In stop-start applications, the battery more sought after than in pure ICE applications. Between the cranking phase, the zero current phase, the recuperation phase, the unloaded alternator phase, the fuel-consuming charging phase, the engine off phase, the after run phase, the sleep phase and the wake up phase, the charging and discharging scenarios are very different and need adequate testing in order to build the most efficiency system.
Hardware in the Loop system from Digatron Firing Circuits
To accurately simulate all of the possible phases of a stop-start system, Ralf Hecke, of Digatron Firing Circuits, developed a “hardware in the loop” (HIL) system. The laboratory test setup included two power circuits: one to simulate the sink, with a high current capacity to mimic the cranking phase, and another to simulate the alternator.
This enabled testing of the zero current phase with both circuits operating at the same time. To test different real-world scenarios, eight drive cycles were simulated with various combinations of probable operating phases and recuperation events.
Hecke’s laboratory work detailed the importance of simulating the sleep phase of the vehicle, which many other testing methods have ignored. Within Digatron’s simulated drive cycles are daily 12-hour periods of a parked vehicle, as well as a two-week parked period every sixth week.
Hecke found situations in which, after a two-week parked period, the relatively small load of the sleep phase would drain a 12 V lead-acid battery’s SOC below the minimum that’s required to enable the stop-start functionality. In this case, the vehicle would operate like a conventional car, burning fuel while idling to charge the battery.
The period of time it takes for the system to return to full stop-start functionality is directly related to the charge acceptance of the battery. And the charge acceptance is a key variable that’s clearly affected by the battery’s age, again highlighting the importance of accurate life predictions.
The test was originally developed to examine the limitations of using 12 V lead-acid batteries in stop-start applications – and the inadequacy of traditional static tests – but the same setup is suitable to test other battery chemistries, energy storage devices, or combinations of them.
Digatron plans to use its new testing system to more accurately compare and contrast the different energy storage options available to stop-start system designers. Through a precise simulation of the interactions between the battery, BMS, and energy management system, Ralf Hecke believes the advantages of using advanced batteries and capacitors will be made clearer.
“People are trying to find an energy storage device that always accepts a lot of energy. Ultracapacitors can do it, and so can lithium-ion batteries. There have also been many developments in advanced AGM and improved flooded batteries for stop-start applications. Some combine the use of lithium-ion batteries or ultracapacitors with lead-acid, so that during the recuperation phase they can always take a lot of charge,” Ralf Hecke explains. “The life expectancy of a stop-start battery depends on the region, but ideally, it is designed to match the life expectancy of a typical vehicle’s battery. “Here in Germany, we average 4-5 years for a battery”.This story was first published in the printed edition of ChargedEV.