This is the world's simplest automatic battery charger. It consists of 6 components, when connected to a 12v DC plug pack. The plug pack must produce more than 15v on no-load (which most plug packs do.) An alternative 15v transformer and a centre-tapped transformer is also shown in the circuit. A centre-tapped transformer is referred to as: 15v-CT-15v or 15-0-15 The relay and transistor are not critical as the 1k pot is adjusted so the relay drops-out at 13.7v. The plug pack can be 300mA, 500mA or 1A and its current rating will depend on the size of the 12v battery you are charging. For a 1.2AH gel cell, the charging current should be 100mA. However, this charger is designed to keep the battery topped-up and it will deliver current in such short bursts, that the charging current is not important. This applies if you are keeping the battery connected while it is being used. In this case the charger will add to the output and deliver some current to the load while charging the battery. If you are charging a flat cell, the current should not be more than 100mA. For a 7AH battery, the current can be 500mA. And for a larger battery, the current can be 1Amp.
Connect the charger to a battery and place a digital meter across the battery. Adjust the 1k pot so the relay drops out as soon as the voltage rises to 13.7v. Place a 100R 2watt resistor across the battery and watch the voltage drop. The charger should turn on when the voltage drops to about 12.5v. This voltage is not important. The 22u stops the relay "squealing" or "hunting" when a load is connected to the battery and the charger is charging. As the battery voltage rises, the charging current reduces and just before the relay drops out, it squeals as the voltage rises and falls due to the action of the relay. The 22u prevents this "chattering".
To increase the Hysteresis: In other words, decrease the voltage where the circuit cuts-in, add a 270R across the coil of the relay. This will increase the current required by the transistor to activate the relay and thus increase the gap between the two activation points. The pull-in point on the pot will be higher and you will have re-adjust the pot, but the drop-out point will be the same and thus the gap will be wider. In our circuit, the cut-in voltage was 11.5v with a 270R across the relay.Note: No diode is needed across the relay because the transistor is never fully turned off and no back EMF (spike) is produced by the relay.
This is the world's simplest automatic battery charger. It consists of 6 components, when connected to a 12v DC plug pack. The plug pack must produce more than 15v on no-load (which most plug packs do.) An alternative 15v transformer and a centre-tapped transformer is also shown in the circuit. A centre-tapped transformer is referred to as: 15v-CT-15v or 15-0-15 The relay and transistor are not critical as the 1k pot is adjusted so the relay drops-out at 13.7v. The plug pack can be 300mA, 500mA or 1A and its current rating will depend on the size of the 12v battery you are charging. For a 1.2AH gel cell, the charging current should be 100mA. However, this charger is designed to keep the battery topped-up and it will deliver current in such short bursts, that the charging current is not important. This applies if you are keeping the battery connected while it is being used. In this case the charger will add to the output and deliver some current to the load while charging the battery. If you are charging a flat cell, the current should not be more than 100mA. For a 7AH battery, the current can be 500mA. And for a larger battery, the current can be 1Amp.
Connect the charger to a battery and place a digital meter across the battery. Adjust the 1k pot so the relay drops out as soon as the voltage rises to 13.7v. Place a 100R 2watt resistor across the battery and watch the voltage drop. The charger should turn on when the voltage drops to about 12.5v. This voltage is not important. The 22u stops the relay "squealing" or "hunting" when a load is connected to the battery and the charger is charging. As the battery voltage rises, the charging current reduces and just before the relay drops out, it squeals as the voltage rises and falls due to the action of the relay. The 22u prevents this "chattering".
To increase the Hysteresis: In other words, decrease the voltage where the circuit cuts-in, add a 270R across the coil of the relay. This will increase the current required by the transistor to activate the relay and thus increase the gap between the two activation points. The pull-in point on the pot will be higher and you will have re-adjust the pot, but the drop-out point will be the same and thus the gap will be wider. In our circuit, the cut-in voltage was 11.5v with a 270R across the relay.Note: No diode is needed across the relay because the transistor is never fully turned off and no back EMF (spike) is produced by the relay.
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