Circuit applied to battery capacity measurement
Time:2022-06-11
Views:2018
Batteries and energy batteries will lose capacity as they age. If the battery or battery capacity is too low, our equipment may also stop working soon. We can use the circuit in Figure 1 to measure the discharge time of the battery. The circuit uses an electromechanical clock and a DVM (digital voltmeter). The battery shall be fully charged before the test. The circuit discharges the battery with a fixed current and measures the time required for the battery to discharge from 100% to 0%.
For example, if the capacity of the battery is rated by the manufacturer and we discharge the battery with a constant current equal to 0.1 times the capacity, it will take about 10 hours for the battery to be fully charged and exhausted. The manufacturers of NiCd (nickel cadmium) or NiMH (nickel metal hydride) batteries set the end of discharge voltage as 1V. At this time, the battery is using 0% of its capacity and is in a flat state. It needs to be charged before further operation. If this process takes less than 10 hours, the battery capacity is lower than the battery manufacturer‘s rating.
Before testing the battery, use a charger to fully charge it. Apply 12V DC to the circuit and set 1V on pin 6 of IC 1b using potentiometer p 2. Set the clock to 12:00. An AA sized 1.5V battery supplies power to the clock via the relay switch S 3.
When we press the momentary button switch S1, the battery under test begins to discharge through transistor Q1 and resistor R1. Use potentiometer p 1 to set the discharge current. The operational amplifier IC 1A maintains a constant voltage across the resistor R 1, thereby providing a stable battery discharge current. Set the DVM to measure the DC voltage and measure the voltage at both ends of R 1. The display shows the discharge current in amperes. For example, 0.25V corresponds to 0.25A. Since the initial battery voltage is higher than 1V, pin 7 of op amp IC 1b is high, transistor Q2 is on, and DPST (double pole single throw) relay coil is active. The relay contact switch S 2 closes and bypasses the start button switch S 1, thereby keeping the discharge process active. Close the relay contact switch S 3 to allow the clock to hold time.
When the voltage of the battery is equal to the discharge end value 1V, the output of IC 1b becomes low and the relay coil is disabled to stop the discharge process. The clock also stopped. To obtain the capacity of the battery, multiply the set discharge current value by the elapsed time. If the discharge current value is small and it takes more than 12 hours for the battery to discharge, the value must be checked every 12 hours after the start of the test and the number of turns from 1 to 12 hours must be remembered.
The circuit also allows us to estimate the self discharge rate of the battery we use or the battery. Charge the battery to 100% of its capacity and measure the battery capacity according to this procedure. Charge our battery again, store it for a month, and then measure the battery capacity again. The difference between these two values is the monthly self discharge rate.
If we stack the batteries together, we should provide a reference voltage higher than the battery discharge termination voltage. If the battery voltage is higher than 12V, use a higher voltage value to supply power to the circuit. In addition, the reference voltage value shall be higher than the discharge termination value of the battery. The specification of the discharge path including transistor Q1 and resistor R1 shall meet the higher discharge current requirements.
The circuit is suitable for batteries or batteries of any chemical composition, including nickel cadmium, nickel hydrogen, lead acid and lithium ion. We can also use this circuit to measure the actual capacity of non rechargeable batteries, such as AA alkaline batteries. In this case, the voltage of the discharged battery shall be equal to the minimum supply voltage of the equipment. Batteries that have passed the test are not suitable for further use, but we can use their capacity information to estimate the capacity of batteries of the same type and manufacturer.
For example, if the capacity of the battery is rated by the manufacturer and we discharge the battery with a constant current equal to 0.1 times the capacity, it will take about 10 hours for the battery to be fully charged and exhausted. The manufacturers of NiCd (nickel cadmium) or NiMH (nickel metal hydride) batteries set the end of discharge voltage as 1V. At this time, the battery is using 0% of its capacity and is in a flat state. It needs to be charged before further operation. If this process takes less than 10 hours, the battery capacity is lower than the battery manufacturer‘s rating.
Before testing the battery, use a charger to fully charge it. Apply 12V DC to the circuit and set 1V on pin 6 of IC 1b using potentiometer p 2. Set the clock to 12:00. An AA sized 1.5V battery supplies power to the clock via the relay switch S 3.
When we press the momentary button switch S1, the battery under test begins to discharge through transistor Q1 and resistor R1. Use potentiometer p 1 to set the discharge current. The operational amplifier IC 1A maintains a constant voltage across the resistor R 1, thereby providing a stable battery discharge current. Set the DVM to measure the DC voltage and measure the voltage at both ends of R 1. The display shows the discharge current in amperes. For example, 0.25V corresponds to 0.25A. Since the initial battery voltage is higher than 1V, pin 7 of op amp IC 1b is high, transistor Q2 is on, and DPST (double pole single throw) relay coil is active. The relay contact switch S 2 closes and bypasses the start button switch S 1, thereby keeping the discharge process active. Close the relay contact switch S 3 to allow the clock to hold time.
When the voltage of the battery is equal to the discharge end value 1V, the output of IC 1b becomes low and the relay coil is disabled to stop the discharge process. The clock also stopped. To obtain the capacity of the battery, multiply the set discharge current value by the elapsed time. If the discharge current value is small and it takes more than 12 hours for the battery to discharge, the value must be checked every 12 hours after the start of the test and the number of turns from 1 to 12 hours must be remembered.
The circuit also allows us to estimate the self discharge rate of the battery we use or the battery. Charge the battery to 100% of its capacity and measure the battery capacity according to this procedure. Charge our battery again, store it for a month, and then measure the battery capacity again. The difference between these two values is the monthly self discharge rate.
If we stack the batteries together, we should provide a reference voltage higher than the battery discharge termination voltage. If the battery voltage is higher than 12V, use a higher voltage value to supply power to the circuit. In addition, the reference voltage value shall be higher than the discharge termination value of the battery. The specification of the discharge path including transistor Q1 and resistor R1 shall meet the higher discharge current requirements.
The circuit is suitable for batteries or batteries of any chemical composition, including nickel cadmium, nickel hydrogen, lead acid and lithium ion. We can also use this circuit to measure the actual capacity of non rechargeable batteries, such as AA alkaline batteries. In this case, the voltage of the discharged battery shall be equal to the minimum supply voltage of the equipment. Batteries that have passed the test are not suitable for further use, but we can use their capacity information to estimate the capacity of batteries of the same type and manufacturer.
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