Issue link: https://resources.mouser.com/i/1437660
18 ADI | Energy Storage Solutions Figure 7 shows a comparison of the long-term drift for a bandgap voltage reference IC and a buried Zener voltage reference IC. The initial measurements are calibrated for 0mV of error. Ten years of measurement drift is predicted from drift after 3000h at 30°C. The picture clearly shows a much better stability of the Zener reference over time, at least 5X better than bandgap reference. Similar tests for humidity and PCB assembly stress show the superior performance of the buried Zener over the band gap voltage reference. Another limiting factor for accuracy is noise. A car battery is a very harsh environment for electronics because of the electromagnetic interference generated by the electric motor, the power inverter, the DC-to-DC converters, and other high-current switching systems in an EV/HEV. The BMS should provide a high level of noise rejection to maintain accuracy. Filtering is the classical method used to reduce unwanted noise, but there is a trade-off between noise reduction and speed of conversion. Because of the high number of cell voltages to be converted and transmitted, the conversion time can't be too slow. SAR converters might be the preferred choice, but in a multiplexed system, speed is limited by the settling time of the multiplexed signal. In this case, sigma-delta (Σ-Δ) converters can be a valid alternative. The ADI measurement ICs use sigma-delta analog-to-digital converters (ADCs). With a sigma-delta converter, the input is sampled many times during a conversion, and then averaged. The result is built-in low-pass filtering to eliminate noise as a source of measurement error; the cutoff frequency is established by the sample rate. The LTC6811 uses a third-order sigma-delta ADC with programmable sample rates and eight selectable cutoff frequencies. Figure 8 shows the filter response for the eight programmable cutoff frequencies. Outstanding noise reduction is achieved by enabling measurement of all 12 battery cells as fast as 290μs. A bulk current injection test, where 100mA of RF noise is coupled into the wires connecting the battery to the IC, showed less than 3mV of measurement error. Cell Balancing for Optimized Battery Capacity Battery cells, even if accurately manufactured and selected, show slight differences from each other. Any mismatch in capacity between the cells results in a reduction of the overall pack capacity. To better understand this point, let's consider our example where the cells were kept between 10 percent and 90 percent of the full capacity. The effective lifetime of a battery can be significantly shortened by deep discharge or overcharging. Therefore, the BMS provides undervoltage protection (UVP) and overvoltage protection (OVP) circuitry to help prevent these conditions. The charging process is stopped when the lowest capacity cell reaches the OVP threshold. In this case, the other cells are not fully charged and the battery is not storing the maximum allowed energy. Similarly, the system is stopped when the lowest charged cell hits the UVP limit. Also, there is still energy in the battery to power the system, but, for safety reasons, it can't be used. It is clear that the weakest cell in the stack dominates the performances of the full battery. Cell balancing is a technique that helps overcome this issue by equalizing the voltage and SOC among the cells when they are at full charge. Cell balancing has two techniques: Passive and active. With passive balancing, if one cell becomes overcharged, the excess charge is dissipated into a resistor. Typically, there is a shunt circuit that consists of a resistor and a power MOSFET used as a switch. When the cell is overcharged, the MOSFET is closed and the excess energy is dissipated into the resistor. The LTC6811 balances each monitored cell using an internal MOSFET to control the individual cell charge currents. The internal MOSFETs enable compact designs, ADC filter programmable ranges and frequency response 8 Long-term drift comparison between buried Zener diode bandgap voltage references 7