Charging controller for 12V lead-acid
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.
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.