Bought this for my car to keep a couple of my portable batteries charged. I take one with me in my work truck and leave the other in the parking lot at work and change them the next morning. So far it has kept them charged and almost refilled them (about 94-99%). My batteries have 22000 mAh. I have them hanging on my rear quarter window and the battery has a 3 foot USB cable. My only complaint is that it warped a bit due to the heat. This early spring we had a few days in 85-90F but it's still working great.