Battery state-of-charge and state-of-health monitoring is normally thought of as technology that's most appropriate for hybrids, but at the SAE Convergence conference this week, BMW made a good case for including it on all motor vehicles. According to ACAD (the German equivalent of AAA), battery failures account for a full 53% of roadside electrical failures, and the situation only gets worse as increased electrical loads and fuel-saving techniques such as idle start/stop come into play.
The idea here is simple - by measuring voltage, current, and temperature, the amount of charge remaining in the battery can be assessed. Based upon this information, additional energy can then be provided by the charging system (via an increase in commanded idle speed), or non-essential loads (such as heated seats) can be momentarily disconnected until overall demand on the charging system decreases. Additionally, degradation of the battery over its usable life can also be monitored, with the potential to warn users of an impending battery failure before someone is forced to walk home.
The trick here is performing the measurements over a wide range of current (a few tens of milliamps during key-off and a draw of several hundred amps during cold-weather cranking) at an economical cost with a device that easily integrates with existing DIN-standard batteries. A rather slick piece of electronics consisting of a shunt, an application-specific integrated circuit (ASIC) based on a ARM7 core, 16-bit analog-to-digital conversion, local area network (LIN) serial communication, and a rugged package was developed by BMW and its suppliers. The device is now proliferating throughout the automaker's lineup, with Audi and Mercedes-Benz now also using the device.
[Source: BMW, Hella]
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