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Single Cell Charger Using LM317 Voltage Regulator

In this electronics project, I will demonstrate how to make a Single Cell Charger utilizing the popular integrated circuit LM317.

This charger offers an adjustable output ranging from 1.2V to 5V and includes a pre-set constant current feature, ensuring the safe charging of various cell types such as Li-Ion, Li-Pol, Ni-Mh, and Ni-Cd.

Let's make it!

Components for Single Cell Charger

The following components are required to make this charger circuit:

Name Value Qty.
U1: Adjustabe Voltage Regulator LM317 IC 1
Q1: NPN Transistor BC547 /2N2222 /CD945 1
R1: Resistor 1kΩ, 1/4W 1
R2: Resistor 330Ω, 1/4W 1
R3: Resistor RX 1
POT1: Trimmer Potentiometer 1kΩ 1
D1: LED 3MM 1
D2: Diode 1N4007 1
J1, J2: Connector 2-Pin Screw Terminal Block 2
Aluminium Heat Sink 20MM 1
Veroboard 50MM x 10.5MM (W x L) 1
Connecting Wire - -
Soldering Iron Kit - -

To select the appropriate resistor R3 for the specific type of cell being charged, an estimation can be made based on the desired charging current. For example, if a charging current of 200 mA is desired, the value of the resistor R3 can be chosen as 4.7 ohms.

The calculation for determining R3 is as follows:
RX = 0.95 / IMAX
[Where RX represents the current limiting resistor, 0.95V represents the total voltage drop across the transistor Q1 (base-emitter) and the diode D2, and IMAX represents the maximum charging current.]

And, the minimum power (P) rating of resistor R3 can be calculated using the formula: P = VOUT x (IMAX)2. [Where VOUT represents the output voltage.]

For safety reasons, it is recommended to include a fuse that is appropriately sized and connected in series with the cell.

Single Cell Charger Circuit Diagram

Schematic of the single cell charger circuit using lm317 voltage regulator is shown below.

After Soldering Single Cell Charger Circuit

I arranged the components and soldered the single cell charger circuit onto a Veroboard following the schematic. Here, you can see the complete circuit board after the soldering.

Testing & Demo: Single Cell Charger

The charging process occurs in two stages - first in the current mode and then in the voltage mode.

Charging characteristic (Voltage vs Current) graph of single cell charger
Initially, the charger operates in the current mode where the current remains constant while the voltage gradually increases. Once the target voltage is reached, the charger transitions into voltage mode, where the voltage is held constant and the current gradually decreases towards zero. During this stage, the current is minimal, indicating that the cell is fully charged.

You can charge various cells or batteries using this charger with a voltage range of 1.2V to 5V. Here, I am performing the testing of the single-cell charger with 18650 - 3.7V 2200mAh, 10180 - 3.7V 100mAh, and Ni-Cd AA - 1.2V 600mAh cells.

To test the circuit, I provided a 12V DC power source and adjusted the regulated output to the safe charging voltage level of the cells.

The typical voltage of an 18650 Li-Ion cell is 3.7V, and its full charged voltage is 4.2V. I selected a resistor with a value of 4.7 ohms for 200mA charging current, and then adjusted the potentiometer to reach a voltage of 4.2V. The charging process is completed within 6 hours (approx).

Similarly, the voltage of a 10180 Li-Ion cell is 4.2V, but the safe charging current is 9.5mA. Therefore, I replaced the resistor R3 with a value of 100 ohms. The charging process is completed in 5hr 22min.

An Ni-Cd cell has a voltage of 1.2V and a full charged voltage of 1.55V. I replaced the resistor R3 with a value of 10 ohms for 95mA charging current. The charging process is completed in 4hr 53min.

Why is charging process slow? - In general, a slow charging process is considered good for cell or battery health and charging efficiency. This can be advantageous for preserving the cell's capacity and performance over time. Increasing the charging current (up to 1.5A) can speed up the charging process, and it is recommended to charge the batttery/cell at 20-35% rate of its capacity with an overcharging protection circuit for faster and safe charging.

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