So at 12.7v fully charged, you push ~v2.1v each cell. USB runs 5v, hence universal serial bus standard. Amperage is also a big deal. Older phones ran charged on 500-700ma. Phones now are in the 2000-2500ma range. It takes a downstep converter to transform 12v to 5v. This uses a very small amount of electric current. Technically, that battery is very slowly draining even with nothing plugged in bc of resistors and capacitor discharge. There's a sealed unit in the bottom that's providing the 5v, 2a power to each cord. You can still use this battery in a car (or solar setup), but I totally wouldn't recommend using it with the hood shut on an ICE as heat is the enemy of lithium ion batteries.