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17 Saving Power for the Future | ADI The Importance of an Accurate Measurement Accuracy is an important feature for a BMS and it is critical for LiFePO 4 batteries. To understand the importance of this feature, let's consider the example in Figure 5. To prevent overcharge and discharge, the cells of the battery are kept between 10 percent and 90 percent of full capacity. In an 85kWh battery, only 67.4kWh are available for normal driving. If there is a measurement error of 5 percent, to continue to operate the battery safely, the cells must be kept between 15 percent and 85 percent of their capacity. The total available capacity has been reduced from 80 percent to 70 percent. If accuracy is improved to 1 percent (for LiFePO 4 batteries 1mV measurement error translates into 1 percent SOC error), the battery can be operated now between 11 percent and 89 percent of full capacity, with a gain of 8 percent. With the same battery and a more accurate BMS, automobile mileage per charge is increased. Circuit designers rely on datasheet specifications to estimate the accuracy of a cell measurement circuit. Other real-world effects often dominate the measurement error. Factors affecting the measurement accuracy are: Initial tolerance Temperature drift Long-term drift Humidity PCB assembly stress Noise rejection All of these factors should be taken into account in order to deliver very high performance. Measurement accuracy of the IC is primarily limited by the voltage reference. Voltage references are sensitive to the mechanical stress. Thermal cycling during PCB soldering stresses silicon. Humidity is another cause of silicon stress as water is absorbed in the package. Silicon stress relaxes over time, leading to long-term drift of the voltage reference. Battery measurement ICs use either a band gap voltage reference or a Zener voltage reference. IC designers use an NPN emitter-base junction operating in reverse breakdown as a Zener reference. Breakdown occurs at the surface of the die, where the effects of contamination and oxide charge are most pronounced. These junctions are noisy and suffer from unpredictable short- and long- term drift. The buried Zener places the junction below the surface of the silicon, well away from contamination and oxide effects. The result is a Zener with excellent long-term stability, low noise, and relatively accurate initial tolerance. For that reason, Zener references are far superior for mitigating real-world effects over time. The LTC68xx family uses a laboratory grade Zener reference, a technology ADI has perfected over 30 years. Figure 6 shows the drift over temperature of the battery measurement IC error for five typical units. The drift in the full automotive range of –40°C to +125°C is less than 1mV. 6 5 LTC6811 measurement error vs. temperature. Battery charge limits WIRES are also prone to pick up noise, leading to the REQUIREMENT for additional filtering.