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
battery chargers up to 10A rating
& SLA charging
& float charging
& adjustable parameters
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
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
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.