Playing With Power

About a year ago, a friend sent me an interesting story titled, “ People Are Using Old Laptop Batteries to Build Their Own Versions of Tesla’s Powerwall .” Being an engineer and lover of tinkering, I read the article with the zeal of a grad student. Tesla is making DC electricity sexy, but their high price tags ( just under $500 per kilowatt-hour ) are hard to justify. Apparently other DIYers felt the same, and had been creating their own using recycled laptop cells. I had both the time and thirst for knowledge, so I dove right in. For the uninitiated, many laptops until recently were powered by batteries built with six or more cells slightly larger than a AA. These cells, 3.7 volts nominal, are combined in parallel to grow their capacity and in series to boost voltage. After some usage, however, one or more cell will fail, effectively rendering the entire battery useless. These batteries are then recycled, and a new one installed. Each of these “bad” batteries, however, still co