How to build Lithium Polymer Battery Charger

Here in this tutorial we will build a automatic overcharge cut-off Lithium Polymer Battery Charger or LiPo battery charger. This LiPo battery charger could be useful to those who uses lithium polymer battery for robotics, motors, quad copter etc. And the components for this diy lithium polymer charger are readily available. If you in electronics circuit design work then maybe you already have the parts. The battery charger circuit uses adjustable voltage regular IC LM317, any standard op-amp like TL072, LM548, LM324 etc, a standard BJT transistor like 2N3904, BC547, 2N222 etc.

Lithium Polymer Battery Charger Circuit and Operation

Circuit Diagram

The following is the automatic overcharge cut-off Lithium Polymer Battery Charger Circuit Diagram.

Lithium Polymer Battery Charger Circuit Diagram
In the above circuit LM317 IC is used. The LM317, a versatile voltage regulator chip widely applied in electronic setups, comes equipped with safety measures. It restricts its outputs to a maximum of 1.5 amps, ensuring the battery operates within a safe amp range. When incorporated into a LiPo battery charging system, the LM317 is employed to set the exact charging voltage, a task easily accomplished by tweaking the 10k potentiometer or preset.

Circuit Operation

The above circuit implements automatic overcharge cut-off for safety. The overcharge cut-off circuit is an essential safety feature for charging LiPo batteries. Its primary role is to prevent overcharging, a situation that can lead to battery instability and potential explosions. This circuit relies on an op-amp and a 10k preset to establish the overcharge threshold. Typically, for a 3.7V LiPo battery (a single cell), this threshold is set at 4.2V. To accommodate this, the LM317 output needs adjustment to around 4.8V, factoring in the 4.2V battery level plus the 0.6V allowance from the diode placed at the positive end of the battery. The resistor values for specific output voltage of the LM317 voltage regulator can readily determined by online LM317 Voltage Regulator Calculator.

When the LiPo battery connects to the charging circuit, it draws power from the LM317 to match its current voltage level, typically around 3.6V when the battery is drained. This action maintains pin3 of the op-amp below the fixed reference voltage at pin2 of the IC, resulting in a low signal at pin6 or the IC's output. As the battery charges, its voltage gradually increases until it hits the overcharge threshold set by the 10k preset. At this stage, the op-amp output switches high, activating the indicator LED and triggering the 2N222 transistor connected to the ADJ pin of the LM317. This grounds the ADJ pin, halting the supply to the LiPo battery and preventing overcharging.


Ultimately, our understanding grows: charging a lithium polymer battery might seem intimidating, yet armed with the appropriate tools and know-how, you can accomplish it securely and efficiently. The circuit outlined for a lithium polymer battery charger is straightforward to assemble and includes an overcharge safeguard, shielding the battery from harm caused by excessive charging. When the occasion arises to recharge your lithium polymer battery or LM317 lithium ion battery charger, consider employing this circuit for a smoother experience!

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