Summary of Circuit provides constant-current load for testing batteries
This article describes a circuit for testing AA and AAA alkaline batteries using a constant-current load instead of simple voltage or short-circuit tests. It explains how to calculate the test current (ITEST) using specific resistors and op-amps like the OP484ES to ensure accurate results, such as distinguishing between excellent and poor battery conditions based on output voltage under a 250 mA load.
Parts used in the Battery Testing Circuit:
- 9V battery
- Voltage regulator
- IC1
- IC2 (Operational amplifier)
- Q3 (Transistor)
- R19 (Resistor)
- R18 (Resistor)
- R4 (Resistor)
- R14 (Resistor)
- Analog Devices OP484ES or OP496GS Op Amp
Suppose that you need to test a 1.5V, AA-size alkaline battery. You can apply a short circuit and measure current, or you can measure open-circuit voltage, but neither method properly tests the battery. A suitable test current of approximately 250 mA gives you a more reasonable test. You can use a 6Ω resistive load at 1.5V, which produces an output voltage of 1.46V at an ambient temperature of 25°C if the battery is in excellent condition.
A poor battery might produce less than 1.2V. Given the load, the output current at 1.2V will be 200 mA instead of 250 mA. The battery will have just 80% of a full load current. Instead, you can use the circuit in Figure 1 to produce a constant-current load.
The circuit uses a 9V battery and a voltage regulator to produce a steady power-supply voltage of 5V. From that voltage, the circuit produces a constant sink current, which is independent of the battery’s output voltage, using IC1, IC2, and Q3. Your choice of current depends on battery size. You calculate the sink current of this circuit as ITEST=1/R19×[VCC×R18/(R4+R18)], where ITEST is the current you are testing and VCC is the voltage of resistive divider R4 and R18. The voltage across R19 should range from 0.3 to 0.85V for AAA and AA batteries. Transistor Q3 should be in its active region. Resistor R14 limits Q3’s base current to a safe level.
A suitable choice for the operational amplifier, IC2, is also important. You should use a single-supply op amp with a rail-to-rail input and a rail-to-rail output, such as Analog Devices’ OP484ES or OP496GS.
For more detail: Circuit provides constant-current load for testing batteries
- Why is a short circuit test unsuitable for testing a 1.5V AA battery?
A short circuit test does not properly test the battery compared to applying a suitable test current. - What is the recommended test current for an AA battery?
A suitable test current is approximately 250 mA. - What voltage indicates an excellent condition battery at 25°C?
An output voltage of 1.46V indicates the battery is in excellent condition. - How does a poor battery perform under the specified load?
A poor battery produces less than 1.2V and delivers only 200 mA instead of 250 mA. - What voltage range should exist across resistor R19 for testing?
The voltage across R19 should range from 0.3 to 0.85V. - Which type of operational amplifier is recommended for IC2?
You should use a single-supply op amp with rail-to-rail input and output capabilities. - What is the function of resistor R14 in the circuit?
Resistor R14 limits Q3’s base current to a safe level. - How is the sink current ITEST calculated in this circuit?
ITEST equals 1 divided by R19 multiplied by VCC times R18 divided by the sum of R4 and R18.
