How can I diagnose and fix a dead battery issue in my Audi 80?

My Audi 80 has been sitting unused for a couple of months and now the battery appears to be completely dead. What are the steps I can take to diagnose the issue and fix it myself?

The first step is to check the battery voltage with a multimeter. If it reads less than 12.6 volts, it is likely depleted and needs to be recharged.

If the battery doesn't hold a charge after recharge, it may need to be replaced. Consult the owner's manual or a mechanic for the appropriate battery size and type for your Audi 80 model.

Before replacing the battery, make sure to clean the battery terminals and connections with a wire brush and baking soda solution to remove any corrosion buildup.