Most lead-acid chargers are very basic and simply pump current into the battery until it is switched off. The main problem with this type of charger is that ultimately it will overcharge the battery and may seriously damage it.

Adding a fully automatic Charge Controller to a basic charger will overcome these shortcomings. It will also prolong the life of your batteries and allow a battery to be left on a float charge, ready for use when required.

A typical lead acid battery charger is shown in Fig.1. It comprises a mains transformer with a centre-tapped secondary output. The output is rectified using two power diodes to provide raw DC for charging the battery. A thermal cutout opens if the transformer is delivering too much current.

Battery charging indication may be as simple as a zener diode, LED and resistor. The LED lights when the voltage exceeds the breakdown voltage of the zener diode (12V) and the forward voltage of the green LED (at around 1.8V). Thus the LED begins to glow at 13.8V and increases in brightness as the voltage rises. Some chargers may also have an ammeter to show the charging current.

The charging current to the battery is provided in a series of high current pulses at 100Hz, as shown in Fig.2(a).

The nominal 17V output from the charger will eventually charge a battery to over 16V if left connected long enough and this is sufficient to damage the battery. This is shown in Fig.2(b) where the battery voltage required for full charge (called the cut-off voltage) is exceeded when left on charge for too long. By adding in the Charge Controller, we can do much better.

Fig.3 shows how the Charge Controller is connected in between the charger and the battery. The Charge Controller is housed in a compact diecast aluminium case. However, if your charger has plenty of room inside its case, the controller could be built into it.

In effect, the Charge Controller is a switching device that can connect and disconnect the charger to the battery. This allows it to take control over charging and to cease charging at the correct voltage. The various charging phases are shown in Fig.4.

Fig.1: a typical lead-acid battery charger. It consists of a centre-tapped mains transformer and a full-wave rectifier (D1 & D2). There's also a thermal cutout and a LED indicator to show when the battery is charged.

Fig.2(a): the charging current from the circuit shown in Fig.1 consists of a series of high-current pulses at 100Hz. This can over-charge the battery if the charger is left on long enough, as indicated in Fig.2(b)

The Charge Controller can switch the current on or off or apply it in a series of bursts ranging from 20ms every two seconds through to continuously on. During the first phase, called "bulk charge", current is normally applied continuously to charge as fast as possible. However, with low-capacity batteries where the main charging current is too high, reducing the burst width will reduce the average current. So, for example, if you have a 4A battery charger, the current can be reduced from 4A anywhere down to 1% (40mA) in 1% steps, using the charge rate control.