The presented compact LiPo battery charger is versatile, suitable for charging nearly all standard 2S LiPo battery packs. This circuit, engineered using readily available, cost-effective components, offers a straightforward, secure, and budget-friendly solution. Typically, a 2S LiPo battery has a target voltage of 8.4 V (it’s important to note that the target voltage differs from the nominal voltage, which is typically 7.4 V). LiPo batteries necessitate a specialized charging mode known as the CC/CV method (constant current/constant voltage), with a 1-C charge rate (equivalent to 1 A for a 1,000-mAh battery).
However, it’s worth mentioning that charging at a rate lower than 1-C is entirely safe and won’t harm any type of battery. This design is optimized for batteries with a capacity of 1,000 mAh or higher, and the input power source can be any linear or switch-mode power supply capable of delivering a minimum output current of 1,500 mA at 18 V.
Typically, when a LiPo battery is connected to this circuit, it will reach 95% of its nominal voltage within 60 minutes and attain a full charge to 100% of its target voltage within the subsequent two hours.
The electronics consist of a well-balanced combination of a constant current source and a constant voltage source, skillfully constructed around the widely used adjustable three-pin voltage regulator LM317T (IC1 & IC2). IC1 and R1 collaborate to establish the output current limit, while IC2, in conjunction with R2 and P1, configures the regulated voltage output. Capacitors (C1–C2) are thoughtfully incorporated to enhance circuit stability by reducing undesirable noise.
The remaining electronic components comprise various visual indicators and their accompanying elements. Notably, LED1 (amber) functions as the indicator for “power/battery connection,” LED2 (blue) serves as the “current flow” indicator, and LED3 (red) optionally indicates when the battery is fully charged. This entire circuit can be conveniently assembled on a compact perf board, as illustrated in my nearly completed prototype. It’s essential to attach heat sinks to both ICs, ensuring they are thermally isolated from other circuit components, particularly the TO-220 heatsinks.
Upon completing the construction, supply 18 V to the circuit via the DC_IN jack (J1) and fine-tune P1 until you achieve an accurate voltage of 8.4 V (with a tolerance of ±0.02 V) at the VBAT rail. If you have included the optional “battery-charged” indicator circuitry, an additional adjustment is necessary. First, disconnect power from J1, and then apply precisely 8.4 V (using your laboratory power supply) to the circuit through J2. Proceed to adjust P2 so that LED3 illuminates just as the voltage reaches 8.4 V. Now, the circuit is prepared to charge your 2S LiPo battery (the prototype was tested with a 2,200-mAh 2S LiPo). As always, I’d greatly appreciate your feedback on its performance—please share your experiences in the comments!
LiPo batteries may explode if shorted, overcharged, exposed to high temperature, or otherwise improperly treated.
Note: Although a balancer is required to maximize energy capability and lifetime of the LiPo battery, most chemistries have shown very little drift and are okay to charge without balancing for several cycles. I will try to publish an add-on LiPo balancer for my LiPo battery charger in the near future.