Safety requirements

If overheated or overcharged, Li-ion batteries may suffer thermal runaway and cell rupture. In extreme cases this can lead to combustion. Deep discharge may short-circuit the cell, in which case recharging would be unsafe. To reduce these risks, Lithium-ion battery packs contain fail-safe circuitry that shuts down the battery when its voltage is outside the safe range of 3–4.2 V per cell. When stored for long periods the small current draw of the protection circuitry itself may drain the battery below its shut down voltage; normal chargers are then ineffective. Many types of lithium-ion cell cannot be charged safely below 0 °C.

Other safety features are required in each cell:

· Shut-down separator (for overtemperature)

· Tear-away tab (for internal pressure)

· Vent (pressure relief)

· Thermal interrupt (overcurrent/overcharging)

These devices occupy useful space inside the cells, add additional points of failure and irreversibly disable the cell when activated. They are required because the anode produces heat during use, while the cathode may produce oxygen. These devices and improved electrode designs reduce/eliminate the risk of fire or explosion. Further, these features increase costs compared to nickel metal hydride batteries, which require only a hydrogen/oxygen recombination device (preventing damage due to mild overcharging) and a back-up pressure valve.

These safety issues present a problem for large scale application of such cells in Electric Vehicles; A dramatic decrease in the failure rate is necessary.

Market

· Costs more per watt-hour than other chemistries

· Not suitable for AAA, AA, C or D form factors due to voltage per cell being more than 2 volts (i.e. 3.7 volts), though most devices designed for a voltage that is a multiple of 1.5 can run safely on a voltage that is 30% higher.

Specifications and design

· Specific energy density: 150 to 250 W·h/kg (540 to 900 kJ/kg)

· Volumetric energy density: 250 to 620 W·h/l (900 to 1900 J/cm³)

· Specific power density: 300 to 1500 W/kg (@ 20 seconds and 285 W·h/l)

Because lithium-ion batteries can have a variety of cathode and anode materials, the energy density and voltage vary accordingly.

Lithium-ion batteries with a lithium iron phosphate cathode and graphite anode have a nominal open-circuit voltage of 3.2 V and a typical charging voltage of 3.6 V. Lithium nickel manganese cobalt (NMC) oxide cathode with graphite anodes have a 3.7 V nominal voltage with a 4.2 V max charge. The charging procedure is performed at constant voltage with current-limiting circuitry (i.e., charging with constant current until a voltage of 4.2 V is reached in the cell and continuing with a constant voltage applied until the current drops close to zero). Typically, the charge is terminated at 3% of the initial charge current. In the past, lithium-ion batteries could not be fast-charged and needed at least two hours to fully charge. Current-generation cells can be fully charged in 45 minutes or less. Some lithium-ion varieties can reach 90% in as little as 10 minutes.

Battery pack life

Li+ batteries last longer if not deeply discharge (depleted) before recharging. The smaller the depth of discharge, the longer the battery will last.

Batteries may last longer if not stored fully discharged. As the battery will self-discharge over time, its voltage will gradually reduce, and when it is depleted below the low-voltage threshold of the protection circuit (2.4 to 2.9 V/cell, depending on chemistry) it will be disabled and cannot be discharged any further until recharged. It is frequently recommended to store batteries at 40% charge level.

The rate of degradation of Lithium-ion batteries is strongly temperature-dependent; they degrade much faster if stored or used at higher temperatures. They may be stored in a refrigerator.

Date: 2015-12-11; view: 1125