The size of the wrench needed to remove and replace a car battery depends on the size of the battery terminal nuts. The two most common sizes for these nuts are 10mm and 13mm. To determine the size of your cars battery terminal nuts, you can consult your cars owner manual or use a digital caliper to measure the nuts diameter. If your car has 10mm battery terminal nuts, you will need a 10mm wrench, and if it has 13mm battery terminal nuts, you will need a 13mm wrench. Using the wrong size wrench can damage the battery terminal nuts, making it difficult to remove or replace the battery in the future.
In addition to a wrench, you may need a ratchet and socket set to remove the old battery from your car. The size of the socket will depend on your car model, but its usually between 12 and 14mm. A battery brush or terminal cleaning tool will also come in handy to clean off any corrosion on the battery posts before you install the new one.
Its worth noting that some batteries have side-terminal nuts, which require a specialized wrench.