Natural speed, please
To assure continuous protection of the batteries, the BMS must be powered and monitor the batteries continuously whether the UPS is turned on, turned off, during shipment and storage. This presents storage and non-use issues. Li-Ion batteries have a very low self-discharge rate. However, when the UPS is not being powered from utility power, the BMS must be powered from the batteries it is protecting. This accelerates the discharge rate, requiring the batteries be recharged far more frequently. With some BMS designs, recharging occurs every few months even while in storage. Like lead-acid batteries, should Li-Ion batteries be allowed to become too discharged, they can suffer internal damage, requiring their costly replacement. Their replacement can become even more costly due to the dangers associated with handling high-capacity Li-Ion batteries. They typically must be replaced by trained factory personnel, requiring the entire UPS to be returned to the manufacturer.
Federal regulations also dictate that all equipment containing high lithium content batteries be subjected to rigorous testing prior to any equipment being shipped. The test plan requires UPSs to be subjected to worst-case tests. These tests are designed to assure the UPS has protections designed-in to protect the lithium batteries and assure proper safeguards are functioning to prevent a fire during transportation. The second tier of testing is performed with the UPS packaged inside its shipping carton including all associated packing materials. These tests include vibration, shock, and drop testing. All test results are reviewed and if deemed acceptable, signed off by a DOT inspector. After receiving this approval all equipment shipments must be shipped at Class 9 Hazardous Materials and clearly identified as containing lithium batteries. Handling instructions must be placed on the outside of the shipping container along with the telephone number that can be contacted in case of an emergency. The staff of both the shipper and recipient shipping departments must be properly trained in the handling of hazardous materials and lithium batteries. The proper fire suppressant equipment must also be in place.
Lithium-Phosphate (LiFePO4) battery technology offers the UPS manufacturer the best hope. The technology appears to be one of the safest lithium battery chemistries due to its improved thermal and chemical stability. It is much more difficult to ignite a Lithium-phosphate battery in the event of mishandling. They have a better cycle life than other Lithium-ion chemistries and are more recyclable. Recycled phosphate can be used as fertilizer. Further, there is presently one manufacturer of Lithium-phosphate batteries that has designed their batteries with internal fusing, eliminating the need for BMS protection circuits. Due to this they have received a waiver from the DOT and can ship the batteries without a Class 9 hazardous materials declaration. However, only four 12 volt batteries may be connected in series without a BMS to provide equalization. Due to the characteristics of Lithium-Phosphate type batteries, the VRLA battery chargers found in some UPS designs can be used without modification.
At the present time, the costs, shipping, and safety issues associated with the proper implementation of high-capacity Li-Ion batteries precludes their use in the majority of UPSs and only viable in vertical markets willing to pay the extra costs associated with Li-Ion batteries. Currently, this consists of the military and a limited number of scientific markets. The Li-Ion battery’s high power density and associated reduction in battery pack size and weight are paramount to the higher costs associated with the UPS.
