Rechargeable batteries, sometimes referred to a secondary batteries, are often the preferred choice over non-rechargeable primary batteries, and although initially more expensive, they offer long-term cost benefits. Various different types are available, some of the most popular being lead-acid batteries, NiMH batteries and lithium batteries. Rechargeable batteries are available in a wide range of sizes, voltages and storage capacities. The choice of battery is determined by the intended use, with each type having both advantages and disadvantages. NiMH batteries give good all-round performance and are available in many sizes, including most popular consumer battery sizes. Lithium-ion (Li-ion)and Lithium-Polymer (LiPo) batteries offer high energy density coupled with compact dimensions and light weight. Both are often used as battery packs in pouch format, and LiPo batteries do not require a rigid outer shell, enabling them to be used in space-critical situations. Lead-acid batteries offer high storage capacities and the potential for high-pulse currents but are relatively large and heavy, and therefore best suited to static installations. Each type of rechargeable battery has specific charging requirements, and in choosing a battery charger it is essential to only use a type specifically designed for both the type and number of cells being used. Primary batteries are still preferred in some situations.