I borrow my roommate’s every now and then since this May. During the time I use the car I left the lights on and drained the battery for 4 times (evenly spread). Since winter is coming we think it’s better to change the battery. I googled it and most people say the average battery will last about 5-6 years. His battery is about 3-4 years old when I first start using the car. But since I’ve jumped it for so many times I’m wondering should I pay half for the battery? I want this to be fair for both of us but I almost know nothing about cars so could someone with more experience help me here? Or maybe what do you think is the fair way to do this.