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Due to IEDs, RPGs and the penetrating power of the AK47, the military is installing a higher level of armor on vehicles to offer as much protection for the soldier as possible. This makes every pound removed from the shelter’s contents by the equipment contractor valuable. With that in mind, around two years ago, a major prime contractor tasked Falcon Electric to design a lightweight, rugged UPS and Frequency Converter system to provide power backup and protection for their electronics to be installed in a military Humvee-mounted shelter.
The contractor wanted a minimum of 15 minutes of battery backup power, which meant the standard 120-pound lead-acid battery system was out of the question. They further requested that the entire battery bank, containing the batteries, charger and battery management system, be packaged inside a 3.5-inch high (2U) rackmount enclosure. The UPS and battery system needed to withstand a large amount of shock and vibration, as well as pass the Army’s Munson Road Test and other military standards.
Lithium-Ion Polymer Technology
After researching the available battery technologies compatible with our UPS technology, the Falcon engineers determined that the Lithium-Ion Polymer battery offered the best solution. The team had a mature UPS/Frequency Converter technology that, with some minor redesign and major repackaging, would exceed the requirements and provide up to a 5 kVA output while weighing only 75 pounds. This part of the project was completed in about six months to product qualification. Since the Lithium-Ion Polymer battery system would take substantially longer to complete, the UPS was qualified using the standard Valve Regulated Lead-Acid (VRLA) battery bank.
Obtaining the batteries was the first roadblock encountered in designing the Lithium-Ion Polymer battery system. The leading manufacturer of this battery technology is located in China, and all of their available cell production was allocated for a substantial period of time. To compound the problem, the Army planned to purchase only a few hundred systems, so the battery manufacturer had little incentive to pull the battery cells required from their existing large quantity customer orders. Going to a second manufacturer was not an option as the required battery cells were single source. After meeting with the battery manufacturer many times, they finally recanted, but required that all battery cells for the entire contract be purchased on a one-time buy. They were willing to supply the engineers with enough battery cells for development purposes before we purchased the remainder of the cells.
Saving Space with Flexible Packaging
The Lithium-Ion technology is based on Lithium-Ion Polymer battery design. The lithium salt electrolyte is not suspended in an organic solvent like the lithium-ion design, but is contained in a solid polymer composite. This makes the battery much safer than the typical Lithium-Ion design. The individual battery cells are typically packaged in flat, flexible packs and are easily packaged into a small portion of the battery bank, allowing space for the charger and battery management system (BMS) PC board.
The battery charger design process was straightforward and accomplished in a couple of months. The charger design incorporated an off-the-shelf AC to DC and two DC/DC converter modules that provided the necessary 72 VDC charge current. Since these small, readily available modules were used, the battery charger circuit board only required a small space down one side of the enclosure.
Lithium-Ion Polymer batteries demand that a sophisticated microprocessor-based, BMS be incorporated to precisely monitor and control the charging of each individual cell used inside the system. The system contained 80 batteries. The BMS also continuously monitors for over-current conditions and switches the individual cells, or the battery bank output off, in the event of a short circuit. Without the proper battery management system protection, over-charging or a direct short across an unprotected high-capacity lithium-polymer battery system could result in igniting the lithium inside the batteries.
Lithium burns at a very high temperature and requires special fire fighting and safety techniques. As a result, the Department of Transportation (DOT) has implemented regulations governing transportation of Lithium-Ion polymer battery cells and packs having lithium content over a specified amount. The regulations governing lithium battery transportation are DOT TITLE 49 CFR and UN ST/SG/AC.10/11. To meet DOT requirements and receive DOT approval to ship or transport the system, the BMS must be active and monitoring the system whether it is in storage or transport.
After some research, the team decided that the BMS portion of the project required knowledge of lithium batteries that they did not have in house, so they outsourced the BMS development. The Falcon engineers worked with the outsourced team and within several months came up with what initially appeared to be a working BMS board. It then took several more months to get the bugs out and archive the performance demanded.
At this point the engineers at Falcon finished a battery system product that they could not ship anywhere until they conducted all required DOT and UN testing on 16 finished units, the shipping container and its packing materials. The testing and subsequent DOT certifications took about three more months. The lithium-ion polymer battery systems along with Falcon’s UPS were then shipped to the contractor for all of the required Army testing and approvals.