Simple Battery Saver Circuit

Some devices such as calculators and remote controllers contain a special circuit designed to safeguard the batteries.

If someone accidentally leave the device ON this circuit helps prevent the batteries from draining too quickly.

However the batteries will still start to slowly lose power as time passes even with this circuit.

The Circuits Goal:

One minute after powering ON or when the battery voltage drops below the set operating level the battery saver circuit under discussion immediately cuts the supply current.

A series regulator FET, T1 is used in the circuit allowing a maximum current of 150 mA.

If the connected equipment is going to use more than 100 mA it is advised to make a use of a more robust FET than the BS250.

Circuit Working:

Parts List:

When the device is accidentally left ON, the circuit described here prevents excessive battery drain by acting as a battery protection system.

The Schmitt trigger NAND gate N3 is in a high state when it is powered on.

This happens as a result of resistor R6 maintaining the inputs to N4 at a logic low level when capacitor C2 is first drained.

In this state the P channel FET T1 is active allowing current to flow across the circuit and power the devices that are included.

Capacitor C2 charges through resistor R3 when T1 is turned on.

For one minute the charging process keeps on.

The voltage across resistor R3 drops to a level that allows the Schmitt trigger NAND gate N3 to detect a logic low signal at pin 1 after a minute.

FET T1 is turned OFF as a result of N3 changing its output state.

By avoiding any oscillation of N3 caused by the fluctuating voltage across R3 the locked function that N2 provides guarantees stability.

The R-C network R1 to R2 to C1 sends a strong pulse to N2s output during power down removing any remaining charge in capacitor C2.

Switches S2 to S1 then quickly turn the circuit back on while automatically saving battery power.

Battery voltage is carefully monitored by NAND gate N4 and diode D3 resistors R5 and R6.

The integrated circuits supply voltage level is connected to the N4 trigger threshold level.

N4 detects a logic low level at the junction of R5 and R6 to N4 when the battery voltage is quite high.

Diode D3 ensures sure that the input voltage to N4 is kept at a safe level in the case of a drop in battery voltage avoiding incorrect triggering.

Formula:

The following formula calculates the batteries lifespan based on the devices battery capacity, average current consumption and discharge safety factor.

Battery life = Capacity / Consumption × (1 – Discharge safety)

here,

• Hours (h) is a unit of measurement used to indicate how long a battery will last between charges.
• The entire amount of electrical charge that a battery can store is its capacity which is expressed in milliampere hours (mAh) or amp hours (Ah).
• The average current consumption of the device measured in milliamperes (mA) or amps A is called consumption.
• The term discharge safety explains why completely draining a battery is not recommended.

How the formula functions:

Capacity by Consumption Division:

This stage calculates the potential lifespan (capacity / consumption) of the battery in the case that it may be completely drained.

Discharge Safety Factor:

The statement (Discharge safety) introduces a safety buffer.

We multiply by this factor to account for the desired reserve capacity that should not be used.

Important Note:

Although the above technique provides an estimate a batteries actual lifespan can differ based on a number of factors including temperature, running programs, network usage, screen brightness and others.

A common suggestion is the discharge safety factor.

In certain battery datasheets a different suggested discharge level is sometimes listed for the longest battery life.

Although this is not always the case this calculation considers that the current usage is constant.

Different device operation patterns may lead to variations in current draw.

How to Build:

Below is the process of how to build Simple Battery Saver Circuit:

Connect the power source:

• Ensure that the batteries positive and negative terminals are connected to the right places on the circuit.

NAND gates with Schmitt triggers:

• The circuits logic parts N3, N4 and N2 are constructed using Schmitt trigger NAND gates.
• Follow the circuit descriptions steps for connecting the inputs and outputs.

FET T1 with a P channel:

• As per the pin layout connect the P channel FET T1.
• Make sure the schmitt trigger NAND gate N3 output is connected correctly.

The resistors connections:

• As indicated by the circuit diagram connect the resistors R1, R2, R3, R5 and R6 to their proper positions.
• The circuits specific needs decide the resistor values.

Capacitors Connections:

• As shown on the circuit diagram connect capacitors C1 and C2 in the right places.
• The values of the capacitors are chosen by the particular needs of the circuit.

Diode D3 Connection:

• In case the battery voltage drops than connect diode D3 to provide a safe input voltage to N4.

S1 and S2 switches Connections:

• To control power and reset the circuit connect the S1 and S2 switches as necessary.

Testing:

• Test the circuit to make sure it works properly before sealing it.
• Verify that everything is operating as it should including automated shut off after a minute, correct power down and reset options.

Modifications:

• Modify the resistor and capacitor values as needed to meet the particular needs of your application.

Conclusion:

To conclude the circuit automatically turns off the supply current when the battery voltage drops below a certain threshold or after one minute of powering on.

Through monitoring and controlling of the power supply to the connected devices it guarantees effective battery usage and guards against overdischarge.

References:

Battery saving self-poweroff circuit