Smart Sensor Addresses Cell Safety, Diagnostics, and Balancing of EV Batteries
October 1, 2019
| By: Reno Rossetti
PhD., EE, Maxim Integrated
Remember the line in the Crocodile Dundee movie: “That’s a knife? This is a knife!” In the electric vehicle world that line translates to “That’s a battery? This is a battery!” Electric vehicles (EVs) are powered by huge battery banks, constructed of long strings of batteries in series (Figure 1). These battery banks, typically made up of lithium-ion (Li+) cells, can achieve operating voltages higher than 800V. However, the materials of this battery chemistry can be damaged if overcharged. Each cell voltage must be monitored and, if necessary, appropriate control methods must be applied to avoid overvoltage. Excessive cell leakage current, overvoltage, undervoltage, and extreme temperature can all lead to weaker performance or even catastrophic failure.
Figure 1. Electric Car and Its Battery Pack
The typical EV battery depicted in Figure 2 is made of 6720 Li+ cells managed by eight control modules. Each cell has a capacity of 3.54Ah, adding up to a total battery nominal energy storage of 100kWh (3.54Ah x 4.2V x 6720 cells). The series of 96 rows, each made of 70 cells in parallel, add up to a battery voltage of 403.2V (96 rows x 4.2V), with a capacity of 248Ah (100kWh/403.2V or 3.54Ah x 70 columns).
Figure 2. Daisy-Chained Battery Pack Stack and Control
This allows an EV to travel 300 miles at a speed of 50mph for 6 hours before exhausting the battery. The EV motor will draw an average current of 41A (248Ah/6h). In the daisy-chain configuration of Figure 2, all the control modules communicate serially with a central microprocessor via IC1. Isolation is required between the microprocessor and the first module, and from one module to the next. The data link must reliably operate in noisy high-power battery environments, where both the high dV/dt supply noise and common-mode current injection (induced by electromagnetic fields) are present.
Unfortunately, when it comes to battery packs, many things can go wrong. Weakened performance or even catastrophic failure can be triggered by excessive current leakage, high or low voltage, and extreme temperature of the cells. The manifestation of these faults varies with the battery cell configuration. In a series stack of cells, voltage variations are more readily spotted, while in parallel configurations, the leakage current becomes amplified. In a mix of series-parallel configurations, like the one in Figure 2, deviations in leakage current are more readily measured while voltage deviations induced by a single bad cell are attenuated and require measurements with a higher level of accuracy.
Now, you can’t really overcharge Li+ and lithium-polymer battery chemistries without damaging their active materials. In a string of cells in series, you have to monitor the state-of-charge (SOC) of each cell voltage. If needed, you must apply appropriate control methods to avoid overvoltage due to overcharge. Cells in parallel tend to be self-balancing since the parallel connection holds all the cells at the same voltage to prevent runaway voltage of a single cell. Accordingly, in a matrix of cells such as in Figure 2, the monitoring proceeds by a row of cells, rather than a single cell. Each module shown contains all the electronics necessary to perform balancing by means of arrays of switches and with resistors that are connected across the cell nodes.
The accuracy of the cell voltage measurement is important for safety reasons as well as for maximizing the battery capacity. Every millivolt of inaccuracy ultimately translates into diminished battery utilization. Accuracy is one of the parameters that weighs heavily in the battery’s state-of-health (SoH) and SOC.
Automotive Safety Integrity Level (ASIL) is a risk classification scheme defined by the ISO 26262 standard. There are four levels of risk identified by the standards ASIL A through ASIL D, with the latter corresponding to the lowest level of risk. An ASIL-compliant IC is equipped with the necessary diagnostics to detect specific fault conditions.
An automotive smart sensor data acquisition IC addresses concerns such as cell safety, diagnostics, and balancing. The MAX17843 is a programmable, high-voltage battery monitoring interface with extensive features for safety. The analog front-end (AFE) combines a 12-channel voltage-measurement data-acquisition system with a high-voltage switch-bank input. Each of the eight modules in Figure 2 is powered by a single IC. Two auxiliary analog inputs can be used to measure external thermistor components. A negative temperature coefficient (NTC) thermistor can be configured with the AUXIN analog inputs to accurately monitor module or battery-cell temperature. A thermal-overload detector disables the on-board linear regulator to protect the IC. A die-temperature measurement is also available.
To learn more, check out the MAX17843EVKIT, the evaluation kit that allows you to assess this part for your next automotive design. This blog post was adapted from a design solution, “Monitor Your Electric Vehicle’s Battery with a Smart Sensor.”