Owning a car is mostly a series of small, annoying bills that keep the engine turning. You probably don't think about your battery until a freezing morning when the car won't start and you're late for work. It is better to handle this while the weather is still mild and you are in control of the timing.
Most shops charge around $220 for a decent battery and the labor to swap it. You can usually find the best prices in late autumn before the first frost hits and demand spikes. While you can't negotiate shop rates like you would a car price, you can manage how that $220 hits your bank account.
This plan only works if you clear the balance within the 18-month window. If you still owe money after that time, the interest rate will jump to the standard high rate on whatever is left, making a simple repair much more expensive.