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LIR2032 Coin Cell Charger

A small, cheap ($2 BOM), open source charger for LIR2032 and 20mm rechargeable coin cells built from an MCP73831.

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This is a very small and affordable lithium coin cell charger. It'll charge several 20mm type coin cell batteries. Total BOM for me was $2 for one. I picked up 3 boards from oshpark for $7.70. The PCB is available to order from the oshpark links below. All files, including BOM to reproduce this project have been included and linked to in my github repository.

I had the need for an LIR2032 coin cell charger to power a few projects. Unfortunately (or fortunately depending on how you look at it) , the charger I had ordered off Amazon didn't work, so I decided to build my own.

I took the guts of the Solder Sniffer 9000 project, which consisted of an MCP73831 for the charge controller, and modified it a bit to charge an LIR2032.

The main change needed was the value of R3, which is responsible for setting the max charge current. For the LIR2032, the max charge current is 35mA. By using the supplied formula in the MCP73831 datasheet (I = 1000V/R) I was able to find that a 28K resistor would give me the charge current needed. To get a decent connection to the USB socket, I tinned each of the pads a bit.

The value of R3 can be adjusted from anywhere between 28K and 67K. Of course, the higher the resistor, the less charge current and thus the longer the battery will take to charge. 

I wanted the charger to be as compact and simple as possible. For this reason I chose to create the USB plug out of the PCB to save on cost. I also shrank the total footprint of the board to just a little wider than the coin cell holder. 

mcp73831.pdf

datasheet for MCP73831

Adobe Portable Document Format - 823.05 kB - 01/09/2018 at 16:11

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ms-excel - 917.00 bytes - 01/08/2018 at 16:28

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Adobe Portable Document Format - 20.36 kB - 01/06/2018 at 19:40

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  • 1 × MCP73831 Power Management ICs / Power Supply Support
  • 3 × 470 Ohm resistor (0805 package)
  • 1 × 10uF capacitor (1206 package)
  • 1 × 4.7uF capacitor (0805 package)
  • 2 × green LED's (0805 package)

View all 8 components

View project log

  • 1
    Tin the ground solder pad under the battery holder BT1.

    Tin the ground solder pad under the battery holder BT1. This step is required in order to ensure a good connection when the battery is inserted. All that should be required here is an even distribution of a thin layer of solder on the square ground pad under BT1.

  • 2
    Tin the 4 USB plug pads.

    By tinning the 4 USB pads, we ensure a solid connection to the USB socket. Although only the 2 outer pads are actually used by the PCB to provide voltage, it is a good idea to tin the middle 2 pads as well. 

  • 3
    Solder power status LED indicator.

    We first want to solder on the PWR status indicator LED (LED3), as well as the 470 ohm series resistor (R4) in order to test that our connection to the USB port is solid. Once these two components are soldered, test them by inserting the USB connector into an available USB port on a computer or hub and check that the power indicator LED (LED3) is lit. 

    If LED3 lights, move onto the next step. If it does not light, recheck that the USB plug is properly tinned as well as the solder joints of R4 and LED3.

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