This article introduces the concept of deep discharging in batteries. Deep discharging has the potential to destroy the batteries you use in projects that range from uninterruptible power supplies to a remote-controlled car. Let’s explore important terms, how different batteries react to deep discharging and a simple circuit that can protect your battery-powered projects from malfunctioning.
What is Deep Discharging?
Deep discharging — more commonly referred to as over-discharging — occurs in a battery when it has been discharged at its full capacity. When a battery is charged, it consists of potential electric energy stored and when it is discharged, you are reversing the charging process and using electric potential energy stored to drive the electric components. Every battery has a cut-off point; this point is a voltage at which the battery has been completely discharged. Manufacturers sometimes specify cut-off voltages for various discharge rates.
The cut-off voltage is very sensitive to discharge rates. If the battery has a high discharge rate, it will have a lower cut-off voltage and vice versa. Also, the cut-off voltage of a battery is sensitive to operating temperature.
In deep discharging, the amount of electric discharge is actually 1.5 to 2 times as great as the capacity of the battery. So when the battery undergoes over-discharging, it is very difficult to recharge it because the internal resistance of the cell has increased. All these batteries have a pretty high price tag on them, so users should keep in mind over-discharging to protect the load and battery.
Depth of Discharge
Another term that is worth recalling here is the depth of discharge. A battery's depth of discharge is the percentage of the battery's potential that has been discharged relative to the overall capacity of the battery. If the battery’s full capacity is 15kWh and you discharge 12kWh, the depth of discharge is 96%.
When the alkaline batteries are deep discharged, they are prone to leaking. The reasons are discharged by load or self-discharge. Self-discharging is a phenomenon in which a battery's stored electric potential is reduced due to internal chemical reactions reducing the shelf life. Thus, deep discharging is something to avoid, as it can harm the load and battery itself.
But some batteries are designed to deeply discharge regularly and these batteries are known as deep cycle batteries. These batteries regularly deep discharge using most of their capacity. For a deep cycle lead-acid battery, the depth of discharge is 50%. These types of batteries are used in UPS, traffic signals, remote applications, and off-grid power storage applications.
Deep Discharge Protection Circuits
For deep discharge protection, we need to identify the cut-off voltage of the battery. After that, we need to design a circuit in which, when the battery reaches the cut-off voltage level, a switch disconnects the load from the battery. For cut-off voltage identification, we will choose a Zener diode. A Zener diode in reverse bias condition will act as an open circuit when its cathode is applied with voltage below breakdown voltage. When the cathode voltage is above breakdown voltage, it starts conduction.
We will use a Zener diode with Zener voltage equal to the cut-off voltage of the battery. When the battery voltage drops below the cut-off voltage, the Zener diode will cut off the conduction and obstruct the flow of the base current of NPN transistor, thus disconnecting the battery from the load.
For selecting the value of Zener diode series resistor use this equation:
- Va=Source Voltage
- Vb=Zener Voltage
- Where Iz=PzVz
- Pz=Max.power dissipation of Zener diode
- Vz=Zener voltage
Protection Circuit for 12V Batteries
12V batteries are widely used by makers in DIY projects, solar power supplies, etc. To protect the load and battery from over-discharging, we will build a protection circuit for 12V batteries.
We will use a similar circuit, but change the resistance and make it variable for the desired range of operation. When the voltage is below 8V here, there will be no conduction through the diode and no base voltage effectively disconnecting the battery from the load.
ICs Used for Over-Discharge Protection
There are two ICs available that are well suited for this application. ICs make our work easier when we are dealing with a complex circuit and have to regulate the power for load. Instead of many different circuits, we can use a single IC. A single IC can monitor as well as control the power flow.
The LTC2960 IC is a high voltage, two input voltage monitor suited for multi-cell battery applications. The IC supervises the input voltage continuously and when the voltage is low, the RST pin goes high, indicating low battery voltage.
Users can provide non-inverting as well as inverting input to the IC. This IC is widely used for power monitoring, controlling, and protection in portable battery-powered equipment and other systems.
Another widely used IC for our application is the LT1495. It is a low power op-amp which can work in extremely low supply currents. It is generally used in 3V, 5V, and +- 15V supplies. It also provides reverse battery protection up to 18V. It can regulate a wide voltage range of 2.2V to 36V. The main advantage of using the LT1495 is that another protection circuit requires high current, while a circuit made by this IC takes a current of less than 4.5uA.
Note: The above circuit will cut-off the load at 3V.
How to Charge an Over-Discharged Battery
When the battery is over-discharged, the internal resistance increases. It discharges 1.5 to 2 times the capacity of the battery making it difficult to recharge them. First of all, you should check the present voltage level of the battery, which is lower than the cut-off voltage.
Let's take an example of a widely used 4.2V Li-Po battery when the voltage reached below 3.7V there is damage in the internal resistance of the battery. The damage becomes significant below 3.0V. Charging these batteries take tons of time.
For charging, a smart charger is a mandatory requirement. Initially, for 0-3V of the battery, charge at 1/20 C charge rate. Do not leave the battery unattended and also touch it frequently and check if it's not getting hot. If so, lower the charge rate. When the battery is charged at 3.0V, increase the charge rate to 1/10 C rate and after 3.7V, you can charge it at higher 1/2C rate until 4.2V.
The procedure is almost the same for most of the batteries. One needs to charge very slowly and then gradually increase the charge rate. The battery can then be used for applications. However after restoring, also notice if the battery puffs or heats much, if so the battery can’t be restored anymore.