Standard wet cell batteries are still a common lead-acid battery type in use today. They usually are not sealed, and electrolyte can be added through holes in the top casing of the battery. Gel cell batteries are sealed and cannot be re-filled with electrolyte; therefore, they have a smooth top with no access holes. AGM (Absorbed Glass Mat) batteries are becoming very popular in the evolution of lead-acid batteries. They are sealed like gel cell batteries but are typically smaller and can be recognized by their more compact size. Lithium batteries are the smallest size vehicle battery and are also drastically lighter than standard, AGM, or gel batteries. Lithium batteries, or more typically LiFePO4 (Lithium Iron Phosphate) batteries, are lithium-ion based and offer good safety characteristics along with resistance to vibration.