Reserve batteries separate the electrolyte from the rest of the battery so that the electrolyte remains inert. There are four basic types of batteries: primary, secondary, reserve, and fuel cell batteries. Reserve batteries are often used as emergency equipment batteries or for long-life munitions. A reserve battery functions by keeping the electrolyte of the battery separate from the plates so that the battery is inert. When the battery is activated, the cells fill with electrolytes and the battery will operate at full power almost instantly. The shelf life for reserve batteries can last for years or even decades. This longevity, however, comes at a price. The cost for reserve batteries is significantly higher than standard batteries.