Method for connecting battery units of a battery system

Abstract

The invention relates to a method for interconnecting battery units of a battery system, wherein, when all battery units are on the same open-circuit voltage curve (LK, ELK), the battery units are interconnected at the same state of charge (SOC), wherein otherwise, when charging the battery units, the battery units are first interconnected at a discharge open-circuit voltage (OCVEL) and the battery units not on the same open-circuit voltage curve (LK, ELK) are disconnected when a charging open-circuit voltage (OCVL) is reached, wherein the remaining battery units are then interconnected and charged at the same state of charge (SOC), wherein, when discharging, the battery units are interconnected at a minimum SOC delta (ΔSOC) and a minimum OCV delta (ΔOCV) between the battery units.

Description

The invention relates to a method for interconnecting battery units of a battery system according to the preamble of claim 1 and an electrically powered vehicle according to the preamble of claim 3. In a battery system consisting of several switchable, parallel battery units, individual battery units may be sporadically shut down by software, for example, due to malfunctions in the battery and / or an electronic control unit, exceeding safety limits, etc. These malfunctions often persist for an extended period, making immediate reconnection impossible. To minimize disruption for the user, the operation of the remaining system is maintained, causing the differences between the states of charge (SOC) of the disconnected and connected battery units to increase over time. To avoid high peak current loads on individual battery units due to an uneven overall current distribution, it is recommended to connect them when all battery units have the same open-circuit voltage (OCV).For cell chemistries with a very flat OCV curve and / or high hysteresis (e.g., LFP - lithium iron phosphate), the low gradient results in a high available SOC delta within the range of approximately identical OCV values, including determination and measurement uncertainties. This is amplified by the OCV hysteresis in cases where connected and disconnected battery units lie on different OCV curves. Consequently, considering only identical OCV values, very high SOC differences between battery units may occur at the moment of connection. This SOC delta manifests itself during subsequent operation as an inhomogeneous current distribution, particularly since individual battery units reach their SOC limits much more quickly, where the OCV increasingly rises or falls and / or the impedance increases.This complicates the prediction of the current distribution, which depends on both factors; particularly when operating near the current limits, this increases the risk of repeated shutdowns due to current limit exceedances. Furthermore, individual battery units limit the total current to maintain their voltage limits. Considering hysteresis, identical states of charge (SOC) may not correspond to identical overvoltage capacities (OCV), creating an optimization problem. US 2023 / 0184815 A1 describes a battery management system that may include a battery monitoring device for detecting the voltage of each reference battery cell and a plurality of battery cells with a common flat area, and for detecting the current of the battery pack, as well as a control circuit for stopping charging of the battery pack, initializing a cumulative current value, and determining a state of charge (SOC) of the cell group as equal to the sum of the SOC of the reference battery cell and the predetermined value when the voltage of the reference battery cell reaches a reference voltage during the discharge of the battery pack. The invention is based on the objective of providing a novel method for connecting battery units of a battery system and a novel electrically operated vehicle. The problem is solved according to the invention by a method for interconnecting battery units of a battery system with the features of claim 1 and an electrically operated vehicle with the features of claim 3. Advantageous embodiments of the invention are the subject of the dependent claims. A method for interconnecting battery units of a battery system (particularly those with a flat open-circuit voltage curve and large hysteresis) is proposed. According to the invention, when all battery units are on the same open-circuit voltage curve, the battery units are interconnected at the same state of charge. Otherwise, during charging, the battery units are first interconnected at a discharge open-circuit voltage, and the battery units not on the same open-circuit voltage curve are disconnected when a charging open-circuit voltage is reached. The remaining battery units are then interconnected and charged at the same state of charge. During discharging, the battery units are interconnected at a minimum state-of-charge (SOC) delta and a minimum open-circuit voltage (OCV) delta between the battery units. During discharge, the activation point is optimized to minimize both the OCV delta and the SOC delta between the battery units. During charging, the strategy is adjusted so that all battery units initially operate on the charging OCV curve. Activation then occurs at the same SOC. The solution according to the invention enables the simultaneous optimization of multiple arguments. This avoids temporarily high loads on individual components (contactors, cells, etc.), thereby increasing their service life and preventing repeated shutdowns of battery units. Furthermore, the controllability of the battery system can be improved by avoiding unpredictable inhomogeneous current distribution and thus repeated shutdowns of battery units. Additionally, the total current available over the entire depth of discharge (DOD) can be increased (avoiding system operation with individual current-limiting units due to voltage limit compliance). Finally, the charging time can be reduced. Exemplary embodiments of the invention are explained in more detail below with reference to drawings. Figure 1 shows a schematic diagram of the open-circuit voltage of several battery units in a battery system as a function of the state of charge when the battery units are connected during charging; Figure 2 shows a schematic diagram of the open-circuit voltage of several battery units in the battery system as a function of the state of charge when the battery units are connected during discharging; Figure 3 shows a schematic view of a method for selecting a strategy for connecting the battery units; Figure 4 shows a schematic diagram of a scaled SOC delta and a scaled OCV delta as a function of the state of charge of the connected battery units at connection, taking into account constraints; and Figure 5 shows a schematic diagram of costs as a function of the state of charge of the connected battery units at connection. Corresponding parts are marked with the same reference symbols in all figures. Fig. 1 is a schematic diagram of the open-circuit voltage (OCV) of several battery units in a battery system, for example, a high-voltage battery system of an electrically powered motor vehicle, in particular a commercial vehicle, a bus, or a passenger car, as a function of the state of charge (SOC) when the battery units are connected during charging. A charging curve (LK) and a discharging curve (ELK) are shown. Fig. 2 is a schematic diagram of the open-circuit voltage (OCV) of several battery units in the battery system as a function of the state of charge (SOC) when the battery units are connected in series during discharge. The charging curve (LK) and the discharging curve (ELK) are shown. In a battery system consisting of several switchable, parallel battery units, individual battery units may be sporadically shut down by software, for example, due to malfunctions in the battery and / or an electronic control unit, exceeding safety limits, etc. These malfunctions often persist for an extended period, making immediate reconnection impossible. To minimize disruption for the user, the operation of the remaining battery system is maintained, causing the differences between the state of charge (SOC) of the disconnected and connected battery units to increase over time. To avoid high peak current loads on individual battery units due to an uneven overall current distribution, it is recommended to reconnect them when all battery units have the same open-circuit voltage (OCV).For cell chemistries with a very flat OCV curve and / or high hysteresis (e.g., LFP - lithium iron phosphate), the low gradient results in a high available SOC delta ΔSOC within the range of approximately identical OCV values, including determination and measurement uncertainties. This is amplified by the OCV hysteresis in cases where connected and disconnected battery units lie on different OCV curves. Therefore, considering only identical OCV values, very high SOC differences between battery units at the moment of connection can occur. This SOC delta ΔSOC manifests itself during subsequent operation as an inhomogeneous current distribution, particularly since individual battery units reach their SOC limits much more quickly, where the OCV increasingly rises or falls and / or the impedance increases.This complicates the prediction of the current distribution, which depends on both factors; particularly when operating near the current limits, this increases the risk of repeated shutdowns due to current limit exceedances. Furthermore, individual battery units limit the total current to maintain their voltage limits. Considering hysteresis, identical states of charge (SOC) may not correspond to identical overvoltage capacities (OCV), creating an optimization problem. In a state-of-the-art strategy S1 for connecting battery units during charging, the battery units are connected according to a charging curve LK when they have the same open-circuit voltage OCV. A disadvantage of this method is that a permanently unbalanced current distribution between the battery units can occur. The method is difficult to control, carries an increased risk of spontaneous shutdowns of battery units, limits the possible system current, and increases the charging time. In another strategy, S2, for connecting battery units during charging, the battery units are connected according to a charging curve LK when they have the same state of charge (SOC). A disadvantage of this is a highly unbalanced peak current during connection. The resulting short-term loads on the battery units can damage battery components. Furthermore, the risk of spontaneous shutdowns of battery units increases. Fig. 3 is a schematic view of a method for selecting a strategy for interconnecting the battery units. In step ST1, some battery units in a battery system are connected together, and others are separated from it. In step ST2, it is checked whether all battery units are on the same open-circuit voltage curve LK, ELK. If so, the battery units are connected together in step ST3 at the same state of charge (SOC). Otherwise, in step ST4, it is checked whether the battery units are charging. If so, in step ST5, the battery units are first connected together at a discharge open-circuit voltage OCVEL. As soon as a charge open-circuit voltage OCVL is reached, they are disconnected. Subsequently, in step ST6, the battery units are connected together at the same state of charge (SOC) according to strategy S2. If the battery units are not being charged, then in step ST7 the battery units are connected together at a minimum SOC delta ΔSOC and a minimum OCV delta ΔOCV between the battery units. This can be determined using an optimization function. In a simulation to optimize between the objectives of strategies S1 and S2, complete charging and discharging procedures of a battery system consisting of six battery units were simulated. A constant voltage phase was performed at the end. A simulated battery management system selects the battery unit with the highest and / or lowest open-circuit voltage (OCV), calculates the maximum permissible current, and multiplies this by the number of battery units to determine an overall current limit. A battery unit was added at different state-of-charge (SOC) deltas (ΔSOC) (from S1 to S2 in equidistant steps). An optimization function was solved in discrete steps. The following results were obtained during the charging process. Strategy S1 (same open-circuit voltage) The added battery unit had a significantly higher state of charge (SOC) and, although it consumed less power, reached the upper end of the SOC range faster than the other battery units. Therefore, a single battery unit limits the total charging current and increases the charging time. The greater the differences in the state of charge (SOC) when switching on, the greater the continuous inequality of the currents. Although the open-circuit voltage (OCV) is the same during connection, a small peak current occurs because transient resistances dissipate during a resting phase and are not initially present when the battery unit is connected. The lower resistance results in higher currents. Strategy S2 (same state of charge SOC) A high initial peak current was observed. Since charging time is the primary parameter perceived by the user, the charging strategy can be chosen more freely: For example, one battery unit (or several battery units) is first connected and charged until a charging open-circuit voltage (OCVL) is reached, and then switched off again. Subsequently, the remaining battery units are charged using strategy S2 (same state of charge, SOC). The same observations were made for the discharge process as for the charging process. However, unlike the charging process, a strategy that initially only engages one or a few battery units might not be accepted by the user due to initial limitations in drive power. Therefore, an optimal point between the two objectives of strategies S1 and S2 should be found, which can be easily determined by a battery management system. The following cost function was used: is the Euclidean norm. ΔSOCmax and ΔOCVmax are manually configurable constraints. Fig. 4 is a schematic diagram of the scaled SOC delta ΔSOC and the scaled OCV delta ΔOCV as a function of the state of charge SOC of the connected battery units at the time of connection, taking into account the limitations ΔSOCmax and ΔOCVmax. For example, the battery units that were not connected had a state of charge (SOC) of 10% and were on the charging curve LK for the open-circuit voltage (OCV). The discharge process began for the connected battery units when the state of charge (SOC) reached 100%. Fig. 5 is a schematic diagram of the costs J as a function of the state of charge (SOC) of the connected battery units at the time of connection. For example, an optimum OPT was determined, according to which the other battery units were switched on when the already switched-on battery units had reached a state of charge (SOC) of 25%. Reference symbol list ELK discharge curve, open-circuit voltage curve J Cost LK charging curve, open-circuit voltage curve OCV open-circuit voltage OCVEL discharge open-circuit voltage OCVL charge open-circuit voltage OPT optimum S1, S2 strategy SOC charge level ST1 to ST7 step ΔOCV OCV delta ΔOCVmmax limit ΔSOC SOC delta ΔSOCmax limit

Claims

A method for interconnecting battery units of a battery system, characterized in that, when all battery units are on the same open-circuit voltage curve (LK, ELK), the battery units are interconnected at the same state of charge (SOC), wherein otherwise, when charging the battery units, the battery units are first interconnected at a discharge open-circuit voltage (OCVEL) and the battery units not on the same open-circuit voltage curve (LK, ELK) are disconnected when a charge open-circuit voltage (OCVL) is reached, wherein the remaining battery units are then interconnected and charged at the same state of charge (SOC), wherein, when discharging, the battery units are interconnected at a minimum SOC delta (ΔSOC) and a minimum OCV delta (ΔOCV) between the battery units. Method according to claim 1, characterized in that battery units not yet connected to the battery system are connected to the battery system during discharge when the battery units already connected to each other reach a predetermined state of charge (SOC) of, for example, 25%. Electrically powered vehicle with a battery system consisting of several interconnectable battery units and a battery management system, characterized in that the battery management system is configured to carry out the method according to claim 1 or 2.

Images