Sunday, May 17, 2009

USB Coin/Button Cell Battery Charger

I have designed many small footprint PIC projects (such as, pocket watches and wristwatches) but I cannot make them really portable. To make them portable, I need small power sources. Of course, Coin Cell battery would be the smallest DC source that I can buy. The problem is that a Lithium button cell provides 3 V. which is not enough to drive my projects. I thought about using DC-DC step-up converter to boost 3 V. to 5 V. However, it's a little bit complex to add DC-DC converter to the projects. Moreover, my projects consume a lot of power as they consist of many LEDs, a button battery will not last for a day. So, I stopped my think at that point.

Just recently, I have found a rechargeable coin cell battery at It provides 3.7 V. at 200mAh. I don't know that my projects will work at 3.7 V. or not. But, I want to give it a try. For portability, I want to charge the battery from my computer's USB port. So, I designed a USB coin cell battery + charger breakout board. Like many simple battery charger, I use MAX 1555 as the controller of the charger.

The schematic:

The single sided PCB (40mm x 30mm):

The LED goes off when the battery is fully charged (but the charger is still charging).
Please note that the schematic/PCB are just the design and I haven't made it yet. The parts are ordered and I will update when I complete the hardware.


RicardoATB said...

I didn't know this MAX1555. Great info!

P.S.: Actually, I'm building a battery charger (AAA or AA) using LM317 as the current limiter.


Anonymous said...

hello... hapi blogging... have a nice day! just visiting here....

spcmicro said...

Did you ever get this working?

Punkky said...

I just got the parts ready. The experiment will be conducted very soon. Please check back.

dutchflyboy said...

Nice project! Did you get this working?

I'm also trying to get a charger to work, but I don't know enough about electronics. Do you know any way to adapt this to charge other types of batteries/other voltages?

Eschel said...

Hello how is it going with the project I am making my own little project with a light using 2x 2016 3.6 v coin batteries, and would love a charger my email is


Eschel Jacobsen

D. Gallot said...

For really small load, you can use the stepup converter from XP Power.

Does not required any external components !