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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Charging controller for 12V lead-acid batteries

Many low-cost chargers for 12V lead-acid batteries are little more than a step-down transformer and bridge rectifier. As a result they tend to overcharge a battery if left connected to it for an extended period.

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This simple charging regulator circuit is connected between the charger and the battery and is easily adjusted to prevent the battery being overcharged beyond the optimum level between 13.8V and 14.0V.

P-channel power Mosfet Q1 is connected as a switch in the positive battery lead, controlled by LM311 comparator IC1. IC1 monitors the battery terminal voltage and compares a portion of the battery voltage from trimpot VR1 with a fixed +7.2V at the inverting (-) input. This reference voltage is derived from the battery voltage via a shunt regulator circuit using zener diodes ZD2 and ZD3.

When the battery voltage is below 13.8V, the voltage at TP1 will be lower than 7.2V. Therefore the output of IC1 will switch low, drawing current through LED1 and its series 100W and 1kW resistors and thus providing forward bias for Q1. Q1 switches on, allowing charging current to flow into the battery.

As soon as the battery terminal voltage rises to 13.8V (or the preset voltage), TP1 rises above 7.2V and this causes the output of IC1 to switch high. The current through LED1 falls to zero and Q1 is switched off, preventing any further charging. The 47kW resistor between pins 2 & 7 of IC1 provides about 0.5V of hysteresis so that the battery voltage will need to drop by 500mV or so, before the circuit can turn back on to provide more charge.

Zener diodes ZD1 and ZD4 together with the 22W resistor are added to prevent over-voltage damage to either IC1 or Q1.

Note that although Q1 is being used here as an on-off switch, it has an on-resistance of about 0.2W and therefore needs to be provided with a good heatsink because of the heat dissipation during charging.

We suggest a finned extrusion heatsink like the Jaycar HH-8566, which has a thermal resistance of 2.2°C/W. You’ll need to provide electrical insulation between the device tab and the heatsink though, because the Mosfet tab will be at positive battery potential.

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