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power loss for each shunt. Meanwhile, the third auxiliary channel can take either fast measurements of selectable buffered inputs or auto-round-robin (RR) measurements of two configurable inputs, stack voltage, die temperature, supply voltage, and reference voltage. When any of the LTC2949’s three data acquisition channels are configured for fast mode (782µs conversion time and 15-bit resolution), the LTC2949 can synchronise its battery stack voltage and current measurements with cell voltage measurements from any LTC681x multicell battery monitor to infer individual cell impedance, age and SOH. With this information, stack battery life can be assessed, since the weakest cell ultimately determines the SOH of the entire stack.
SOH is a point in the life cycle of a battery (or battery stack) and a measure of its condition relative to a fresh battery, it is important to use accurate EV BMS monitors to not only maximise driving range, but to minimise unexpected battery failures.
Digital Dominance The LTC2949’s digital features include an oversampling multiplier and accumulators, producing 18-bit power values, and 48-bit energy and charge values — reporting minimums and maximums, as well as alerts based on user-defined limits. This unburdens the BMS controller and bus from the task of continuously polling the LTC2949 for voltage and current data, and the additional task of performing computations based on the results. By taking power samples at the oversample ADC clock rate (predecimation filter) instead of multiplying averaged values, the LTC2949 accurately measures power in the presence of current and voltage variations far beyond its conversion rate, with signals up to 50kHz.
Because the LTC2949 tracks the minimum and maximum values
Figure 1: Distributed EV BMS monitoring topology using LTC6811-1s and an LTC2949
www.electronicsworld.co.uk February 2021 07
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