Charge indicator gauges lead-acid batteries

-May 27, 1999

Although rechargeable, sealed lead-acid cells are uncommon in portable applications, they are a good choice for standby applications, such as emergency lighting and burglar alarms. A key advantage to using these batteries is that you can determine the amount of remaining charge by measuring the open-circuit voltage. This technique is invalid for NiCd or NiMH cells. Figure 1 shows the relationship between the amount of remaining charge versus the open-circuit battery voltage. This curve is accurate to approximately 10%, provided that you have not charged or discharged the battery for at least 24 hours. A simple circuit measures the open-circuit voltage, such as the expanded-scale voltmeter circuit in Figure 2, which follows the curve in Figure 1.

Figure 1 The curve of remaining charge versus open-circuit battery voltage for a sealed lead-acid battery is accurate to approximately 10%, if you haven’t charged or discharged the battery for at least 24 hours.

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Figure 2 To measure a sealed lead-acid battery’s open-circuit voltage, an expanded-scale voltmeter circuit uses an op amp and reference to provide the necessary gain and offset to drive an analog or digital-panel meter, or optionally an ADC.

Sealed lead-acid batteries are available in several sizes, from a single D size (2.5 Ahr) to multicell rectangular battery packs. These cells can provide high output currents and years of reliable backup power. Other desirable features include relatively simple charge requirements and low self-discharge. The low self-discharge and ease of determining the remaining charge make sealed lead-acid batteries an ideal choice for flashlights and portable lighting. The low self-discharge, which is approximately 5% per month at 25°C, means that a rechargeable flashlight using sealed lead-acid cells will still have usable capacity of approximately 30% after one year of inactivity. NiCd and NiMH cells lose approximately 30% of their charge per month. A flashlight using NiCd cells requires a trickle charge when not in use to ensure reliable power when necessary. Without trickle charging, NiCd cells will completely discharge after three to four months of inactivity.

With the range switch in Figure 1 in the one-cell position, the panel meter doesn't move until the input voltage exceeds 1.930V. Full scale corresponds to an input voltage of 2.130V. The op amp and reference provide the gain and offset for driving a digital panel meter, an ADC, or an analog meter with the meter scale calibrated from 0 to 100% of remaining charge. A rotary switch allows you to use the meter circuit with multicell battery packs containing one to six cells. You can measure other cell quantities by selecting the appropriate resistor divider values.

The circuit configures the op-amp section of IC1, which also includes an unused comparator, as an inverting gain-of-five amplifier. This configuration produces a 1.000V change at the output for a 200-mV change at the input. The negative terminal of the battery connects to the op amp's inverting input resistor. To accomplish the 1.930V offset, IC1's internal 1.200V reference, R2, and R3 generate a current that flows into the op amp's summing node (Pin 2). The op-amp output drives a standard 50-µA analog panel meter with a scale from 0 to 100%. You can also use a 1V full-scale digital panel meter or an ADC (Figure 2). The 8-bit ADC, IC2, uses the 1.2V reference voltage of IC1 for the ADC reference, giving a full-scale output (8 bits) for a 1.2V input. If you use the ADC, the op amp's gain must increase from 5 to 6 to provide an output of 1.2V from the op amp for a 200-mV change at the input. To make this change, you simply increase the value of R4 to 600 kW. You can also use analog meters ranging from 100 µA to 1 mA, if you reduce the values of R5 and R6.

Calibrating the circuit requires an adjustable voltage source, preferably with coarse and fine voltage adjustment and a digital voltmeter. With three AA cells for power and the range switch in the one-cell position, apply a precise -2.130V to the input at point A. Connect a DVM to the op amp output (Pin 1) and adjust R3 for a 1.000V reading on the DVM. Next, adjust R6 for a full-scale reading, 100%, on the analog meter. Decreasing the voltage source by 100 mV to -2.030V should drop the DVM reading to 500 mV and drop the analog meter to midscale, or 50%. Dropping the voltage source an additional 100 mV to -1.930V results in a DVM reading near 0V and a corresponding meter indication of 0%. Because of minor resistor and offset-voltage errors, the output may not exactly equal 0V, but may be a few mV positive. For this application, this value is more than adequate. Resistor values of 1% provide the best accuracy and stability, but you can use a standard 16.2-kW 10% resistor that measures approximately 100W high for R1. You can use Table 1 to verify other ranges.

The circuit does not require a power switch because the op-amp section of the circuit draws extremely low quiescent current (12 µA). Battery life should equal the shelf life of the battery, which is several years. The op amp's input also includes overvoltage and reverse-voltage protection. (DI #2359)

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