Simplest Automatic Battery Charger Circuit using One Transistor

There is a super basic way to build an automatic battery charger using just a few parts.

This circuit is like a simple machine for charging batteries by itself.

It uses only a transistor a resistor and an LED.

It is great for charging things like 9V batteries and even has a light to show when it is working.

However, this basic charger is like a starter tool.

It does not have fancy features to perfectly protect the battery, so it is best for simple charging tasks.

Circuit Working:

Parts List:

This circuit represents the simplest form of an automatic battery charger.

It consists of just six components and is designed to be used with a 12V DC plug pack that outputs more than 15V on no load which is common for most plug packs.

The relay and transistor in the circuit are not critical components, as the 1k preset is adjusted to ensure that the relay switches off at 13.7V.

To improve the accuracy of the switch on and switch off voltages the 10k resistor can be replaced with a 12V zener diode.

This zener diode can be made up of two 6V zener diodes or any combination of diodes in series, including regular diodes with a 0.6V voltage drop.

The plug pack can have a current rating of 300mA, 500mA or 1A depending on the size of the 12V battery being charged.

For example, a 1.2Ah gel cell battery should be charged with a current of 100mA.

However, this charger is designed to maintain the batteries charge rather than rapidly charge it so the exact charging current is not critical.

If the battery is connected while in use the charger will provide some current to the load while also charging the battery.

For a fully discharged 12V battery, the charging current should not exceed 100mA.

For a 7Ah battery a current of 500mA can be used and for larger batteries a current of 1Amp can be used.

Most 12V plug packs output 15V to 16V on no load which is utilized in this circuit to charge the battery.

The circuit takes advantage of the plug packs poor regulation to charge the battery without causing the plug pack to overheat.

Setting up the charger involves connecting it to a battery and using a digital meter to adjust the 1k preset so that the relay switches off when the battery voltage reaches 13.7V.

A 100 ohm 2 watt resistor can be connected across the battery to observe the voltage drop.

The charger should switch on when the battery voltage drops to around 12.5V.

This voltage is not highly critical and is determined by the circuits hysteresis which is influenced by the load connected to the transistors collector.

The 22u capacitor prevents the relay from squealing or hunting when a load is connected to the battery and the charger is active.

As the battery voltage rises, the charging current decreases and just before the relay switches off it may produce noise due to the voltage fluctuations.

The 22uF capacitor reduces this noise.

To increase the hysteresis and thus decrease the cut in voltage a 270 ohm resistor can be added across the relay coil.

This increases the current required to activate the relay widening the gap between the switch on and switch off voltages.

However, the preset will need to be readjusted accordingly.

In this circuit a diode across the relay is not required because the transistor is never fully turned off so no back electromotive force EMF spike is produced by the relay.

Formulas:

The following is a general formula relate to specific parts of the circuit diagram above:

Voltage Divider:

V_out = V_in * (R2 / (R1 + R2))

where,

• The voltage at the transistors base is known as Vout.
• The input voltage, Vin comes from the power source.
• These are the resistor values, R1 and R2.

Crucial Information:

Safety measures like as heat protection and current limiting are absent from this circuit.

To avoid overcharging or battery damage, a real world battery charger circuit must have these elements.

To ascertain the precise functioning and charging characteristics of this circuit, the values of the constituent parts and the relay connections must be ascertained.

It is advised to use a commercially available battery charger made specifically for your battery type due to the safety constraints.

How to Build:

To build a Simplest Automatic Battery Charger Circuit using One Transistor follow the below mentioned steps:

• Connect the 1k preset in series with the base of the transistor.
• Connect the collector of the transistor to one terminal of the relay coil.
• Connect the other terminal of the relay coil to the positive terminal of the battery.
• Connect the emitter of the transistor to the negative terminal of the battery.
• Connect the 10k resistor in series with the relay coil.
• Connect the 270 ohm resistor across the relay coil.
• Connect the 22uF capacitor from base of transistor to ground.

Note:

• Please note that working with electricity can be dangerous.
• If you are not familiar with electronics or unsure about any step seek assistance from someone with experience.

Conclusion:

To conclude, the simplest automatic battery charger circuit using one transistor provides a basic charging indication for batteries like 9V.

However, it lacks advanced features such as overcharge protection and voltage regulation.

This circuit is easy to build and suitable for basic charging applications where a simple indication of charging status is sufficient.

References:

Build a smart battery charger using a single-transistor circuit