power supply

1.2 kW RC Battery Charger

Submitted by Matt on Sat, 05/09/2015 - 11:00

My drone has a flight time of 15-20 minutes, but nevertheless goes through batteries at a tremendous rate. I can have extras standing by, but after a while it gets to be a bit ridiculous with the amount of batteries (lose track of which ones are dead/alive, have to carry them all around, etc).

Lithium Ion/Poly battery chargers are fairly expensive, and for the most part very very slow. Also consumer level models designed with lots of generalities, not taking advantage of special aspects of a given battery role, so they tend to charge conservatively. And even then they can only charge one "battery" at a time, even though the battery may consist of one or more cells.

I wanted to bypass all of this and use my electrical knowledge and design a super fast charger. My first challenge I had to solve was I needed a high power DC source. You can buy expensive lab versions of these, but a computer PSU works just fine! So I acquired an old 1.2 kW server power supply for the task at minimum expense off of ebay. The power supply indicates that it supplies a rail voltage of +12. My batteries are 3 cell lithium ion batteries have a terminal voltage fully charged of around that. Great! I might not even need to convert DC-DC.

If any of you have ever worked with computer/server power supplies before, you know that you have to get the power supply to turn "on" before you can start pulling power from it. So I had to find the signal pins on the PSU, and make a switch to enable it to turn on at will. After some short searching online, I found the method with little issue. After measuring the terminal voltage of the supply, I came out with 12.4 V. This is almost ideal! The 3 cell batteries have a fully charged voltage of 12.6. So what this means if you are looking at the charge curve of a lithium battery cell, I will be able to charge each cell to about 90%.