This article describes an active load circuit that can be used to simulate a battery in any state of charge. The battery simulator provides a constant-voltage load for a battery-charging circuit, independent of applied charging current. The simulator’s impedance is less than 500 mΩ at all reasonable input frequencies. Best of all, the simulator can never be overcharged, allowing long-term testing and debugging of a charger system without the possibility of battery damage.
Circuit operation
The simulator (Figure 1) uses an LT1211 high speed, single-supply op amp to drive the base of a high gain PNP transistor-stage active load. Power for the LT1211 – a portion of the charging current – is supplied through a diode so the op amp and reference can survive brief periods of zero charging current. The op amp is configured for a DC gain of four, so the voltage on its noninverting input is one fourth of the voltage that the load box is set to. With S1 open, the load-voltage adjust range will be from 10 V to 20 V, and with S1 closed it will be approximately 3.5 V–10 V. Low voltage operation could be improved by replacing the top LT1004-2.5 with an LT1004-1.2 and reducing R1, the reference bias resistor, to 1k. The 510 Ω and 1.1k resistors are required for high frequency stability; they suppress a 1 MHz oscillation. The 1N5400 diode and 4-amp fuse protect the circuit from reverse voltages.
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| Figure 1. | Schematic diagram of the battery simulator. |
Results
The battery simulator circuit has been tested “swallowing” currents from 30 mA to 3 A with the output voltage essentially unchanged. When simulating a battery, the voltage adjust can be increased until the charger thinks the battery is fully charged and reduces the current into the simulator. Conversely, as the voltage is adjusted down, the battery charger may think the battery is becoming discharged and increase the current into the simulator.
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| Figure 2. | Current absorption capacity of the battery simulator at 5 V and 15 V. |
Figure 2 shows the circuit’s capacity for current absorption at two voltages, 5 V and 15 V, from 50 mA to 3 amps.
