energy that has been delivered, the output voltage of the multiplier can either be integrated, using an analog integrator circuit with a very long time constant or it can be sampled at frequent intervals by an A/D converter, with the Wh readings accumulated in a computer. Figure 1 is a circuit of a wattmeter that uses an analog multiplier to measure the actual power (volts × amperes) being delivered by the battery at a given instant. When a battery is life-tested with a prescribed load, its terminal voltage may be a poor indicator of how much of its energy has been discharged.
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