Lithium-ion batteries have a number of advantages over traditional valve-controlled lead-acid batteries (VRLA), which are widely used in UPS today. Much longer life, smaller size and weight, faster recharge times, and lower prices have made lithium-ion batteries an attractive energy storage technology. This article provides brief answers to the most common questions about lithium-ion batteries and their use by UPS and is intended to help the user decide which battery technology is best suited for use in backup power supplies.
What is a lithium-ion battery and how does it differ from a lead-acid battery?
Simply put, a battery is an electrochemical device that stores energy and releases it as electricity. Batteries are usually connected in series, parallel, or in combination to achieve any voltage and current. This simple description applies to both lead-acid and lithium-ion batteries. Each battery contains a cathode (positive electrode) and an anode (negative electrode), which are located in the electrolyte. The electrolyte acts as a catalyst for an electrochemical reaction that leads to charging and discharging as ions flow from one electrode to the other. It also prevents the reverse flow of electrons accumulated at the anode to the cathode inside the battery itself when there is no load. The chemical reaction results in a charge potential difference (i.e., voltage) between the cathode and the anode when electrons accumulate on the anode. When a load is connected by a wire to the battery terminals, a current is induced that discharges the battery as a stream of electrons (i.e. current) from the anode (negative terminal) to the load, and then to the cathode (positive terminal). The chemical composition of the battery changes as this ion flow occurs until more electrons are applied to the anode, which leads to battery discharge. The battery can be recharged using an external power source to reverse the flow of electrons through the electrolyte from the cathode to the anode.
|Property||Lithium-Ion batteries||Lead-acid (VRLA)|
|Energy density, W/Kg||70 - 60||15 - 50|
|life Cycle||10 - 15 years||4 - 6 years|
|Number of charge/discharge cycles||>1000||200 - 400|
|Recharge time||1 hour||6 - 12 hours|
Table 1. Typical performance ranges for the type of battery used in the UPS today.
So, the main difference between a lithium-ion battery and a lead-acid battery is the chemical composition of the materials used in the electrodes and electrolyte. Most modern lithium-ion batteries use metal oxide for the cathode and carbon-based material for the anode. The electrolyte solution is a lithium salt dissolved in an organic solvent. A lead-acid battery, on the other hand, uses lead dioxide for the cathode, lead anode, and a form of sulfuric acid as the electrolyte. This chemistry largely determines the battery’s operational capabilities. Table 1 highlights some of the high-level performance differences between the two types of batteries used in UPS applications. However, it is important to understand that there can be large differences in performance between one battery and another of the same type due to differences in chemistry (for example, in the composition of the electrolyte and electrodes) and the general type and quality of materials used, as well as the design of the cell. This makes it difficult to generalize or generalize the characteristics of lithium-ion batteries unless their specific application and design is intended.