If at all possible, remove the battery from the vehicle. Clean the battery and terminals using a solution of baking soda and water if there is any concern of electrolyte outside of the battery. Make sure that nothing enters the battery during cleaning. You can also use this same solution to clean the battery compartment of the vehicle to help neutralize any electrolyte that may be present. Rinse with clean water and dry thoroughly.
After the battery is cleaned, inspect for any signs of damage or extraordinary wear that may have occurred while in service. If you have any concerns about the condition of your battery, you should seek the advice of a mechanic or a battery specialist.
Upon determining there is no concern of damage, for conventional batteries, check the electrolyte levels. The level of electrolyte must be maintained above the minimum and at or below the maximum level line on the side of the battery. Check only when on a flat level service. If you need to increase the level, carefully add distilled water avoiding any overfill. Once the levels are adjusted, charge the battery referring to the manufacturers instructions.
AGM batteries (also referred to as VLRA or Maintenance Free) do not require you to maintain the electrolyte levels. AGM batteries must never be opened once in service or permanent damage and failure will occur. As with the conventional batteries, once you have cleaned and inspected the battery, charge it per the manufacturers instructions.
With the batteries in a full state of charge, you may wish to store it in a cool dry area away from children and pets. You may also choose to reinstall it in the vehicle. Either way, allow yourself access to the battery so you can periodically check your state of charge, or simply attach a battery charger/ maintainer to it.
Maintaining your battery state of charge during extended periods of storage is essential to insure the maximum service life is delivered.
Link to: Spring Startup - Detailed article.