Make a note of your presets or attach an auxiliary battery to the car when changing your battery cables. Otherwise, you'll need to reset your clock and reprogram your radio presets after attaching the new battery cables. New battery cables will keep your car starting properly, so you should check them as part of a routine inspection and replace them when they appear worn.
Locate the battery on the driver's side close to the inside wall on the Ford Focus. You shouldn't have to remove the bar that holds in the battery, since the top posts are easy to reach.
Loosen the bolt on the black negative battery cable first with a box wrench or cable clamp pliers. Carefully wiggle the cable end off the post. Repeat the process on the red positive cable. Remove both cables from the battery, even if you're replacing only one of them.
Follow the negative cable to where it's grounded to the wheel well. Remove the end of the cable with the wrench. Track the positive cable to its connection on the driver's side of the engine and remove it with the wrench.
Attach the new positive cable first, reversing the process in Step 3. Install the negative cable last and start the car to ensure that the connections are working on your Ford Focus.
When purchasing replacement cables, always buy cables of the same thickness or thicker than the cables that you're removing. Buying a less expensive, thinner cable may cause problems in your electrical system. Clean the battery posts with a small wire brush or cable cleaner while you have the cables off.
Never work on your battery while the car engine is running. Never smoke while replacing your battery cables, as there may be fuel fumes in the engine compartment that could ignite.