In this era of portable electronic devices, most electronics run on batteries. A battery stores charge and then supplies it to power any electronic device. The use of batteries requires proper care and handling. A major problem with using batteries is their over-discharge and overcharge. Both issues affect battery life and cost the end user unnecessarily. These issues are also often ignored by consumers. As batteries are attractively priced, this ends up increasing the maintenance cost of any electronic device.
In this electronic project, a zener diode based circuit will be designed to protect a battery from over-discharging. When a battery is charged, its terminal voltage, that is, the voltage between the battery's anode and cathode, increases. At full charge, the terminal voltage reaches a peak value that is an indication of 100% charge. As the battery is connected to an electronic device and begins to discharge, the terminal voltage begins to drop. The percentage or level of charge of a battery is therefore estimated by its terminal voltage. If a battery's terminal voltage falls below a lower limit, the battery begins to die prematurely. This degrades the battery's recharging capacity as well as its efficiency. Therefore, there must be a protection circuit that can monitor the charge level of the battery by detecting the terminal voltage and protect the battery from over-discharging by cutting the battery's connection with the electronic device.
In this electronic project, a power circuit is designed which will detect the lower limit of terminal voltage by the use of a suitable Zener diode and cut off the connection of the battery with the charging device by the use of a relay. The circuit also includes an LED indicator section that will illuminate the LED when the battery has discharged below the threshold and needs to be recharged.
Specifically, in this project, two lithium-ion batteries connected in series will be used as a power source. In most commonly used portable electronic devices such as laptops, smartphones and others, lithium-ion batteries with 3V terminal voltage lower limit are used, but some manufacturers design lithium-ion batteries with a lower cut-off limit of 2, 7V too.
In this project, batteries with a cut-off limit of 3.1 V are used for power supply. Thus, using two batteries in series, set the cutoff limit to 6.2 V. Thus, a zener diode of reverse peak voltage of 6.2 V is used to detect the cutoff limit in the circuit design. The diode will be used to drive the switching transistors that will operate the relay. As the battery terminal voltage will be below 6.2 V, the diode will enter the conduction state, activating the switching transistors and changing the relay state to cut off the power supply to the charging device. After understanding the working of this project, protection circuits for other cut-off limits can also be designed by properly selecting the zener diode and relay with the same circuit.
Required components
Fig. 1: List of components required for the battery over-discharge protector
Block Diagram –
Fig. 2: Battery over-discharge protector block diagram
Circuit Connections –
The circuit designed in this project has the following circuit sections –
1) Zener diode circuit to detect battery cut-off terminal voltage
2) Transistor circuit to operate the relay
3) Diode circuit for reverse current protection
4) LED indicator circuit for battery discharge indication
1) Zener Diode Circuit – A zener diode is connected in series with the battery such that the cathode of the zener diode is connected to the anode of the battery and the anode of the zener diode is connected to the base of the switching transistor. The purpose of connecting the diode in this way is to operate it in reverse bias condition. Until the terminal voltage of the battery is above the cutoff threshold and the reverse voltage peak of the zener diode, the zener diode will remain in a conducting state, but as the terminal voltage will fall below the cutoff and the reverse voltage peak of the zener diode , it will be turned off.
2) Transistor circuit – The transistor circuit is used to operate the relay. Transistors are used as high side switch in the circuit where two stages of transistors operate as logic inverters. The anode of the zener diode is connected to the base of transistor Q1, the emitter of transistor Q1 is connected to ground while the collector of the transistor is connected to the anode of the battery. The base of transistor Q2 is connected to the collector of transistor Q1, so the collector voltage of transistor Q1 will be switching transistor Q2. The emitter of transistor Q2 is grounded and the collector of transistor Q2 is connected to the relay coil that controls the power supply to the load device.
3) Diode Circuit – A diode circuit is connected parallel to the relay coil for protection against back current of the charging device. Back current from a high current load can permanently damage the battery, so this diode circuit is used for back current protection.
4) LED indicator circuit – The LED indicator circuit is connected to the NC point of the relay. When the transistor circuit switches the relay to the NC point, the LED is forward biased as the anode of the LED is connected to the NC point of the relay and the cathode is connected to ground. A current limiting resistor is connected in series with the LED to prevent any damage to the LED from excessive voltage.
How the circuit works –
Fig. 3: Battery over-discharge protector prototype
The circuit is based on the operation of the zener diode. A zener diode, when connected in reverse bias configuration and its cathode voltage is below its breakdown voltage, then the zener acts as an open circuit. But when a voltage above the breakdown of the zener is applied at its cathode terminal, the zener starts conducting from the cathode to the anode in reverse bias condition. As the zener diode can also work in reverse bias, this feature of the zener diode is used to detect the cut in battery voltage level.
There are two Li-Ion batteries connected in series, so they have a final full discharge voltage of 6V. Therefore, for safety, the cut-off voltage can be 6.2V and therefore a zener of 6. 2 V is used in the circuit.
When the two lithium-ion batteries are connected to the load, there can be two cases as follows –
The terminal voltage of the battery may be above 6.2 V- When the battery voltage is above 6.2 V, the cathode of the zener diode (D1) will be above 6.2 V. In this case, the zener diode will break and will start conducting from cathode to anode terminal (as shown in the image below). As the base of transistor Q1 is connected to the zener anode (as shown in the image below). Therefore, the base of transistor Q1 will start conducting and act as a closed circuit. Thus, all the collector current gets shorted and current will start flowing from the collector of Q1 to its emitter and finally to ground. Therefore transistor Q1 works as a logic inverter. When the zener diode is in a conducting state and there is sufficient voltage at the base of the BC547 transistor, the collector voltage is consumed as is. When the zener diode is in a non-conducting state and there is not enough voltage at the base of the transistor, the collector current is short-circuited to ground through the emitter and the collector voltage drops.
Fig. 4: Circuit diagram of Zener diode section of battery over-discharge protection
As the base of transistor Q2 is connected to the collector of Q1 but the potential on the collector of Q1 is almost zero because all the current is grounded, so conduction of Q1 will ground the base of transistor Q2 and transistor Q2 will be in non-conducting state . When the collector of transistor Q2 provides ground to one end of the relay, only then will the relay be energized. But as Q2 is in OFF state, its collector is at the battery voltage potential, so the relay will not be activated and the NO (normally open) LED pin of the relay will also remain in the OFF state. The NC pin (normally closed) of the relay contains the charging circuit that will remain connected to the battery.
Fig. 5: Circuit diagram showing operation of battery over-discharge protection high side switch
The other case can be when the terminal voltage of the battery can be below 6.2 V. When the battery voltage drops below 6.2 V, the zener diode will no longer remain in the conducting state. Now the zener diode will block the current through it due to reverse bias which will also cut off the base current of Q1.
But in practice it is observed that although the zener diode should not conduct current below 6.2 V, but it does conduct some current (in microamperes) that flows from its cathode to the anode, this current is the Zener leakage current. When considering the BC547 transistor, when the voltage between the base and emitter is between 0.65 V and 0.7 V, the transistor acts as a short circuit. The transistor (BC457) has a minimum gain of 110, so the base of the transistor needs much less current to conduct. As the current in the base of the transistor begins to increase, it acts as a variable resistance, the value of this resistance begins to decrease as the current increases.
Therefore, in this experiment, transistor Q1 has high gain and will amplify the leakage current from micro amps to milliamp current. Therefore, current in milliamps will begin to flow from the collector to the emitter. The zener leakage current will also turn on Q1. But in this state, Q1 is not fully ON, as the voltage from base to emitter so far does not reach 0.65 V. This leakage current will be zero when the battery voltage is below 5.9 V, but to turn off the battery at 6.2V, another transistor switching stage with transistor Q2 is used to achieve an accurate cut-off at 6.2V voltage.
Transistor Q2 provides low voltage indication and also disconnects the battery charge when the battery voltage is below 6.2V.
Fig. 6: Circuit diagram showing operation of battery over-discharge protection high side switch
The base of transistor Q2 is connected to the collector of transistor Q1. Now, with a voltage below 6.2 V, transistor Q1 will conduct, but not in its full saturation state. This means that the voltage difference between the collector and emitter of Q1 is much smaller, but has enough voltage to drive the base of transistor Q2.
Fig. 7: Circuit diagram showing practical operation of the battery over-discharge protection high side switch
Therefore, transistor Q2 will start conducting and the collector to emitter voltage of transistor Q2 will be almost zero as all the current will be drained to ground. This will activate the relay and the load will be disconnected from the battery and the LED circuit connected to the NO pin of the relay will start receiving power and the LED will start glowing indicating excessive battery discharge. Therefore, from the above explanation, it can be concluded that the use of a relay on the collector of transistor Q1 would have caused early switching of the charging circuit before the end of the battery discharge voltage. This is why another switching transistor stage with transistor Q2 is connected to set the precise cutoff voltage to 6.2V.
Fig. 8: Circuit diagram showing relay operation in battery over-discharge protection
Complete circuit diagram (below 6.2V)
Fig. 9: Circuit diagram showing complete operation of battery over-discharge protection
Use of series resistance (R1) with zener diode and other components
A zener diode requires a series resistance that limits current flow through it above its current rating, which will prevent the zener diode from overheating and being damaged. With the use of series resistance, the zener can provide a regulated voltage at the output.
Resistors R2 and R3 are connected to the collector of both transistors and resistor R4 is connected in series with the LED. The purpose of these resistors is just to limit the transistor and LED current. This will prevent any damage to the components.
Selecting zener resistance in series of diode (R1)
In this project, the zener diode used has a rating of 6.2 V/250 mW. The series resistance of the zener diode can be calculated by the following equation –
R1 = (Vs-Vz)/Iz
Where Vs = maximum supply voltage
Vz = zener voltage
Iz = zener current
To calculate R1 the Zener current must be calculated by the following method
Maximum power dissipation of the zener diode, Pz = 250mV
Zener voltage, Vz = 6.2V
Maximum zener current, Iz can be calculated as follows
Pz = Vz * Iz
Iz = Pz/Vz
Iz = 0.25/6.2 V
Iz = 40 mA (approx.)
Since the 3.7V Li-ion battery charges up to 4.2V, the full charge voltage of two Li-ion batteries (in series) is 8.4V.
So here the maximum battery supply voltage, Vs = 8.4 V
Zener voltage, Vz = 6.2 V
Zener current, Iz = 40 mA
Now, from the above equation, the resistance can be calculated as
R1 = (Vs-Vz)/Iz
R1 = (8.4-6.2)/0.040
R1 = 55 ohms
But in the experiment, the resistance R1 is more than 55 ohms. It's 80 ohms just to be safe. Zener series resistance selection must be chosen wisely so that it does not allow current exceeding the Zener rating. As more current will permanently damage the zener diode.
The different voltage readings obtained from the circuit are summarized in the following table –
Fig. 10: Table listing voltage readings at different sections of the battery protection circuit
From the above practical observations, it can be analyzed that the practical voltage at which the battery disconnects from the load is 6.27 V. Therefore, the battery will be disconnected when the battery voltage of each lithium-ion battery is at approximately 3.15 V.
Use of diode (D3)
As the relay internally has an inductor coil, this coil stores some charge when the relay is activated or energized. When the relay is de-energized, the polarity of the relay is reversed and a reverse current will flow from the coil, which can damage the circuit. Therefore, a diode (D3) is used across the relay to prevent circuit backcurrent when the relay is de-energized. This diode is known as a fly back diode or freewheeling diode. The inductor will discharge through this diode and this will prevent the other circuit from receiving any counter current.
It is important that the relay's nominal voltage is lower than the battery's cut-off voltage. For example, if a 9V relay is used in the circuit, it will never be energized at 6.27V. That is why 5V relay is used in the circuit.
Circuit diagrams
| Circuit-diagram-battery-over-discharge-indicator-protection- |  |