Constant Voltage (CV) charging refers to the process in which the charger maintains a steady voltage output, allowing the battery to draw the maximum current until it reaches the preset voltage level.
CV charging is commonly used in laboratory settings for charging batteries. However, in the OEM equipment market, constant current (CC) chargers with sophisticated charging algorithms or smart charging circuits are also employed.
To understand CV charging, let’s delve into basic electric circuit theory, which states that the current in a circuit is directly proportional to the voltage potential or voltage difference between two points (Ohm’s Law).
Ohm’s Law describes the relationship between voltage difference (V), current (I), and resistance (R), and is expressed as V = IR. Here, V represents the voltage difference, I represents the current in amperes, and R represents the resistance in ohms.
By applying Ohm’s Law, one can determine the total resistance (RT) of a circuit by dividing the voltage (V) by the current (I): RT = V / I. For instance, in a parallel circuit with a voltage of 12 volts and a total current of 3 amps, the total resistance would be 4 ohms (RT = 12 volts / 3 amps = 4Ω). In theory, during a CV charge, there is no limit on the charging current, although many manufacturers provide guidelines and publish limits.
During the CV charging process, as the charging continues at a constant voltage, the charging current gradually decreases. This decrease occurs because the potential difference between the charger’s output voltage and the battery terminal voltage decreases as the battery charges. In other words, the charging current is highest at the beginning of the charge cycle and gradually reduces towards the end. In a CV charge circuit, the battery acts as the current-regulating device. It draws only the amount of current required to reach full charge. Even when the battery theoretically reaches a 100% state of charge, it continues to draw small currents to compensate for standing/parasitic losses.
Let’s consider an example to illustrate this concept further. Suppose the battery being charged has an internal resistance of 8mΩ (0.008Ω) when fully discharged to an end voltage of 1.75 volts per cell (10.5V for a 12V battery) under load. When fully charged, the battery’s internal resistance is 4mΩ (0.004Ω). Additionally, when the load is removed from the battery during discharge, the voltage jumps back to a higher value of approximately 2 volts per cell (12V for a 12V battery). This jump is known as the back electromotive force (EMF) or counter electromotive force (CEMF). When the charging process begins, the charger measures this voltage at the battery output terminals. This voltage influences the initial charge current surge and the final charging level. Considering these factors, if we decide to charge the battery using a constant voltage of 2.4 volts per cell (14.4V for a battery), and assuming the battery’s internal resistance when fully charged is 4mΩ (0.004Ω), we can estimate the finishing current as the battery approaches a 100% state of charge.
In reality, the charging process is dynamic. As soon as charging begins, the terminal voltage of a discharged battery starts rising to match the charger’s output voltage. Over time, one would expect the battery voltage to eventually equal the charger’s output voltage as the voltage difference in the charging circuit approaches zero. This would result in the charge current dropping to zero. However, due to the internal electrochemistry of the battery, it continues to draw small charging currents even when fully charged. The previous example demonstrates how the battery acts as a current regulator in a constant voltage charging regime, gradually reducing the current flow in the circuit to match its own state of charge. Hence, even if the current limit on the charger is set at 350 amperes, the battery would experience an initial inrush current of 300 amperes before it tapers off and finally drops to 50A towards the end of the charge.
Dear friends, this concludes our exploration of constant voltage charging, unveiling its mysteries. Armed with this newfound knowledge, you can make informed decisions when selecting the right charger for your batteries. Embrace the power of constant voltage charging and embark on thrilling adventures with your lithium batteries!
Stay charged, stay curious, and conquer the world of constant voltage charging like true voltage warriors!
