A while ago a co-worker gave me a few dead laptop batteries that he was cleaning out, so I took them apart. Some cells were good while other's were bad. I charged most of them fully and then ran them through a constant current load. I got 6 cells that are good and have a capacity of about 2000mAh each. I decided to build a project around them, and since I was going camping in june, what better way to utilize them than to make a power bank.
Now I realize that I can just buy one, stick the batteries in, and be done with it. But where's the fun in that?
First IC on 2nd board went poof. :( Thought I reflowed correctly and checked for shorts. Nope, nothing. Unless pins are shorted on the IC itself, Im getting full 5V on the Vbattery terminal. I may try another IC and I have one more board left.
If it comes down to it and I cant successfully solder a QFN I will have to redesign the board with another IC and add in the load sharing option. There still is the issue of soldering the boost board which is also a QFN package. That doesnt have as many pins so it should be easier..I hope.
The first charger is done, but Im terrible at documenting my projects. I had to reflow the package twice to get one of the pins soldered correctly. After that it was fine. 2nd Charger went together well, and powered on correctly. Problem is it shows 4.30V. Welp. That means pin 6 (Feedback) isnt connected. I'll have to remove the chip, put on a small bit of solder and then reflow it..again.
QFN packages are a pain. Im doing this without any flux or paste. Just plain solder. Maybe I should invest in some flux and paste..
It started with an Idea or rather..a problem. We live in a technologically advanced society where we carry small devices with us all the time. They need energy to do so. Being in a remote location poses a problem. Sure it has a battery, but that battery will run out in due time. I could lug a generator to the site..but then I need gas and it also will pollute the area with noise. Solar is the obvious option.
I had done a previous project where I wanted to charge my phone via Solar. Well it worked, until the boost IC blew for unknown reasons. Luckily someone's truck was nearby and I could charge my phone overnight using an adapter.
So I started from there. I wanted to also do a total rebuild of my solar panel, as it really didnt give enough power. It is made of 10 semi broken 3x6 cells. Gives about 4.5V at 2A. Not really enough for charging your phone without a boost converter. Something also happened to the panel during the last few tests as now it seems to give only 500mA. I suspect there is a break in one of the cells.
The new panel will consist of 12 3x6 solar cells. I tested an individual cell and it gives about 435mV @ 2A. Open Circuit Voltage is around 575mV. This was around Noon in the winter sun. The whole panel should give 7V Open circuit and 5.22V under full load.
Construction Will be of Wood and Plexi or Acrylic. It will be somewhat water tight, but not fully (so NEMA 3/4/12). It really wont sit outside all the time either. I do plan on sealing it with caulk.
Its my first time soldering a QFN package..luckily I build a small hotplate with a PID controller to help with that. I could use a skillet..but I used that for rocket fuel. It seemed to work well, but I had to do it twice as a few pins didnt have enough solder on them. The first board I did was based on Microchips MCP73871 Lithium charger with Load Sharing. I chose this IC because I wanted to be able to charge the battery and draw from the solar panel at the same time. The IC itself is limited to 1.8A. I am charging at 500mA so that leaves a little over a amp available. The panel can only put out around 5V @2A at max sun light, but this was tested in the winter sun. 1A to charge a phone is plenty since there are two batteries.
The area I will be in isn't your normal camping site..its pretty much near a beach on a open field so I will probably get a higher than normal output. Now the question is, what will I do with this thing after camping is done and over? The batteries will be there and they will be charged. I will have a system in place that effectively can charge batteries for free.