it was not until the arrival of other chemistries, such as nickel-cadmium (NiCd) later in the twentieth century, that batteries became practical for heavier-than-air aircraft. Modern batteries are mostly rechargeable types based on lithium technologies. Lithium polymer (LiPo), a type of lithium-ion batteries (LIB), have long been applied in unmanned flight for their light weight and rechargeability. However, their energy density limits their application mostly to being drone batteries. Increasing maximum time of flight by simply designing larger aircraft using larger batteries is inefficient, because of the payload-range compromise. After a certain increase in battery weight, there are diminishing returns through the mass penalty not outweighing the increase in battery specific energy. There is a similar trade-off between the maximum range and number of passengers. Computational tools have been used to model this trend, predicting that a small-scale electric aircraft of average weight (1500 kg) and average energy density (150 Wh/kg) could travel a range of ~80 mi with one passenger, ~60 mi with two, and less than ~30 mi with three. In 2017 the power available from batteries was estimated at around 170 Wh/kg, 145 Wh/kg at the shaft including the system efficiency, while a gas turbine extracted 6,545 Wh/kg of shaft power from an 11,900 Wh/kg fuel. In 2018 Lithium-ion batteries including packaging and accessories were estimated to give 160 Wh/kg while aviation fuel gave 12,500 Wh/kg. In 2018 the specific energy of electricity storage was still only 2% of aviation fuel. This 1:50 ratio makes electric propulsion impractical for long-range aircraft, as a mission for an all-electric, 12-passenger aircraft would require a six-fold increase in battery power density. That said, battery-electric motors have a higher efficiency (~90%) than most jet engines (~50%), which can be further exploited through emerging battery chemistries. To be feasible for electric aircraft application, it is essential that power storage be improved. Energy density is widely recognized to be the bottleneck for zero-emission electric powertrain. Another limitation is the discharge rate due to demand-pack energy ratio and sensitive mission segments, as the discharge C-rate for take-off is 4C while it is almost 5C for landing. Electric aircraft have additional heat generation and end-of-life needs, requiring novel thermal management strategies, power-fade capabilities and battery pack failure modes. As of 2019, the best Li-ion batteries achieved 250–300 Wh/kg, sufficient for a small aircraft, while a regional airliner would have needed a 500 Wh/kg battery pack