Questions raised about lithium-ion cell’s claim to fame

Inaccurate estimates of a lithium-ion battery’s remaining juice can come from failing to fully charge and discharge the battery, a new study reports. The surprising finding, published April 14 in Nature Materials, could affect the up-and-coming electric vehicle industry, given the need for accurate estimates of how far a car owner can drive before needing to recharge.

Some rechargeable batteries, such as nickel-cadmium and nickel-metal-hydride batteries, lose capacity over time when a user repeatedly recharges them without allowing them to fully discharge. But lithium-ion batteries, which are widely used in laptops, smartphones and electric cars, were thought to be unaffected by a user’s charging habits.

Physicist Tsuyoshi Sasaki of Toyota Central R&D Labs in Japan and colleagues at the Paul Scherrer Institute in Switzerland measured the voltage across one of the electrodes as a battery repeatedly charged and discharged. After a charge that wasn’t complete, the next charge produced a surprising voltage spike. A sensor might measure that voltage blip and overestimate how much juice is left in the battery.

Fortunately this effect is manageable, says Paul Braun, a materials scientist at the University of Illinois at Urbana-Champaign who was not involved in the study. There is no long-term impact on the battery’s capacity, Sasaki’s team found, and fully charging the battery eliminates the anomalous voltage readings. Braun says understanding this phenomenon will allow car battery manufacturers to design better battery packs and sensors that accurately measure miles until empty.