Simple Safe Lithium Cell Charger

In this electronics project, I make a simple but safe charger circuit for Lithium cells with a well-known integrated circuit LM317. This charger circuit very simple to make and cost-effective. It has an adjustable output feature (around 1.2 to 5 V), so the circuit can be charged any single lithium cell battery like Li-Ion and Li-Pol.

Lithium cell charger

Charging characteristic

Charging characteristic graph of Lithium cells
The charging characteristic graph of lithium cells (li-ion/li-pol).

Charging takes place first in the current mode then voltage mode. When the voltage rising, the current becomes constant. After reaching the target voltage (Vmax), the charger goes into voltage mode, now the voltage is constant and the current asymptotically approaches zero. At this moment the current is small, the cell is charged.

The target voltage of Li-ion and Li-Pol accumulator is usually 4.2 V but the target voltage is not the same as the nominal voltage, it is generally 3.7 V.

The lithium cell does not need to be charged the full 4.2 V, because it decreases its lifetime. If you reduce the target voltage to 4.1 V, capacity drops by 10% but lifetime (the number of cycles) will increase almost twice.

When using the lithium cells in gadgets or any circuit, they should never discharge below 3.4 to 3.3 V. Cells Li-Ion and Li-Pol cell do not like storing in a charged or discharged state, they should be stored partially charged.

Simple safe Lithium cell charger Circuit diagram

Schematic of simple safe lithium cell charger circuit shown below.

Circuit explanation

In this charger circuit, I used an LM317 ic which serves as the voltage stabilizer. Lithium-Ion and Lithium-Polymer batteries are quite demanding on the accuracy of charging voltage. If you want to charge the cell to full voltage (usually 4.2 V), it is necessary to adjust this voltage to with +/- 1% accuracy. By the way, lithium cells are very sensitive to overcharging. If you charge the cell at 90% capacity (4.1 V), it is sufficient a little less accuracy (around 3%).

LM317 circuit provides a relatively accurate voltage stabilization which adjusts the target voltage by trimmer R4. The adjustment of the output voltage of the charging circuit without connecting the cell, because the target voltage corresponds to the output voltage with no load.

Stabilization of output current is not as critical as the stabilizing output voltage, so it is sufficient to stabilize it by a shunt resistor R2 and NPN transistor Q1 (I used BC547).

If the drop of shunt resistor R2 reaches approximately 0.95 V (the total voltage drop of the NPN transistor's B-E and diode 1N4007) transistor starts to open.

This reduces the voltage on the pin adj of the LM317 ic and so it stabilizes the current and current is depending on the value of resistor R2.

Select the resistor R2 according to the type of charged cell. For example, charging the current 200 mA, I used the value of the R2 resistor is 4R7. The value of R2 is calculated: R2 = 0.95 / Imax. For safety reasons, it is good to connect appropriately dimensioned fuse in series with the cell.

The supply voltage of this charging circuit should be in the range of about 9 - 24V. If the input voltage is too high, increases the power loss of circuit LM317 and too low would not allow the proper operation (it is necessary to count the voltage drop on a shunt and minimal voltage drop for the integrated circuit).

LM317 circuit should be placed on a sufficiently large heat sink. The charger is resistant to short-circuit at the output and LM317 in the worst case (short circuit) dissipates a power loss: P = Vin x Imax. The maximum allowable power loss of LM317 ic in the TO220 version is 20W.

After soldering

See the following images of this charger circuit after soldering in a PCB.

Lithium cell charger circuit
Lithium cell charger circuit
Lithium cell charger circuit
Lithium cell charger circuit

Lithium-Ion and Lithium-Polymer cells may explode if a shorted, overcharged, charged or discharged with too high currents, exposed to high temperature, mechanically damaged or otherwise improperly treated. Everything you do at your own risk, for any injury to health or property I do not take responsibility!

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