Short Description

This circuit controls the cahrging of a battery from a power supply using On-Off, or hysteresis method. It connects the power supply directly to the battery if the battery voltage is lower than a threshold and keeps it connected for about 1.1 sec, after which it checks the battery voltage again. This process continues until the battery voltage stops falling below the threshold. The threshold for the voltage comparison must be selected to be the float voltage of the battery according to its type.

This charge controller is also suitable for charging a battery with solar panels if the maximum voltage and current conditions are met.

Working Principle

There is a 555 timer connected as a monostable multivibrator. The trigger terminal (pin 2) is connected to the battery voltage via a voltage divider with such a ratio that the 555 is triggered if the battery voltage falls below 13.3 V. Once the 555 is triggered, it keeps on for about 1.1 second and keeps the power supply conencted to the battery through a TIP147 PNP trasnsistor.

During the 1.1 sec charge pulse, current flows form the charger to the battery freely. When the charge pulse ends, the 555 turns off and the charger is disconneted from the battery. However, if the battery is not near its full state, its voltage would fall as soon as the pulse ends and the 555 timer would be triggered again for another 1.1 sec. If battery capacity is lower than about 80%, the charger remains almost continuously ON, because the 555 timer triggers immideately after each 1.1 sec charge pulse.

As the battery fills up, its voltage doesn't drop immidelately below the threshold after the end of the 1.1 sec charge pulse. So after each charge pulse, there is some time for which the voltage of the battery is above the threshold and the charger is off. This makes a sort of PWM wave having a constant on-time (1.1 sec) and the off-time increases as the battery fills up. 

An LED indicates the state of the charger, so when the battery is near its full charge, the LED starts pulsing instead of beaing constatnly on. The off-time between the pulses increases as the battery fills up, thus visibly indicating the state of charge of the battery. When the battery is completely charged and the threshold voltage is corerctly set according to the abttery type, the LED gives a 1.1 sec pulse after every 10-15 sec and the battery gets an equivalent of float charge.

If a load is connected to the battery, the charge LED may light up constantly instead of pulsing even when the battery is near its full charge. 

Specifications

This charger is intended to be used with small to medium sized 12 V flooded lead acid batteries from a ~ 14 V charger or a solar panel.

Constraints and Precautions

This charge controller connects the power supply directly to the battery, so care must be taken in the selection of the power supply. 

Supply Current Limit

It is very important that the power supply must have a current limit. This current limit defines the battery charge current in the bulk charging phase. The general rule of thumb is that the current limit must be 1/10 of the battery capacity. For example, a 2 A power supply would be suitable with a 20 Ah battery. Highers currents up to 1/5 C can be used with flooded batteries on the condition that their temperature and water level must be continuously monitored.

Solar panels can inherently supply only a limited current, so they are a much easier choice as long as their maximum current is in accordance with the battery capacity as described above.

Supply Voltage

If a solar panel is used as the charger, then small and meduim sized panels usually have maximum voltage up to 22 V, which is suitable for this charge controller. However, if a power supply is being used, then its nominal voltage must be 14-15 V. If the nominal voltage of the power supply is less than this range, it may not charge the battery completely and if it is higher, the supply may be overly stressed by the reduced voltage at its output (13-14 V) when the battery is being charged.

Minimum Battery Voltage

The electronic components in this circuit are powered from the battery, so it must have at lease some charge in it to operate this circuit. It cannot start from an empty battery. For reliable operation, the battery must have at least 7 V when this circuit is connected. However, if a power supply is being used to charge the battery, it may be overly stressed due to such a low voltage at its output.

Thermal Considerations

The swithcing element in this circuit is a TIP147 operating at a maximum of 1 Hz. It is not expected to heat up greatly if the charging current is below 5 A. However, its heatsink may be hot to touch after continuous operation. Care must be taken to appropriately judge the heatsink's temperature before handling it.

User Manual