LIR2450 USB Charger

Description

UPDATE: Charging works, LEDs do not.

I started doing a lot with low power devices (Arduino, BLE 4.0) and coin cells are a convenient power source. Lately I found out that there are even rechargeable coin cells. That's why I needed a charger. There are only a few models out there which were either expensive or I didn't trust the charge current (too high). So I made my own.

It is based on the MCP73831. Order three PCBs for 13.60$ at OSHPARK: https://oshpark.com/projects/roORU8h6

There are two charge modes:
- fast (45mA)
- slow (25mA)

Max charge current for a coin cell is usually between 80% and two third of it's capacity. So you could charge a 100mA coin cell with 80mA. I wanted to increase the lifetime of my coin cells and reduced the charge current. If you want to have different currents here is the equation to calculate R_SLOW/R_FAST:

mA = 1000 / R in kΩ

Example: 1000/20kΩ

Components

1 × MCP73831 SOT23-5 Power Management ICs / Power Supply Support
1 × 0603 LED Green (3.1V voltage drop)
1 × 0603 LED Orange (2V voltage drop)
1 × 0805 Resistor 200Ω
1 × 0805 Resistor 320Ω

View all 11 components

Discussions

Chris wrote 09/10/2015 at 12:25

Forgot to update it here: The circuit is charging the coin cell but the led indicator does not work. I am not sure if it's because I got a cheap chip from Aliexpress or what else ... The circuit is almost the exact copy of the reference implementation from the data sheet. The charging LED is on all the time and does not turn off. However the chip is charging correctly. Once the cell is full the amperage going to the cell is almost zero.

Are you sure? yes | no

davedarko wrote 09/10/2015 at 13:35

thanks for the quick reply! I'll see what my chinese clones will do :)

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Alex wrote 09/10/2015 at 13:59

the ICs (probably also china-clones) I did get do work fine (with normal lipo- batteries). And also the charge LED is going off if ready

Are you sure? yes | no

davedarko wrote 09/10/2015 at 14:05

the datasheet said LOW when charging and HIGH when done, hmm...

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Alex wrote 09/10/2015 at 15:08

When you use an led between VDD and STAT, the led is on when charging.

Are you sure? yes | no

davedarko wrote 09/10/2015 at 15:15

Aahh, I thought your chip was putting it out differently from what you expected... makes sense now.

Are you sure? yes | no

davedarko wrote 09/10/2015 at 12:11

so, does it work? because I've copied your design (checked back with the datasheet as well) for a LIR2032 and was wondering about any updates.

Are you sure? yes | no

jaromir.sukuba wrote 09/10/2015 at 13:51

There is nothing in this circuit that shouldn't work. It is basic circuit from MCP73831 datasheet, as per manufacturers recommendation; I used it in a few projects before, some of them are here on HaD.io

Are you sure? yes | no

davedarko wrote 09/10/2015 at 14:04

That was actually me trying to bring someone to update his project ;) I started with the datasheet as well but got the idea here. Later I compared the resistor values with his and took the formula from above to get values for a standard 2032 version.

Are you sure? yes | no

LIR2450 USB Charger

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