The compact LiPo battery charger introduced here can be used to charge almost all standard 2S LiPo battery packs. The circuit, designed around generally available low-cost components, is very simple, safe, and economical. The typical target voltage of a 2S LiPo battery is usually 8.4 V (note that target voltage is not the same as the nominal voltage, which is 7.4 V typical), and LiPo batteries require a special type of charging mode that uses the CC/CV method (constant current/constant voltage) with 1-C charge grade (1 A for a 1,000-mAh battery). However, charging at a lower-than-1-C rate is perfectly safe and will not damage any battery type. The design is optimized for batteries of 1,000 mAh or higher, and the input power source can be any linear/switch-mode power supply capable of catering an output current of minimum 1,500 mA at 18 V. Normally, a LiPo battery connected to this circuit will be charged to 95% of its nominal voltage within 60 minutes and charged up to 100% of its target voltage within two hours thereafter.
The electronics is, in fact, a perfect blend of one constant current source and one constant voltage source built around the popular adjustable three-pin voltage regulator LM317T (IC1 & IC2). Here, IC1 and R1 set the output current limit while IC2, with R2 and P1, sets the regulated voltage output. Related capacitors (C1–C2) are used to increase the circuit stability by reducing unwanted noise. The rest of the electronics is a bunch of visual indicators and their supporting components. LED1 (amber) is the “power-/battery-connected” indicator; LED2 (blue) is the “current flow” indicator; and LED3 (red) is the optional “battery-charged” indicator. The entire circuit can be constructed on a small perf board (see my nearly finished prototype). Both ICs must have heat sinks, and the TO-220 heatsinks should be isolated from other components of the circuit.
After construction, feed 18 V to the circuit through DC_IN jack (J1) and adjust P1 to get precisely 8.4 V (±0.02 V) at the VBAT rail. If you added the optional “battery-charged” indicator circuitry, one more adjustment is needed. Remove power from J1 and feed exactly 8.4 V (from your lab power supply) to the circuit through J2. Next, adjust P2 so that LED3 just lights up at 8.4 V. Now the circuit is ready for charging your 2S LiPo battery (prototype was tested with a 2,200-mAh 2S LiPo). As usual, I would love to hear how it works for you — let me know 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.