Correct battery care can mean a long service life. Most starting batteries and all gel batteries are considered maintenance-free. That means they have no vent caps. They experience minimal gassing and are hermetically sealed. There is never a need to add water. (Never attempt to add water to a sealed battery. Opening the battery will dry out the electrolyte and void the warranty.)
Deep-cycle and dual-purpose batteries will require some maintenance. As the battery provides deep-cycle power, it releases oxygen and hydrogen gases into the atmosphere. This needs to be replaced periodically by uncovering the vents on top of the battery and adding water to the electrolyte reservoir. Be sure to follow the manufacturer's recommended procedures because under- or over-watering can limit your battery's service life.
Charging is critical to the extended life of the battery. Be sure to follow all instructions provided on your battery charger. Gel batteries have special charging requirements and should only be charged with a reliable, temperature-compensating, voltage-regulated charger. Never use a trickle charger to recharge a gel battery.
Be careful to never overcharge a gel battery. Overcharging any battery will cause it to gas and lose moisture. Gel batteries are designed with one-way vents to release gas in case of overcharging. But because gel batteries are sealed, there is no way to replace this water if it is lost into the atmosphere. Follow the manufacturer's charging instructions closely.
Always wear eye protection when working on or near batteries. Be sure to ventilate the battery area. Dangerous hydrogen gas will collect in enclosed spaces around the batteries. The smallest spark could cause a dangerous explosion.
In addition to watering and charging, it's important to keep batteries and cables clean and corrosion-free. Corrosion protection spray, terminal protectors and battery cleaners are available from battery manufacturers.