Self Discharge Battery Protection Circuit

This article for Self Discharge Battery Protection Circuit teaches how to make one easy circuit to protect car battery (lead acid type) when not using.

This circuit help battery live more long time and keeps it healthy.

WARNING: Making lead battery for car can be dangerous.

Try to do this with one big person who know how to work safe with car battery.

Circuit Working:

Parts List:

This circuit is very good for saving lead acid battery when not in use for long time but still need to work.

It gives small charge to battery and let battery discharge slowly by itself and by circuit.

When battery charge goes down and charger starts again by itself when it reaches special charge level (SoC).

In circuit Schmitt trigger T1, T2, diode and Zener diode D6 help to set SoC level to stop charger.

Resistor R2 gives needed hysteresis.

Relay test is done by using steady power voltage to see if relay works without main power and is with no battery plug.

Set battery voltage between 13.6V and 12.5V to make this self discharge protect circuit working good.

To make ON point more correct add 1N4148 diode with D6.

To change OFF point try to change R2 resistor maybe use 100 ohm.

We can also use battery charger instead of bridge rectifier and transformer.

But be careful if battery is very low below 10V it does not work because relay does not starts.

First charge battery to more than 10V and then connect to circuit.

We can also add one switch with relay to turn OFF main power.

If want to protect two 12V battery then we can make coil voltage, D6 Zener voltage and transformer voltage double.

Use fuse to protect all parts from short circuit and connect two battery in series to terminal.

Also put 1A fuse to protect transformer side.

No need for smoothing capacitor here battery does that job.

Formula:

Below is formula for Self Discharge Battery:

Self discharge current = mAh / current

here,

Self discharge current show how much battery loses power by itself.

It is in mA (milliampere) or µA (microampere) this happen because of battery chemical inside.

mAh (milliampere hour) means battery is full power and how long it can give current.

More explain:

Self discharge is percent loss per month.

Example: 3% per month for lithium battery which mean battery loses 3% even not in use.

Battery company datasheet show this information.

For example: battery of 1000 mAh with 3% per month, even if there is no use of battery to lose 30 mAh in one month.

How to Build:

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

Collect parts:

• Take all parts listed above and use circuit diagram to know what we need.

Understand diagram:

• Look carefully at circuit diagram it shows how to join parts and where to put each one.

Connect transformer and bridge rectifier:

• Join transformer and bridge rectifier same like in diagram and be sure wires and polarity are correct.

Join Zener diode and resistor:

• Now connect R2 resistor and Zener diode D6 like in diagram.
• We can change R2 value if needed or use 100 ohm preset for easy setting.

Add Schmitt Trigger T1 and T2:

• Put Schmitt trigger parts T1, T2 and Zener in place as in diagram.

Add 1N4148 diode:

• To fix ON voltage correct join 1N4148 diode in series with D6.

Connect relay:

• Before add relay to board and check its coil pins are right.

Optional switch:

• If we use a switch to turn OFF power then connect it in parallel with relay contact.

Fuse protection:

• To stop short circuit use fuse.
• Put 1A fuse in transformer main line to protect it.

Connect battery:

• Join batteries to circuit in series across terminals.

Testing time:

• Before using battery test circuit with regulated power supply.
• See if relay goes ON and OFF as it should.

Make changes:

• Set voltage between 13.6V and 12.5V to test ON/OFF.
• Change ON/OFF values if needed.

Be safe:

• Always be careful with electric parts and follow safety rules.
• Ask help from expert or person who knows.

References:

Self-discharge

Analog circuit for battery discharge protection