I’m making a portable Bluetooth speaker and need some help with how to charge the battery pack.
I have made a 3 cell Li-ion pack with a BMS to get a 12V battery for the speaker. My thought was to charge the pack over USB with the help of a 5-12 Volt step-up converter. I’ve set the converter to output 12.6 Volts and then connected it to the charge points on the BMS. This all works and the battery is charging just fine. But…
The step-up converter gets really hot, even though I’m only using a 1 amp USB-port as the input power. What am I doing wrong? Or is that just how it is, that it gets really hot? I’ts so hot that it hurts when I touch it and I don’t feel comfortable putting it inside a sealed enclosure (the speaker box).
Please help me. Is it drawing too much current? How can I limit that? Or is my design just not possible?
Thanks! 🙂
Best Answer
It is not too difficult to design a real Good Charger to charge 3S LION or similar batteries using a 5V USB supply. There are two things you will have to keep in mind:
(1) Each cell has to be monitored “individually” so as NOT to exceed 4.2V
&&
(2) When condition in (1) is satisfied than only you apply a charge pulse.
You will need a minimum of at least 6 tiny (3x N channel && 3x P channel) MOSFETS (or 3x NPN && 3x PNP transistors for switching in each cell at a time. You will also need a micro-controller with at least 4 Analog inputs and 6 digital outputs for Mosfets switches and 2 digital outputs for LED indicators. ( more I/O pins for anything especial you wish to accommodate.) Atmega328p (Arduino Nano) is very much suitable for this project since it has 8 analog inputs and over 13 digital I/Os. About 15 tiny resistors will also be needed.
Programming with Arduino IDE is very simple since it uses the best language ..C, C++. NOTE: this charger would be much better than anything you can buy in the market since here you will be doing all that is required by LIONs charging and checking the individual cells properly in sequence. Commercial chargers don’t normally do these activities since they use cheap devices with fewer I/Os
Also don’t worry about all the misinformation you find flowing on the web about ‘constant current’ etc, we have experimented extensively with LION type of batteries at our university and found that they charge very well with varying currents.
You don't need any voltage booster circuit since you will be working with individual cells separately and 5V USB supply is enough to charge a 4.2V cell. The micro-controller will take care not to switch any other than the required Mosfets. Programming these types of projects is a real fun.....
Good Luck with your project.