Some electronics, like remote controls and calculators, have a special circuit to protect the batteries.
This circuit helps keep the batteries from draining too quickly if you accidentally leave the device on.
But even with this circuit, the batteries will still slowly lose power over time.
Objective of the Circuit
The discussed battery saver circuit automatically interrupts the supply current either one minute after power on or when the battery voltage falls below the specified operational level.
The circuit employs a series regulator FET, T1 which allows a maximum current of 150 mA.
It is advisable to use a more robust FET than the BS250 if the connected equipment is expected to draw more than 100 mA.
Circuit Working:
Parts List:
| Type | Description | Quantity | Notes |
|---|---|---|---|
| Resistors | 1M | 1 | 1/4 watt |
| 10M | 1 | 1/4 watt | |
| 2.2M | 1 | 1/4 watt | |
| 22k | 1 | 1/4 watt | |
| 3.3k | 1 | 1/4 watt | |
| 100k | 1 | 1/4 watt | |
| Capacitors | Ceramic 1nF | 1 | |
| Electrolytic 22µF 25V | 1 | ||
| Semiconductors | Mosfet BS250 | 1 | |
| IC 4093 | 1 | ||
| Diode 1N4148 | 2 | ||
| Zener Diode 6.8V | 1 | ||
| Miscellaneous | ON/OFF Switch | 1 | |
| Push to OFF switch | 1 |
The described circuit serves as a battery protection mechanism in simple devices preventing excessive battery depletion when the device is inadvertently left on.
Upon power up the schmitt trigger NAND gate N3 is in a high state.
This is because capacitor C2 is initially discharged and the inputs to N4 are kept at a logic low level through resistor R6.
The p channel FET T1 is activated in this state allowing current to flow through the circuit to power the connected equipment.
When T1 is activated it causes capacitor C2 to, charge through resistor R3.
The charging process continues for one minute.
After one minute the voltage across resistor R3 becomes, low enough for the schmitt trigger NAND gate N3 to detect a logic low level at its pin 1.
This triggers N3 to change its output state turning off FET T1.
The locked function provided by N2 ensures stability by preventing potential oscillation of N3 due to the changing voltage across R3.
During power down the output of N2 receives a high pulse from the R-C network R1 to R2 to C1 clearing any residual charge in capacitor C2.
The circuit can then be promptly turned on again with switches S2 to S1 conserving battery power automatically.
Diode D3 resistors R5 and R6 and NAND gate N4 effectively monitor the battery voltage.
N4 trigger threshold level is related to the supply voltage level of the integrated circuit IC.
When the battery voltage is relatively high N4 identifies a logic low level at the junction of R5 to R6 to N4.
In the event of a drop in battery voltage, diode D3 ensures that the input voltage to N4 is maintained at a secure level preventing false triggering.
Formula:
Below formula uses the devices battery capacity, average current consumption, and discharge safety factor to determine how long the battery will last.
Battery life = Capacity / Consumption × (1 – Discharge safety)
here,
- Battery life: Usually expressed in hours (h), this is an estimate of how long the battery will last between charges.
- Capacity: Measured in milliampere hours (mAh) or amp hours (Ah), capacity is the total amount of electrical charge that the battery is capable of storing.
- It shows the maximum current the battery is capable of delivering over a specific period of time.
- Consumption: This is the devices average current consumption expressed in milliamperes (mA) or amps A.
- This shows the amount of current the gadget gradually drains from the battery.
- Discharge safety: This phrase explains why it is not advised to empty a battery entirely.
- In order to maintain battery health and longevity, this factor, which is normally between 0.1 and 0.2, represents a safety buffer of 10% to 20% of the batterys capacity that should not be used.
How the formula functions:
Dividing Capacity by Consumption: This step determines how long the battery would theoretically survive (capacity / consumption) if it could be emptied fully.
Factor of Discharge Safety: There is a safety buffer introduced by the expression (1 – Discharge safety).
We take into consideration the suggested reserve capacity that ought not to be utilized by multiplying by this factor.
Important Note:
An estimate is given by this formula, temperature, active apps, network usage, screen brightness and other variables may all affect how long a battery lasts in the real world.
The safety factor for discharge is a general suggestion.
For the best battery life, a varied recommended discharge depth may be specified in some battery datasheets.
This calculation makes the assumption that the current consumption is constant, which is not always the case.
Variations in current draw may result from patterns of device operation.
How to Build:
Below is the process of how to build a battery saver circuit:
Connect the Power Supply:
- Connect the positive and negative terminals of your battery to the appropriate points on the circuit.
Schmitt Trigger NAND Gates:
- Use schmitt trigger NAND gates to build the logic elements N3, N4, N2 of the circuit.
- Connect the inputs and outputs as described in the circuit description.
P-channel FET T1:
- Connect the p channel FET T1 according to its pin configuration.
- Ensure proper connection to the schmitt trigger NAND gate N3 output.
Resistors:
- Connect the resistors R1, R2, R3, R5 and R6 to their designated locations based on the circuit diagram.
- The resistor values depend on the specific requirements of your circuit.
Capacitors:
- Connect capacitors C1 and C2 according to their designated locations on the circuit diagram.
- The capacitor values depend on the specific requirements of your circuit.
Diode D3:
- Connect diode D3 to ensure a secure input voltage to N4 in case of a drop in battery voltage.
Switches S1, S2:
- Before sealing the circuit test it to ensure proper functionality.
- Check for the desired behavior such as the automatic, cut off after one minute and proper power down and reset functionality.
Adjustments:
- If necessary, adjust resistor and capacitor values to meet the specific requirements of your application.
Conclusion:
To conclude, the circuit automatically cuts off the supply current after one minute of power on or when the battery voltage falls below a certain level.
It ensures efficient use of battery power and protects against over discharge by monitoring and controlling the power supply to the connected equipment.