The Solar Powered Laptop…

 

After weeks of energy storage research involving flywheels, pumped hydro, and batteries I decided to take a shot at storing energy.  I ordered a bunch of cheap 18650 lithium batteries from eBay along with 16 4 cell battery holders, and 4 mini voltage meters.  I finally decided on an arrangement of 4 cells with 16 18650s in each cell. This provided me with a working voltage range of 12-16.8 volts considering the working voltage range for an individual cell is 3-4.2 volts. Since most laptop batteries use this 4 cell arrangement, I was originally going to just parallel my pack into a laptop battery directly. That project encountered a bunch of other problems, so when I learned about Boost Converters from this guy on YouTube, I changed the project to charge a laptop through its regular charge port.  Since I didn’t have to cut open and mangle any expensive computers, this turned out to be a much better solution anyways. For those of you who don’t know what a boost converter is, it’s a DC-DC step up power converter that uses a transistor and a coil.

I hot glued 4 of the 4 cell holders together to form 1 large cell and repeated this 4 times for a total of 4 large cells capable of holding a total of 64 18650s. I soldered a huge glob of wires together a bunch of times and put them all in series (positive of one to negative of the next).  While I was doing all of that, I used a smart 18650 charger that recorded the mah of all the cheap 18650s that I bought from eBay.  I honestly wasn’t surprised to find the individual cells only were capable of 500mah each even though they were labeled as 2500mah. With 500mah and a nominal 3.7 volts in an arrangement of 4 groups of 16 I built a 14.8 volt battery with 8000mah, which is pretty terrible considering that can be done with 12 high quality cells and I used 64.  I glued a volt meter to each cell and soldered them individually to their respective cell’s + and – then calibrated them via the adjustment on the back according to my multi-meter. The great part of this design is I can replace the crap cells later with good cells in just a few minutes as long as I don’t have a catastrophic fire or something.

I already had 2 100 watt solar panels from a previous project to collect energy with, so I spent some sleepless nights on YouTube figuring out which components I would need to charge up my 4 cell lithium battery pack without starting a runaway toxic electric campfire. I measured the open circuit voltage of 1 of the panels and it was already 20.6 volts on a cloudy day.  I decided to go with a “buck converter” to tame the wildly varying higher voltage of the ~40 volt series solar panels down to 16.8 volts DC. A buck converter is basically opposite of a boost converter, it’s a DC-DC step down power converter that uses a transistor and a coil and has high efficiency (up to 95%).  I attached a pair MC4 connectors onto the ends of 2 wires to connect the buck converter to the panels without any modifications to the solar panels.

I dug around in my closet all the way back to 2008, where I found an old netbook to use as a test subject for the solar laptop charger.  The netbook struggled to even load a webpage in less than 1 minute, but its battery and charging circuit work perfectly and made the perfect test victim.  I plugged it into the boost converter that was set to 18 volts and it actually worked! It charged in about 2 hours to 99%.  The laptops internal charger is tiny and only pulls 1.5 amps max, so the charging speed was limited by the laptop itself considering at times I was pumping over 8 amps @ 16.8 volts into my homemade storage battery from the panels.  The boost converter that was charging the laptop had it’s input connected to my homemade battery so even if the solar panels aren’t connected it can still charge the laptop at full speed until the voltage drops to the lower working limit of 12 volts.

Here is a list of Parts I used for this project:

Panels – https://www.amazon.com/Renogy-Watts-V…

12a Buck Converter – https://www.amazon.com/gp/product/B07…

15a Boost Converter – https://www.amazon.com/gp/product/B01…

Digital Voltmeters – https://www.amazon.com/gp/product/B00…

Battery Holders – http://www.ebay.com/itm/1P-Plastic-Ba…

MC4 Connectors – https://www.amazon.com/gp/product/B00H1M8ASE/ref=oh_aui_search_detailpage?ie=UTF8&psc=1