Natural speed, please
There are substantial additional costs associated with the implementation of Li-Ion batteries in a UPS. First, there is the cost differential between lead-acid and Li-Ion batteries. Good quality Absorbed Glass Mat (AGM) VRLA batteries are very inexpensive and readily available. Their low cost is one of the reasons UPS manufacturers use them in their products. A typical 12 volt, 7 Ah VRLA battery designed to provide a five-year service life costs UPS manufacturers about $9.00. The same 12-volt Lithium-Phosphate battery costs more than $100, ten times the price. If a typical 6 kVA on-line UPS had a 240 VDC battery bus voltage, it would require 20 individual 12 VDC batteries to be installed. The cost to the manufacturer for lead-acid batteries maybe as low as $180, while the cost for equivalent Li-Ion batteries would be $2,000. This example is very conservative. A custom lithium-battery system for the same 6 kVA UPS might cost more than $10,000.
However, the cost of the Li-Ion batteries alone is not the total cost associated with proper implementation. In order to meet UN and DOT transportation regulations, one needs to incorporate adequate over-current protection and properly recharge the Li-Ion batteries. A specialized intelligent battery management system (BMS) must be incorporated into the UPS design. The BMS typically consists of a circuit board assembly or assemblies containing microprocessor battery monitoring, battery protection, recharge control and battery cell equalization circuits.
The primary function of the BMS is battery short circuit protection. Lithium cells have a very low cell resistance and a high energy storage capacity. A short-circuited string of cells or batteries could be catastrophic and result in an explosion or very high-temperature lithium fire. A typical BMS processor monitors the battery current and has a large semiconductor switch circuit that can be opened to prevent an excessive over-current condition. The second function of the BMS is to assure the proper recharging of the lithium cells. Cells can receive an initial high-rate charge current to reduce recharge time, followed by a long cell equalization charger to balance the cell voltages to within a few millivolts. The equalization charge is followed by the charge voltage being turned off to prevent cell overcharging and possible cell damage. The BMS also monitors to assure the charger voltage does not run away to the point of causing the cells to overheat and catch fire. The cost of the BMS board is substantial and has to be added to the cost of the Lithium-ion batteries and UPS. As the UPS market is very competitive, these added costs would price most UPS products out of most markets except military and specialized scientific applications. The cost of adding a BMS board to a UPS can range from $100 to more than $1,000.
