In today’s world, lithium batteries power a wide range of portable devices, from smartphones and laptops to various electronics. To maintain their lifespan and avoid safety hazards such as overheating or explosions, it’s critical to follow industry-standard charging protocols. Typically, lithium-ion or lithium-polymer batteries are charged at a rate of 0.5C to 1C. In this article/video, I present a versatile dual lithium battery charger, where the charging current (C rate) can be easily adjusted by changing a single resistor. All you need is a 5V power source, such as a mobile charger, and a USB Type-C cable.
For designing the schematic and PCB layout, I used Altium Designer 22 along with the SamacSys component libraries (via the Altium Plugin). To ensure high-quality PCB fabrication, I sent the Gerber files to Wellcircuits and sourced original components from Componentsearchengine.com. For precise current/voltage measurement during testing, I used the Siglent SDM3045X multimeter. Let’s dive in and explore the details!
• Download the Gerbers or order 10 high-quality boards for just $5
• To order a fully assembled PCB (including free shipping), please contact: anson@wellcircuits.com
Circuit Analysis
Figure 1 presents the schematic of the charger. At the core of this design are the MCP73831 ICs from Microchip [1].
Figure 1: Schematic diagram of the Lithium-Ion/Lithium-Polymer USB Type-C Charger
The MCP73831 series is a highly advanced linear charge management controller, optimized for space-constrained, cost-effective applications. Available in both DFN (2mm x 3mm) and SOT-23 packages, these devices are ideal for portable solutions. The MCP73831/2 follows the USB power bus specifications and supports constant-current/constant-voltage charging with selectable preconditioning and termination. The voltage regulation options are fixed at 4.20V, 4.35V, 4.40V, or 4.50V to cater to various battery types, while the charging current is adjustable with a single external resistor. Additionally, thermal regulation ensures that charging performance remains stable even under high power or ambient conditions.
The MCP73831 offers a flexible current range from 15mA to 500mA, which suits most applications. The key difference between the MCP73831 and its sibling, the MCP73832, is the inclusion of a charge status pin. For this project, I chose the MCP73831. Figure 2 below illustrates the complete charging cycle for a 100mA battery, as referenced from the datasheet.
Figure 2: Charging cycle of a 100mA battery (source: MCP73831 datasheet)
In the schematic, capacitors C1, C2, C3, and C4 help to filter supply noise. D1 is an LED indicator for verifying proper Type-C cable connection, with R1 limiting current to the LED. The CON1 is a Type-C USB connector, while resistors R2 and R4 set the charging current for the two battery connectors (H1-P1 and H2-P2). Diodes D2 and D3 show the charging status, with the ON LED indicating a completed charge. The charging current can be calculated using Equation 1. For maximum current (500mA), a 2K resistor is required.
Equation 1: Charging current calculation formula for IC1 and IC2
PCB Layout
Figure 3 shows the PCB layout, which uses a two-layer design. After preparing the Gerber files, I ordered 10 high-quality PCBs from Wellcircuits. The design employs mostly SMD components, with the exception of the battery holders/connectors.
Figure 3: PCB layout of the Lithium-Ion/Lithium-Polymer USB Type-C Charger
During the design process, I realized I lacked the component libraries for IC1 and IC2 [2]. To resolve this, I used the IPC-rated SamacSys component libraries, which I imported into my design using the free SamacSys tools and services. There are two ways to integrate these libraries: either by visiting componentsearchengine.com or by using the SamacSys CAD plugin for direct integration into your design environment. Figure 4 shows the full list of supported CAD software, and since I use Altium Designer, I installed the libraries via the Altium plugin (Figure 5). A 3D view and assembly drawings of the PCB are shown in Figure 6.
Figure 4: Supported CAD software for SamacSys plugins
Figure 5: Using the SamacSys Altium plugin to import component libraries
Figure 6: 3D view and assembly drawings of the PCB
Assembly and Testing
Figure 7 displays the assembled PCB board. The USB Type-C connector is the most challenging component to solder, requiring a hot air station and solder paste. However, the rest of the components are relatively easy to solder. If you’re a beginner or prefer not to assemble the PCB yourself, fully assembled boards are available for purchase.
Figure 7: Assembled PCB board of the Lithium-Ion/Lithium-Polymer USB Type-C Charger
I configured the charging current for one channel to 450mA (using a 2.2K resistor) and the second channel to 100mA (using a 10K resistor). The first channel charges a lithium-ion battery, while the second channel is used for a small 200mA lithium polymer battery. Figure 8 shows the voltage measurements during the charging cycle using the Siglent SDM3045X multimeter as a reference [5].
