Design and implementation of a fuzzy logic-based state-of-charge meter for Li-ion batteries used in portable defibrillators
A fuzzy logic-based state-of-charge meter is being developed for Li-ion batteries for potential use in portable defibrillators. ac impedance and voltage recovery measurements have been made which are used as the input parameters for the fuzzy logic model. The load profile for the Li-ion battery packs comprises a continuous 1.4 A constant current discharge periodically interrupted by 10 A pulses. As the battery is cycled the available capacity diminishes and so the number of 10 A pulses that may be delivered decreases. Measurements are being made on a total ...view middle of the document...
In recognition of this fact, automated external defibrillators (AEDs) are becoming widely deployed in airports, offices, and among emergency responders, including firemen and policemen. The most common failure of AEDs is associated with failure of the battery. Indeed, recently all AEDs sold by a particular manufacturer were removed from the market due to frequent battery failure in the devices .
Since 1997, Villanova University and US Nanocorp® Inc. have collaborated on the development of patented fuzzy logic-based methods for determining state-of-charge (SOC) and state-of-health (SOH) of batteries  and .
Due to their higher energy densities compared to lead acid and nickel cadmium chemistries, lithium ion batteries are being considered for use in AEDs. The aim of the present project is to design, integrate and develop a fuzzy logic-based SOC/SOH meter for Li-ion batteries to be used in AEDs.
The goal will be to be able to take a Li ion battery pack from storage of unknown condition and using a combination of an interrogation method and fuzzy logic data analysis, estimate both the number of times the battery has been cycled and the number of pulses it can deliver.
There are three commonly used techniques for interrogating a battery, namely coulomb counting, voltage recovery and ac impedance measurements. Each of these is described next in more detail.
1.1. Coulomb counting
In coulomb counting, the charge flowing into and out of the battery is monitored and the SOC estimated by determining how much charge has been removed from the battery compared to how much was available from the previous charging cycle. If the estimated charge capacity is compensated for variations in temperature and discharge rate, the coulomb counting approach can be quite accurate in determining the SOC of a battery. However, it provides little useful information on the SOH of a battery. Nevertheless, this is the standard technique used for battery monitoring in consumer electronic devices employing Li-ion batteries, e.g. laptop computers, camcorders, cellular phones, etc.
1.2. Voltage recovery
In this approach a load is applied to the battery and the voltage depression under load and the temporal recovery of the battery voltage after removal of the load are monitored and used to estimate the SOC/SOH of the battery. Since the battery is pulsed in an AED (to charge the capacitors that deliver the high voltage to the electrodes) this method is preferred since the pulsing circuitry is already built into the AED. This method is the one we used in the present project.
1.3. ac impedance method
The ac impedance approach involves the measurement of the ac voltage response of a battery when a small perturbing ac current is applied to the battery. This is typically done under open circuit conditions but may also be done on-line. Usually a single frequency is used and the resulting battery condition is estimated from the value of the impedance at the single...