Simple Self Discharge Battery Protection Circuit

The construction of a simple circuit to protect a lead acid car battery when it is not in use is described in this article.

The circuit increases the life of the battery and protects its overall health condition.

It can be dangerous to build lead batteries for cars.

Try working with an adult who is responsible or who knows how to handle car batteries safely.

Circuit Working:

Parts List:

This circuit is very useful for preserving lead acid batteries that must continue to operate even when they are left idle for long periods of time.

It charges the battery and allows a slow discharge through the circuit and the batteries internal resistance.

The charging process is restarted when the charger reaches a set state of charge (SoC).

Using a Schmitt trigger T1 and T2 diode and a Zener diode D6 in the circuit as shown in the diagram above sets the SoC level at which the charger deactivates.

The required hysteresis is introduced through resistor R2.

Relay switching behavior is measured using voltages from a regulated power source in order to test the relays operating without mains power and an unplugged battery.

Set the voltage between the battery terminals to 13.6V and 12.5V in order to configure this self discharge battery protection circuit.

Make the ‘ON’ threshold more accurate by connecting a 1N4148 in series with D6.

The ‘OFF’ threshold is adjusted by changing the R2 value perhaps substituting a default of 100 ohm.

As an alternative think about using a battery charger in place of the bridge rectifier and mains transformer.

However keep in mind that it is not possible to connect a fully drained battery below 10V to the circuit since the relay will not charge.

Instead before connecting such a battery charge it to more than 10V.

Adding a switch in parallel with the relay contact and using it to cut the mains is a second option.

The setup can further double the coil voltage D6 hysteresis Zener voltage and secondary voltage of the mains transformer in order to protect two 12V batteries.

Use a fuse to protect components from short circuits and make sure the batteries are connected in series across the terminals.

The transformer primary circuit can also be protected with a 1 Amp fuse.

A smoothing capacitor is not required in this configuration since the battery is the main attraction of the circuit.

Formula:

Below is the formula for Self Discharge Battery:

Self discharge current = mAh / current

here,

• Self discharge current is usually expressed in milliamperes (mA) or microamperes (µA) this is the real current that the battery loses as a result of internal chemical processes.
• Milliampere hour (mAh) refers to a batteries overall capacity or the amount of current it can provide for a certain period of time (hours).

Below is a more complete description of self discharge current:

Self discharge is the percentage loss of capacity over a certain time period and generally expressed as a percentage per month (e.g., 3% per month for lithium-ion batteries).

This information is usually found in battery manufacturers datasheets.

For example even when not in use a battery with a 1000 mAh capacity and a 3% monthly self discharge rate would lose 30 mAh (1000 mAh * 3% of its capacity) each month due to internal reactions.

How to Build:

To build a Simple Self Discharge Battery Protection Circuit follow the below mentioned steps for connection:

Gather the Parts:

• Collect all of the above mentioned necessary parts as per the circuit diagram.

Understanding Schematics:

• Study the schematic diagram closely to understand how each component are connected and placed.
• Put the bridge rectifier and transformer together.
• As shown in the diagram connect the bridge rectifier and mains transformer.
• Check for correct connections and polarity.

Attach the resistor and Zener diode:

• As shown in the diagram connect resistor R2 and Zener diode D6.
• R2 can be changed to suit ones requirements or for simpler tuning a 100 ohm preset variable resistor can be used.

Include Schmitt Trigger Zener T1/T2:

• As shown in the diagram add the Schmitt trigger T1 and T2 Zener to the circuit.
• Add a 1N4148 diode.
• To adjust the ON threshold accurately connect the 1N4148 diode in series with D6.

Connect the Relay:

• Make sure the coil connections are correct before connecting the relay to the circuit.

Optional Switch:

• If a switch is being used to cut the mains connect it in parallel with the relay contact.

Protection of Fuse:

• To avoid short circuits in the components use a fuse.
• To protect the transformers main circuit install a 1 amp fuse.

Connecting the battery:

• Connect the batteries across the terminals in series.

Testing:

• Use a regulated power source to confirm the circuits operation before connecting it to a battery.
• Verify that the relay turns ON and OFF properly.

Modification:

• Set the voltage between the battery terminals to 13.6V and 12.5V to calibrate the circuit.
• Adjust the ON and OFF thresholds slightly if required.

Conclusion:

When dealing with electrical circuits always follow all the safety rules and protections.

Seek advice from a professional or someone with experience if one is unfamiliar with electronics.

References:

Self-discharge

Analog circuit for battery discharge protection