Have you seen this 8.1Ah DeWalt Battery Pack?https://www.tool-rank.com/media/listing/photos/thumbnail/300x300s/05/94/04/1-terrible-soldering-with-tabs-that-are-too-light-73-1455933400.jpg
Let's face it, we can never get enough runtime out of our batteries. Every year the manufacturers pack in more cells, that hold more juice, and we still want more. The desire for more power was seemingly so strong for one DeWalt cordless tools user that he decided to Frankenstein together his own DeWalt 8.1Ah battery.
*Warning, playing around with lithium-ion battery cells can be dangerous to your health. Especially when they explode and burn your face off.
**There is also a good chance that using modified batteries in your cordless tools will void the manufacturers warranty.
After purchasing a few "defective" battery packs on eBay that had both good and bad cells in them, the Mad Dr. used the good cells to make a pair of working 3.0Ah DeWalt Battery Packs. But that still left him with an empty DeWalt battery shell, and he could not have that; so he proceeded to over-fill it with a total of 15 2.7Ah cells from a set of defective USB battery packs.
Once power was applied, this hacked together DeWalt battery came to life with an 8.1Ah capacity at 18.5 volts. In the end, its housing doesn't look very pretty or safe, but it is big, powerful, and works in a DeWalt Charger.
Source - reddit