In a previous post we had a discussion regarding how a cell phone charger works . I hope that was informative enough. But a charger alone cannot charge the LI ion batteries in your phones. They additional circuitry which is embedded inside your phone. Li ion batteries are not safe to play with and thus it also requires a special protection circuit. So today we will have a brief discussion on charging and protection of Lithium cells . So keep reading .
Constant Voltage Constant Current Charging :
A lithium cell is charged using the principle of constant current and constant voltage.So lets see what this principle exactly is.The graph above illustrates the principle.
As shown in the graph , in the initial phase of charging , the current is kept constant and the voltage increases linearly with time. This phase is known as the constant current phase.This continues until the voltage reaches the maximum battery voltage. Once the voltage reaches the peak it remains constant and the current starts decreasing exponentially till cut off. This phase is known as the constant voltage phase.
The constant charging current can be set as per our requirements by making adjustments in the circuit. If you are using an Integrated Circuit for charging then you can program the charging current by varying a specific resistor value. TP4056 is an example of charging IC for a single lithium cell.
Battery protection :
Here is one of the protection circuits that is used in order to protects lithium cells from damage by preventing battery operation at under voltage ,over voltage and over current conditions.
Lithium cells need to be handled with care or it can serious problems and thus it is necessary to protect these cells. But how does this circuit protect the batteries.
Let’s find out .
A battery may get damaged if
- its voltage goes below a threshold voltage
- Goes above a thresh voltage
- Or it exceeds the current limit
The battery protection circuit is used to prevent the above scenarios.So let’s say the voltage has dropped below the threshold. In that case the IC detects it and pulls the DOUT pin low and thus inhibiting the discharge operation.This is because , the battery will be grounded only if both the transistors are on.But as one of these transistors is turned off the battery doesn’t get grounded and discharging stops. Similarly ff over voltage condition occurs the COUT pin is pulled low and stops the charging. In case of over current ,the IC checks the voltage across the V- pin and if that exceeds the permissible value then the discharging is inhibited.
Dealing with a single lithium cell is not a big pain. But when you have multiple cells then the complexity increases. Also there is a concept called Cell balancing when it comes to multiple cells. All those will be covered in detail later on.