Design Ideas: September 12, 1996

The lead-acid battery charger in Figure 1 works with either gel- or wet-cell, lead-acid, 12V batteries. The charger includes the proper temperature compensation for the charger output voltage as well as a detector that signals when the battery is fully charged.

The best charger for a lead-acid battery is a current-limited voltage source that sources current into the battery until it reaches the voltage setpoint. The charger should then supply just enough current to keep the battery voltage at this value. For 12V, lead-acid batteries, a setpoint voltage of approximately 13.8V at 25°C is typical. However, this so-called optimum voltage setpoint is temperature-dependent. Battery manufacturers recommend a TC of ­22 mV/°C, so that the charger set voltage tracks the TC of the battery.

You can determine the state of charge of a lead-acid battery from its voltage and charging current. When you charge the battery using a voltage source set to 13.8V, the charging current gradually tapers off to a very low value when the battery is fully charged. The design in Figure 1 includes a full-charge detector circuit that lights an LED when the current drops lower than this threshold.

This charger design comprises a 12V, 0.5A wall transformer and an LM2941CT voltage regulator, IC₂. The wall transformer provides the unregulated dc voltage to IC₂, which the circuit uses to charge the battery and hold its voltage at 13.8V. R₃, R₄, and D₃ through D₁₂ set the regulator output voltage. You should adjust R₄ for an output voltage of 13.8V with the battery disconnected. An important characteristic of any wall transformer is poor load regulation. For example, the transformer in Figure 1 puts out 12V when loaded to 0.5A but approximately 17 to 18V with no load. The design exploits this feature, because it means that IC₂ requires no heat sink.

D₃ through D₁₂ establish the negative-TC output necessary to match the battery-terminal voltage. Measurements of some 1N4148 diodes at 1 mA show a TC of ­2.2 mV/°C, so 10 of these diodes in series provide the necessary ­22 mV/°C. R₅ and D₁₃ indicate the presence of power or when a battery connects to the output and provide the minimum required loading for IC₂. C₁ stabilizes IC₂.

A discharged battery has a terminal voltage of about 10 to 12V. If you connect a battery to the charger output, IC₂ fully turns on (saturates) its pass transistor. Under this condition, IC₂ conducts all of the current possible to try to force the battery voltage up to the 13.8V setpoint. Thus, while the battery is below 13.8V, IC₂ acts as a current source. The maximum that the wall transformer can provide at that voltage determines the charging-current limit.

As the battery charges and its voltage rises, the amount of current that the wall transformer can provide decreases. In this case, with a battery voltage of 11.5V and a corresponding transformer output voltage of 12V, a maximum charge current of approximately 0.5A is available. By the time the battery reaches 13.8V, a maximum of only about 250 mA is available. IC₂ remains fully on until the battery voltage reaches 13.8V. Then, IC₂ reduces the charging current as required to maintain this voltage across the battery. At this point, IC₂ is operating in a constant-voltage mode. While IC₂ is in the current-source mode, the voltage drop across this IC is about 250 mV, which means that the power dissipation is less than 0.2W. Because of this low power dissipation, IC₂ requires no heat sink.

While a lead-acid battery charges at a constant voltage, the charging current continually decreases until it reaches its final value, which is typically about 1% of the ampere-hour rating of the battery. By adjusting R₂, you can calibrate this charger to correctly detect full charge of any battery with a capacity as high as approximately 10-Ahr. You can measure the charging current as the voltage drop across R₁. D₁ turns on and shunts current around R₁ whenever this voltage exceeds about 0.2V, which minimizes power dissipation in R₁. Because of D₁, the measured voltage across R₁ is accurate only when the charging current is less than 0.2A. However, because the end-of-charge detection occurs at currents lower than 0.1A, this range of measurement is more than adequate.

IC_1A, a differential amplifier with unity gain, shifts the voltage drop across R₁ to produce a ground-referred signal. IC_1B compares this signal, which is proportional to the charging current, to a reference voltage that R₂ sets. When the load current drops low enough that the voltage at pin 6 of IC_1B drops below pin 5, the output swings high and turns on D₂ to indicate that the battery is fully charged. The best way to calibrate the end-of-charge detection circuit is to let the battery fully charge and then to adjust R₂ just until D₂ comes on. (DI #1922)