Andreas Jossen

Methods for state-of-charge determination and their applications

Abstract State-of-charge (SOC) determination becomes an increasingly important issue in all the applications that include a battery. Former operation strategies made use of voltage limits only to protect the battery against deep discharge and overcharge. Currently, battery operation is changing to what could rather be called battery management than simply protection. For this improved battery control, the battery SOC is a key factor. Much research work has been done in recent years to improve SOC determination. Operational conditions differ for batteries in, for example, photovoltaic applications, (hybrid)-electric vehicles or telecommunications. Hence, a given method for SOC calculation will be more suitable for a certain application than for another. The authors introduce commonly used methods for SOC determination and establish a relationship between the advantages of the different methods and the most common applications. As a main illustration, the analysis of Kalman filter technique for lead-acid battery SOC determination are presented and some results for other calculation methods as well.

Battery management systems (BMS) for increasing battery life time

The life time of the battery depends of many different parameters. One parameter set is the internal battery parameters which are influenced by the battery manufacturers, and the second set is the external battery parameters which are influenced by the battery users. The external parameter can have an enormous influence on the life time. By use of a battery management system (BMS) it is possible to control the external parameters with the target to extend battery lifetime. This paper describes a BMS consisting of a data acquisition system, battery state calculation, electrical management, thermal management, safety/supervisory management, and communication. If the functions of a BMS are optimized to the used battery type and application, it is possible to extend battery lifetime.

The influence of different operating conditions, especially over-discharge, on the lifetime and performance of lead/acid batteries for photovoltaic systems

A study is made of the influence of different operating conditions (i.e., cycling, self-discharge, floating) on the lifetime of different types of lead/acid battery (vented: flat plate, tubular; valve-regulated: gel, AGM) recommended by manufacturers for solar applications. A bench test has been conducted for more than six years and is based on practical photovoltaic operating conditions. The best performance is exhibited by gel batteries and by vented, pasted-plate batteries with electrolyte agitation. The over-discharge of series-connected cells in large solar battery packs influences the lifetime. Results are given for the discharge and over-discharge characteristics of lead/acid batteries, i.e., battery voltage, cell voltage, positive and negative electrode potentials, gassing rate, oxygen evolution, and sulfuric acid density. The same characteristics are also examined for the recharge of over-discharged batteries. In the over-discharge period, inactive PbO2 and Pb are activated and can react in a further electrochemical discharge or/and in a chemical comproportionation reaction.

Reliable battery operation — a challenge for the battery management system

Abstract Advanced batteries, like lithium-ion batteries, are more sensitive in case of irregular operation than conventional batteries. Therefore, the operation of such batteries must be controlled by a management system. The features of a battery management system depend on the application, but in most cases, features like battery state determination, electrical management and safety management are necessary. This paper describes these functions of a battery management system. The use of a battery management system will lead to an increased lifetime and a safer operation of the battery.

The detection of the state of health of lead-acid batteries

The authors have tested, for about 6 years, four different types of lead-acid batteries under cycling, floating and self-discharge conditions. A correlation has been established between the operating data (current, voltage) and the capacity measured during these tests by a fuzzy cluster approach. The data sets were categorised in three fuzzy clusters. These three fuzzy clusters: (1) starting phase (new phase); (2) working phase; and (3) capacity drop phase should able to describe the ageing behaviour of the battery sufficiently. For all fuzzy analysed data, they found a continuous correlation between the fuzzy classifications and the operation time of the battery, which themselves correlate to the development of the battery state of health. This fuzzy technique allows the prediction of the future behaviour of the battery by measuring the operating parameters (current, voltage) without explicitly checking the capacity, if the fuzzy set is determined once as a kind of calibration curve.

Electrical safety of commercial Li-ion cells based on NMC and NCA technology compared to LFP technology

Since a laptop caught fire in 2006 at the latest, Li-ion cells were considered as more dangerous than other accumulators [1]. Recent incidents, such as the one involving a BYD e6 electric taxi [2] or the Boeing Dreamliner [3], give rise to questions concerning the safety of L#i-ion cells. This is a crucial point, since Li-ion cells are increasingly integrated in all kinds of (electric) vehicles. Therefore the economic success of hybrid electric vehicles (HEV) and battery electric vehicles (BEV) depends significantly on the safety of Li-ion cells. Lithium nickel manganese cobalt oxide (NMC) and lithium nickel cobalt aluminium oxide (NCA) are two standard Li-ion cathode chemistries, which are often used for today’s HEVs and BEVs Li-ion batteries. Cells with this two cathode technologies are investigated in detail and compared to cells with the alleged save lithium iron phosphate (LFP) technology. Furthermore only commercially available and mass produced Li-ion cells were tested, in order to get as close to real end-user applications as possible. To ensure comparability, cells with the most common 18650 casing have been used. Furthermore all cells had no built-in resistor with positive temperature coefficient (PTC-device). For each abuse test at least 2 cells have been tested to get to know the statistical dispersion. The spread was in all tests for all measured values of each cell type lower than 11 %. Consequently it can be supposed, that mass produced cells show equal behaviour also in abusive test. The performed electrical safety tests on these cells, involve overcharge, overdischarge and short circuit tests. These tests represent real abuse scenarios and are geared to established standards [15], [16], [17], [18]. To complete these measurements an accelerated rate calorimetry (ARC) test has been carried out, to determine the thermal stability of the cells. As in the literature discussed, the investigated LFP/C cells show a higher thermal stability and are therefore safer, although they do not have any overcharge buffer as the investigated NCA/C and NMC/C cells.

Electrochemical impedance spectroscopy for online battery monitoring - power electronics control

Online diagnostics for monitoring of battery cells in a battery pack is necessary in order to determine the state of the battery pack, like its age and safe operation. It could also provide the possibility to adjust the operation strategy of the battery management system and the load to increase the battery lifetime and safety. Impedance spectroscopy is a well-known measurement technique for electrochemical systems, such as a battery half-cell. Once the method is implemented in the battery management system and performed online during operation it could provide a monitoring system for the whole pack. Current solutions are either inaccurate or too big, expensive and energy inefficient. The presented approach proposes a dual use of the battery charger which incorporates a switched mode amplifier to generate the stimuli current necessary to perform an electro impedance spectroscopy. A suitable control is designed to overcome the non-linearities and instabilities introduced by the output filter and the current crossover effects of the electronic switches. This inexpensive, energy efficient technology could allow impedance monitoring of every cell in the battery pack and make a better prediction of the state of the battery possible.

Hybrid Energy Storage Systems for Electric Vehicles: An Experimental Analysis of Performance Improvements at Subzero Temperatures

Electric vehicles based on high-energy lithium-ion batteries often exhibit a substantial loss in performance at subzero temperatures: Due to slower electrochemical kinetics, the internal resistances of the batteries rise and diminish available power and capacity. Hybrid energy storage systems (HESSs) can be used to overcome these weaknesses. In this paper, the performance of two HESSs, combining a high-energy lithium-ion battery with either a high-power lithium-ion battery or a lithium-ion capacitor, has been investigated experimentally for a driving scenario at various temperatures. Both configurations enable driving at -20 °C, which was not possible without hybridization. The HESS using the high-power lithium-ion battery provides a substantially higher driving range due to its higher energy density. An analysis of different operating strategies has helped to maximize the driving range: Discharging the high-energy battery with a constant current and keeping the high-power cell at a higher state of charge (SoC) extend the driving duration, as the requested driving power can still be provided at a lower SoC of the high-energy battery. In addition to the HESSs, two energy storage systems without hybridization, consisting of different generations of high-energy lithium-ion cells, have been examined to disclose improvements in battery technology. These improvements narrow the benefits of HESSs, as the high-energy batteries have become less reliant on the support of an additional high-power device. Although HESSs lose importance for current lithium-ion battery systems, they can be a valuable option for next-generation lithium batteries, which are expected to provide higher energy densities but exhibit reduced rate capability.

Improving the Low-Temperature Performance of Electric Vehicles by Hybrid Energy Storage Systems

Electric vehicles based on high-energy Li-ion batteries often show a substantial loss in performance at cold temperatures: Due to slower electrochemical kinetics, internal resistances of the battery rise and available power and capacity diminish. In order to overcome these weaknesses, a selection of hybrid energy storage systems (HESS) is investigated here: Different hybrid systems combine a high-energy Li-ion battery with either a double-layer capacitor or a Li-ion capacitor or a high-power Li-ion battery. For these three types of HESS, experimental studies performed at various temperatures reveal available energy under realistic driving conditions. At temperatures of 0 oC and below, an increased driving range can be achieved with two of the three HESS combinations. Depending on the available space for the energy storage system, either the HESS utilizing a Li-ion capacitor or the HESS utilizing a high-power Li-ion battery is found to be the most promising solution for electric vehicle applications.

Development of an All Kapton-Based Thin-Film Thermocouple Matrix for In Situ Temperature Measurement in a Lithium Ion Pouch Cell

Based on the design of a lithium ion battery cell and the resulting thermophysical properties, considerable temperature gradients may form within the cell not only during abusive scenarios. While the temperature gradients during normal operation are mostly negligible for small cells, which are commonly employed in mobile applications, the discrepancy between the temperature inside a battery cell and the cells surface can be significant for larger scaled cells, which are employed, e.g., in automotive applications. Battery management systems that rely on a monitoring of the cells surface temperature may consequently lead to an unfavorable operation of the whole battery in terms of aging and safety aspects. Thus, we hereby present an approach of designing, producing, and incorporating an all Kapton-based temperature sensor for an in situ temperature monitoring of lithium ion pouch cells. First prototypes are developed as a proof of concept when using a common potential for a thermocouple matrix that will later allow for a spatially resolved temperature monitoring within a lithium ion pouch cell. The interactions between the sensor and the cell are investigated and further design steps for improvements in functionality are elaborated.