Abstract
For lithium iron phosphate batteries (LFP) in aerospace applications, impedance spectroscopy is applicable in the flat region of the voltage-charge curve. The frequency-dependent pseudocapacitance at 0.15 Hz is presented as useful state-of-charge (SOC) and state-of-health (SOH) indicator. For the same battery type, the prediction error of pseudocapacitance is better than 1% for a quadratic calibration curve, and less than 36% for a linear model. An approximately linear correlation between pseudocapacitance and Ah battery capacity is observed as long as overcharge and deep discharge are avoided. We verify the impedance method in comparison to the classical constant-current discharge measurements. In the case of five examined lithium-ion chemistries, the linear trend of impedance and SOC is lost if the slope of the discharge voltage curve versus SOC changes. With nickel manganese cobalt (NMC), high impedance modulus correlates with high SOC above 70%.
Highlights
Because of the high demands on the reliability of emergency power supplies in aircrafts, planned take-offs can be delayed
For lithium iron phosphate batteries (LFP) in aerospace applications, impedance spectroscopy is applicable in the flat region of the voltage-charge curve
The frequency-dependent pseudocapacitance at 0.15 Hz is presented as useful state-of-charge (SOC) and state-of-health (SOH) indicator
Summary
Because of the high demands on the reliability of emergency power supplies in aircrafts, planned take-offs can be delayed. After parking for a long time without a power supply, the state-of-charge (SOC) of aircraft batteries drops because of self-discharge. Short maintenance intervals require a reliable method for fast battery diagnosis that reflects at least the upper SOC range. Based on our earlier work [1,2] on SOC determination using electrochemical impedance spectroscopy [3,4,5,6], we have investigated various chemistries of lithium ion batteries under normal ambient conditions and overcharging. For emergency power supplies in aircrafts, LFP combines a couple of positive properties such as inherent safety on thermal runaway in the event of overcharging and overheating. The capacity is prone to degradation if long periods of time are operated either in the upper or lower potential range and at low temperatures [9,10,11]
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