Your car is a tool, not an investment, and it demands constant upkeep to stay reliable. Batteries usually die right when the temperature drops, so getting ahead of the first frost is the smartest move you can make.
A new battery and a quick terminal scrub will run you about $220. Getting this done now avoids the premium you would pay for a tow or an emergency shop visit when you are already running late for work.
Remember that the 0% window is a deadline, not a gift. If you still have a balance after 18 months, the bank will start charging interest on whatever is left, which defeats the purpose of the plan.