18650 Li-Ion Charger

Recently I’ve been interested in finding another medium complexity project to spend some evenings on, something in the complexity range of a decent ~100W bench power supply. However, I still haven’t been able to settle on an idea yet but have been itching to make some more PCBs. I have one on the way for a project that I’m working on with a friend, but that will be a whole other post/write-up in itself.

I’ve always been interested in energy generation and storage and have been toying with the idea of getting some 18650 lithium-ion batteries to play around with. Unfortunately/fortunately, my old laptop of about 5 years has finally refused to charge, so I was able to tear the pack apart and get at the cells. I managed to harvest nine 2.6 Ah cells, three of which read 3.7 V and six of which read around 2.2-2.5 V. Also, a compact charging/control board based on a bq8030 (no datasheet available, but probably a charge controller IC) and bq29330 2-4 series cell protection IC.

18650 Li-Ion Cell

18650 Li-Ion Cell

"Front" of controller board

“Front” of controller board

"Back" of controller board

“Back” of controller board

Giving all the cells a brief constant-current charge, I was able to bring them all to 3.8 V without much issue. This gives me hope, but charging them the way I did (using a 5.0 V USB charger and two multimeters, one for cell voltage and one for current) is inherently dangerous if I can’t be around monitoring them the whole time. If the cell voltage creeps much above ~4.2 V, li-ion batteries like to spew flame and noxious gasses. So, this gave me a great chance to design and send off a quick PCB that should prove handy and enhance the safety of charging li-ions. I designed a quick board around the MCP73833, a simple charge management IC for single li-ion cells with some status pins for driving LEDs. I think the total time between selecting the IC to sending off the Gerbers was something like 3 hours.. so not a very challenging board but a certainly a nice way to spend an evening.

3D rendering of the 18650 charger

3D rendering of the 18650 charger

Schematic here: Li-Ion 18650 Charger Schematics, Rev A

The MCP73833 handles all of the charging, including the following the proper CC then CV profile ending when the charge current diminishes to a ~C/10 rate. I added some LEDs for status indication and a slider switch to allow charging from a standard USB port or a USB wall charger (1000+ mA). The board was sent to fab (through Oshpark again) yesteday, so I expect to receive it in about 3 weeks time. I’ll probably make a quick page in the projects section when I receive/build it.

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