Friday, February 15, 2013

Adjustable LiPo battery charging circuit

So I found myself having a lot of small 1 cell lipo batteries from various devices and gadgets I took apart (broken before or after I disassembled them) and no way to charge them. Recently I found a small boost converter to power small 5V devices. It has a USB female jack on one end and 2 solder pads on the other. It uses anything from 1.5 to 4.5V to output the needed 5V at a maximum of 450mA. The batteries I have would be great but I have no way to use them multiple times. This is the result:
DIY lipo charger
It looks ugly but it does the job great.

Of course the idea is not mine and the only thing I did is to layout the components in a way that is easier to solder them together without an actual PCB. The instructions and component list you can find in this web site and all credit goes to the author. This is the version 3 schematic with LED and variable charging current.
Circuit Diagram

At first I had no idea how big this will be and ended up wasting some space on the perfboard but this is ok as I do need space for the heatsink. One thing that I did is to separate the charging current selector from the main board with the help of some header pins. This allows me to either insert just the right resistor in the female pins and make this more compact or use the add-on board for different batteries.
Close up of the lipo charger
The 2 boards of the lipo charger separated

As you can see the circuit is simple and every component is cheap. The most difficult part was sourcing the right trim potentiometer as I ended up buying 3 or 4 until I found the right one. I ended up doing 2 additional trips to the parts store because in the bin where all the 500 ohm pots were most of them where actually 50 ohm. When I went there the third time I grabbed my multimeter to make sure I will get the right one. Now that I have leftover parts (is there a case when you won't have leftovers?) I need to find another project to use them.

"main" board of the lipo charger
back of the "main" board of hte lipo charger

Don't mind the crooked heatsink it has a notch that prevents it from sitting flush with the LM317. I will replace it soon because I feel it is too small, the LM317 really gets a bit hot to the touch when charging a small 3.7V 200mA lipo battery. It works but to be on the safe side it needs to be bigger (this is 'merica bigger is better). If I use 9V as input then it does not get hot at all, that is a solution as well, just use a closer to the output current.
I am quite happy how this turned out as there are very few overlaps and this could actually be used for a PCB schematic.

current selection board of lipo charger
back side of current selection board of lipo charger
This is very simple, just a board with switches and resistors. I did not have any 1W resistors at hand at first so the 1ohm resistor on switch 6 is 1/4W witch is not good for the current going through it and might burn out. I later added the 1ohm 1W resistor (guess which one) to be on the safe side. I don't have any batteries that need that many miliamps for charging but maybe I will have some in the future.
This is about it for this project, I found it very useful and hope it helps.

Good day to you sir!

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