Battery and energy management in cars and trucks
Physically optimised low ohmic Manganin resistors developed by Isabellenhuette play a crucial role in the measurement of automotive battery currents.
As a result of ever increasing customer demands in relation to comfort and safety in the car and the pressure of competition among car manufacturers, the number of electrical loads in vehicles continues to increase with no end in sight.
Users dream of navigation and infotainment functions or networking their vehicle with their PC; manufacturers dream of x-by-wire functions, driver assistance systems and an autopilot in the vehicle.
The permanent consumption of electrical energy in a mid-range vehicle these days is around 1kW, and according to forecasts this is likely to reach 2 to 3kW or more within the next 10 years.
The situation becomes far more complex when developments in hybrid vehicles are taken into account.
In addition to all these innovations the vehicle of the future is also supposed to become better, safer, more comfortable and, for even better performance, use less energy at the same time.
The condition for the realisation of these developments is an electrical powertrain that provides the electrical energy required for each of the loads without fail whenever it is needed.
This in turn is only possible with a vehicle-wide energy management system in which all energy sources (battery, generator, engine, supercaps etc) and consumer loads (light, steering, stereo system, electrical heating etc) are monitored, controlled and used most effectively.
Therefore there is an essential requirement for a hierarchical subdivision of the loads, with time-dependent or situation-dependent dropping of each load by the electronics.
Although the battery has been viewed and used as no more than a passive energy storage mechanism in the vehicle for almost 100 years, the need for battery management - already standard in laptops and cameras for years - has become increasingly apparent as the number of battery-related breakdowns on the road has surged over the last 10 years.
To be able to calculate battery data such as the state of charge (SOC) and state of health (SOH) requires that the current, voltage and temperature of the battery can be measured to a relatively high level of accuracy.
The first attempts to measure battery current with magnetic sensors ultimately failed due to the extreme requirements involved, because on the one hand the sensor has to be capable of measuring the starter current of 1000A or more, at high speed, and on the other hand it also has to reliably measure the idling current of the vehicle with a resolution of just a few milliamps.
The first technical solution only came with the physically optimised low ohmic Manganin resistors developed by Isabellenhuette, used in combination with ultra-precise evaluation electronics.
Today shunts with values between 50 and 200uohm are used.
Despite the high requirements these can be manufactured at relatively low cost by punching electron-beam-welded composite material (copper-manganin-copper).
Here, the Manganin precision-resistance alloy provides the required low temperature coefficient, good long-term stability and the low thermal EMF against copper.
The latter is an absolute precondition for the required resolution for measuring current of just a few milliamps in the idle (quiescent) state of the car.
The measurement ASIC, also developed by Isabellenhuette in conjunction with a well-known semiconductor manufacturer, in principle provides a highly accurate, offset-free four-channel data acquisition system for measuring ultra-low voltages down to the microvolt range.
This combination bridges the gap between the two extremes in terms of technical requirements on the one hand and the high requirements of the automotive industry on the other hand concerning size, reliability and costs.
The technology is now widely accepted throughout the world.
BMW, together with Hella and Autokabel, was the first car manufacturer to develop an intelligent battery sensor (IBS) for mounting in the pole niche, starting serial production in 2003.
The subsequent of the ASIC has led to major cost savings compared with the discrete solution because the number of parts has been reduced considerably and installation simplified while achieving extended functionality at the same time.
Here, the sensor is connected to the grounding cable as a special battery terminal and communicates with the onboard electronics or the higher level control unit (EPM) via the LIN bus.
It therefore measures the entire charging/discharging current of the battery, and can assign it to the vehicle quiescent current or the consumption current of an individual device or unit when the generator is not active.
By performing a quiescent current analysis defective control units can be detected in advance in production, or the end-of-line test of individual loads can be improved or even be automated.
Similarly, the current/time performance can be used for diagnosing loads such as power steering, starters or simple lamps.
When driving, suitable evaluation algorithms are used to permanently monitor the SOC and SOH, ensuring that the battery is optimally charged at all times.
When not under load, the battery can then also supply electrical loads, hence reducing the no-load speed even further.
On the hand other, a battery which is not completely charged can also store energy temporarily which, for example, can be provided while braking by increasing the generator voltage.
The BMW application has therefore demonstrated that good battery management can not only guarantee the starting capability of the vehicle and increase the life of the battery, it is also suitable for making a contribution to the further optimisation of the entire energy management system in the vehicle and even to further reductions in fuel consumption.
Given the outstandingly successful experience with vehicles equipped with the system to date, BMW will be introducing the IBS in all new vehicle series starting in 2006.
Other vehicle manufacturers such as DaimlerChrysler and Audi will be following soon with similar systems.
As the sensor is switched within the main chassis ground cable, it can be basically used in trucks as well, but in reality the conditions are somewhat different.
Higher power consumption and the dual-voltage 12/24V system, plus special demands in terms of direct switch or relay outputs are not directly transferrable, and so special versions with a resistance of 30 to 50uohm are currently being developed, with the option of measuring both battery voltages.
Nevertheless, future versions of the battery sensor will profit from the further integration of data acquisition, MCU and bus drivers in a single package, which will further reduce the inhibition threshold for other vehicle manufacturers to apply them for serial production as well.
However, to fulfil the medium-term requirement for complete and accurate energy management, the measurement of the battery current alone is not sufficient, because this does not provide information about the current used in the vehicle or that produced by the generator.
One solution here could be to measure the battery and vehicle current simultaneously at a suitable central connection on the high side at which all current flows come together.
In this solution the starter current would, as before, be measured as battery current, and the generator current would be calculated as the sum of the vehicle current and the battery current.
The hardware for such a double sensor would only be slightly more complex and hardly more expensive than today's solution.
It comprises a double resistor (eg 100 plus 200uohm) with practically the same evaluation electronics as for the ground referenced battery sensor.
The idle current flows through both resistors and can therefore be determined much more accurately in the total circuit.
Although operation on the high side requires a separate power supply for the ASIC and a level convertor for the digital communication with the microcontroller, the advantages in terms of safety, reliability, functionality and integration possibilities for other required functions (such as safety shutdown of the battery connection, electronic fuses, shutting down and monitoring of individual main circuits and further diagnosis requirements) will be of greater importance.
The complete unit can be installed compactly and at low cost in a hybrid power package with bus bar terminals.
A high performance MCU provides the data measurement and evaluation functions and the switching functions.
Communication with the onboard electronics is via CAN or FlexRay, which means it is fast enough for a complete diagnosis of all loads on the PC for automotive workshops without the need for major investment in expensive and complex measuring systems.
For the diagnosis of individual main circuits it is conceivable that a multiple resistor positioned upstream of the fuses, with a common high side connection, should work, the measuring terminals of which can be switched to the evaluation electronics on a multiplexed basis in case of failure.
This would enable faults to be found very quickly and even permit the realisation of emergency running properties and self-setting electronic fuses.
Thanks to the availability of high quality shunts and evaluation circuits, shunt based current measurement in intelligent battery sensors is well established and will therefore see increased applications in cars of the future while contributing to further cost reductions.
For complete energy management, however, which meets all future requirements in terms of safety and reliability, the vehicle current must also be measured as well.
A low cost and space saving solution could be provided by using of a double resistor in an energy distribution box on the high-side to measure the battery and vehicle current which could also contain functions such as safety shutdown devices and electronic fuses as well as other measurement functions for individual main circuits.
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