Simple Self Discharge Battery Protection Circuit

This article shows you how to build a simple circuit to protect a car battery lead acid battery when it is not being used.

The circuit helps the battery stay charged for longer and keeps it healthy.

Building circuits with car batteries can be dangerous.

Experiment with adult supervision or someone who knows how to work with car batteries safely.

Circuit Working:

Parts List:

This circuit proves particularly beneficial for maintaining lead-acid batteries that remain dormant for extended periods but still need to function.

It works by charging the battery and allowing slow discharge through both the internal resistance of the battery and the circuit.

Upon reaching a predetermined state of charge SoC, the charger is reactivated initiating the charging process anew.

The circuit, detailed in the schematic below utilizes a schmitt trigger T1 and T2 zener and includes zener diode D6, responsible for setting the SoC level at which the charger deactivates.

Resistor R2 introduces necessary hysteresis.

To test the relays functioning without mains power and an unconnected battery assess its switching behavior using voltages from a regulated power supply.

To set up this self discharge battery protector circuit calibrate the voltage across the battery terminals to be 13.6V and 12.5V.

Fine tune the ‘on’ threshold by incorporating a 1N4148 in series with D6.

Adjust the ‘off’ threshold by modifying the R2 value possibly replacing it with a 100 ohm preset.

Alternatively, consider substituting the mains transformer and bridge rectifier with a battery charger.

However, note that connecting a completely discharged battery below 10V to the circuit is not feasible, as the relay will not energize.

Instead, charge such a battery to over 10V before connecting it.

Another option is to include a switch in parallel with the relay contact and use it to turn off the mains.

The setup can also cater to the protection of two 12V batteries doubling the secondary voltage of the mains transformer the zener voltage of D6 hysteresis and coil voltage.

Ensure the batteries are connected in series across the terminals and use fuse to safeguard components against short circuits.

A 1 Amp fuse can similarly protect the transformer primary circuit.

As the circuit centers around the battery, there is no need for a smoothing capacitor in this setup.

Formula:

Below is the formula for Self Discharge Battery:

Self discharge current = mAh / current

here,

• Self discharge current: 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).

This explains self discharge current in more detail:

Self discharge, which is usually represented as a percentage per month (e.g., 3% per month for Lithium-ion batteries), is the percentage loss of capacity over a given time period.

The datasheets from battery manufacturers include this information.

A battery with a 1000 mAh capacity and a 3% monthly self discharge rate, for instance, would lose 30 mAh (1000 mAh * 3%) of its capacity per month as a result of internal reactions, even while it is not in use.

How to Build:

Building the self discharge battery protector circuit involves assembling the components according to the provided schematic.

Collect Components:

• Gather all the required components listed above.

Schematic Understanding:

• Study the schematic diagram carefully to understand the connections and the placement of each component.

Assemble Transformer and Bridge Rectifier

• Connect the mains transformer and bridge rectifier as per the schematic.
• Ensure proper polarity and connections.

Connect Zener Diode and Resistor:

• Integrate zener diode D6 and resistor R2 as indicated in the schematic.
• Adjust R2 according to your preferences or replace it with a 100 ohm preset resistor for easier tuning.

Incorporate Schmitt Trigger T1/T2 Zener:

• Add the Schmitt trigger T1 and T2 zener to the circuit following the schematic.

Include 1N4148 Diode:

• Connect the 1N4148 diode in series with D6 to fine tune the ‘on’ threshold.

Integrate Relay:

• Attach the relay to the circuit ensuring proper coil connections.

Optional Switch:

• If using a switch for turning off the mains connect it in parallel with the relay contact.

Fuse Protection:

• Insert fuse to protect the components against short circuits.
• Use a 1 amp fuse to protect the transformer primary circuit.

Battery Connection:

• Connect the batteries in series across the terminals.

Testing:

• Before connecting the circuit to a battery, test its functionality using a regulated power supply.
• Ensure the relay switches on and off correctly.

Adjustment:

• Calibrate the circuit by adjusting the voltage across the battery terminals to 13.6V and 12.5V.
• Fine tune the ‘on’ and ‘off’ thresholds if necessary.

Final Check:

• Double check all connections and components to ensure everything is in order.

Connect to Battery:

• Once satisfied with the testing and adjustments connect the circuit to the lead acid battery.

Conclusion:

Always follow safety precautions and guidelines when working with electrical circuits.

If you are not familiar with electronics seek assistance from someone experienced or consult with a professional.

References:

Self-discharge

Analog circuit for battery discharge protection