Best Battery Charger with Auto Cut-Off

Here is another simple school project that can be used to charge any type of general purpose rechargeable battery or any type of mobile battery without having to worry about overcharging. This battery charger has an auto-cutout feature, which is enabled when the battery is fully charged. Once the battery is fully charged, the output from the circuit goes ‘ZERO’. I have done two distinct circuits here the first one is simple and the other one is more complex but has the feature to control current also.

I have been asked by a senior studying in an engineering college to make a battery charger project for him. I am happy to have made this project as his assignment, please note the circuit (Circuit 1) was provided to me by him – not to overlook, there are far better battery charging circuits with auto-cutoff. However, it is much simpler than many of the circuits that I have come across. To make this post more interesting, I have included another battery charging circuit here that can control the voltage and the current of upto 10Amps and can be used to charge high amp batteries.

Best Battery Charger with Auto Cut-Off

This battery charger (Circuit 1 and Circuit 2) can be used to charge a 3.7V, 6V, 9V, 12V batteries. However, the automatic-cutoff feature will work for 6V batteries to 12V batteries. With little bit of adjustments to the circuit (as shown in Circuit 2), it can also perform automatic cut-off operations for 3.7V, 5V and 24V batteries (though these circuits can safely charge 3.7V and 5V batteries, but cutoff feature won’t work).

ABOUT CIRCUIT

For this battery charger circuit (Circuit 1 and Circuit 2) you need a transformer (TX) based on the battery type you need to charge. For example 0-12V transformer is good enough for a 12V battery or 6V Transformer for 3.7V or 5V mobile battery charging.

For a 12V transformer the 12V is the RMS value when we convert it in peak to peak it becomes around 15Volt. So it is good enough to charge 12V batteries – as a 12V battery requires around 14.1V (peak to peak voltage) to charge (See the table below to know more about the voltage required to charge the common batteries – for the current refer to the respective information on the battery).

The circuit is self explanatory and simple to understand. The 220 Volts is stepped down to 12V AC (or as per your requirement) by the step down transformer (TX). The Step Down voltage is then fed to the 1N4007 bridge rectifier and passed through the filter capacitor 3300uf 25V. Till this point it is very simple. The filtered voltage is fed to the regulator LM317 IC pin 3 which can handle a current load of upto 1.5Amps. Preset connected between Pin 1 and GND will be used to set the voltage required to charge the battery. The feedback resistor of 220Ω at the pin 2 and the preset ensure the accurate output voltage.

Simple Battery Charger with Auto Cut-Off

The 10K preset in series to the relay is used to adjust the cut-off voltage. The charging will automatically stop when the voltage across the battery crosses the preset cut-off voltage (set at 10K preset).

HOW TO SET THE CUT-OFF VOLTAGE?

From the chart below see the charging voltage. Now use your multimeter to see the output voltage by connecting the probes to the output. I am using a Solar powered digital multimeter from INDE Enterprises to test and set. Now slowly turn the preset (potentiometer) across the LM317 to the required voltage.

The next step is to set the cut-off voltage. Slowly turn the preset (potentiometer) to a position just less than where the relay is energized. ie. to a point where you just cannot turn the potentiometer anymore without energizing the relay.

Red LED is connected on N/C (Normally Closed Terminal of the Relay), which ensures by glowing that battery is being charged, and when the relay is energised the Green LED turns ON signifying that the battery is fully charged. As soon as battery is drained to level below the preset value the RED LED lights up again.

That’s it and your circuit is ready to charge any battery. It is the best battery charger circuit on the net - END OF STORY!

Battery Charging Voltages

• 12V Battery Charging Voltage 14.1V – 14.8V
• 9V Battery Charging Voltage 11.8 – 12V
• 6V Battery Charging Voltage 7.4V – 8.0V
• 7 Battery Charging Voltage 4.1V – 4.3V

The battery should be charged with the current at the rate of 1/10^th of the battery capacity (for Lead Acid) and 1/3^rd of the battery capacity (for Lithium Ion). Example the Charging current of battery of 4500mA must be 4500mA/10=450mA (To adjust current, I suggest to use my previous circuit).

Battery Charging Current

• 10%-15% of the Current Value (Lead acid)
• 30% of the Current Value (Lithium Ion)

Best Battery Charging Project

Component List

• 4 x 1N4001 Diodes
• 1 x LM317 (Regulator IC)
• 3300uf 25V (Capacitor)
• 220Ω (Resistor)
• 1KΩ (Resistor)
• 2 x 10KΩ (Presets/Potentiomenters)
• 1 x LED (Red)
• 1 x LED (Green)
• Fuse
• TX (Transformer - See Description)
• RX (Relay - See Description)
• General Purpose PCB