Battery Power - Spring 2016 - (Page 28)

Feature Assessment of Performance and Safety on Lithium Ion Pouch Battery Under Various Test Conditions Joshua Su, Sr. Engineering Manager * HZO In recent years, lithium ion polymer cells in pouch format have found increasing use in portable consumer electronics. They are commonly being referred to as lithium polymer cells (hereafter referred as Li-Poly), although these cells are not of the true polymer types1,2. Compared to conventional batteries such as nickel cadmium systems, Li-Poly cell designs provide the highest specific energy (>220 Wh/kg) and energy density (>450 Wh/L) of all commercially available designs3. From a safety and fire protection standpoint, a high energy density coupled with a flammable organic, rather than aqueous, electrolyte has created a number of new challenges with regard to the design, storage and handling of batteries containing lithium ion cells. In order to maximize lifecycle and increase safety, lithium ion cells require the use of a Protection Circuit Model (PCM) when assembled together to form a battery. The PCM will generally provide the battery with overcharge/discharge protection, short circuit protection, current limitation, and voltage and current balancing in each cell. All of these factors are important in obtaining the maximum output and lifetime from a rechargeable lithium battery. Consumer electronic devices have become increasingly more important for accomplishing everyday tasks. Moreover, people who rely on electronic devices tend to carry them along to locations in which the electronic devices can incur damage. One form of damage that can be particularly detrimental to an electronic device is water damage. Water tests conducted in the HZO laboratories show a common failure mode of shorting or corrosion at Li-ion batteries. Failure analysis finds the watertight cells of Li-Poly batteries to be robust, but the imbedded PCMs are vulnerable to shorts and/or corrosion, despite commonly being covered by protective tapes. HZO has developed a CVD process to generate a thin film nanocoating that protects electronic devices and components from damage caused by water, other corrosive liquids, humidity, sweat, dust and debris. The CVD process involves placing the devices in a closed deposition chamber, vacuuming them as high as 10 mTorr, and then introducing vaporized coating gas. The typical cycle time is five to seven hours. To make a Li-Poly battery powered device waterproof using HZO technology, the battery also needs to go through this process. One big concern: can the Li-Poly pouch cell withstand this process? Extensive studies have been carried out to obtain baseline data on the Li-Poly battery safety and performance under different discharge rates, temperatures and pressure conditions by NASA-JSC2,4,5. The NASA studies showed swelling of the pouch under extended low pressure conditions, as well as a loss of capacity. But if the pouch was constrained (not allowed to swell), cells showed no change in charge/ discharge performance under vacuum conditions. There is no report on the cell leakage or venting when under vacuum conditions. The most rigorous vacuuming test condition reported was 0.1 psi (~ 5 Torr) for six hours. As mentioned, the vacuum of HZO CVD process can be as high as 10 mTorr, 500 times stronger than NASA's tests. To assess the Li-Poly pouch cell performance under this vacuum condition and/or with the thin film nanocoating, a set of experiments was carried out and the results of such studies are discussed here. Test Plans for Assessment of Pouch Cells A pool of new 1,500 mAh Li-Poly pouch cell batteries provided by an HZO partner, a major battery manufacturers in China, were tested under different conditions. The tests carried out on the cells included pouch cell leakage test, capacity test, environmental test and life cycle test. Pouch Cell Leakage Test Besides the visual inspection of any electrolyte leakage, venting, explosion or fire on the tested cells, the most common industrial testing is to measure the impedance for a cell pouch. The impedance measurement was performed on a fully charged cell before the vacuuming experiments. A Fluke 115 digital 28 Battery Power * Spring 2016 www.BatteryPowerOnline.com http://www.BatteryPowerOnline.com

Table of Contents for the Digital Edition of Battery Power - Spring 2016

Editor's Choice
Reducing TCO with the Right UPS Architecture and Operating Mode
Decades of Data: Using Analytics to Predict the Future
Top Four Considerations for Battery Management in a Connected World
Conference Preview
Designing and Managing Custom Battery Pack Plastic Enclosures
Assessment of Performance and Safety on Lithium Ion Pouch Battery Under Various Test Conditions
Batteries
ICs & Semiconductors
Charging & Testing
Industry News
Calendar of Events

Battery Power - Spring 2016