Digitally controlled hybrid energy storage system

By Petar Grantcharov, Solution Business Development Manager, Rutronik 

HESS essentially consists of a battery and an ultracapacitor, as well as ultra-fast control and regulation electronics. This combination allows an optimum division of work: the battery, in this case a lithium-ion battery, with its high energy density, supplies the energy for the average power of the application on a permanent basis. The ultracap takes over the short-term peak currents. Thanks to its high performance, it can also supply high currents for a short time without being damaged. It is then fully charged again within a few seconds. Its lifetime of ten years and at least 500,000 charging cycles is many times longer than that of a battery, and with an operating temperature range of -40 to 70°C, it is also significantly less temperature-sensitive than batteries. This also makes the overall system more robust and can even be used at temperatures below 0°C without any loss of performance. However, the key to the high-performance energy system lies in the optimised control circuit. 

Figure 1: Typical load profile of an e-scooter with numerous current peaks above and below the zero line (Source: Rutronik)

Dynamic load profiles with high current peaks 

To understand how HESS works, it is useful to look at the real load profile, e.g. of an e-scooter (Figure 1). Here you can see numerous current peaks, especially when the motors start up. There are also peaks below the zero line that are caused by high recuperation currents. They all have a very steep gradient with a short peak overall, the load profile is extremely dynamic with current changes in the microsecond range.  

In the 48V electrical system, which is increasingly becoming the standard in small vehicles in particular, there is a strong imbalance between the average power and the peak power. For example, electric power steering requires an average power of between 0.25kW in a relaxed driving style and 0.46kW in an aggressive driving style. At the peak, e.g. with electronic steering intervention, 1.5kW is achieved. The ratio of average and peak power is therefore around 1:6 and 1:3 respectively. The gap widens even further with the electric turbocharger: here, average power of 0.53kW (relaxed driving) and 0.71kW (aggressive driving) are offset by peaks of between 7.5 and 10kW – resulting in a ratio of around 1:16 and 1:12 respectively. Across all applications in the vehicle, only between 3.55 and 4.33kW are required on average, depending on the driving style, but between 25.3 and 27.8kW at peak times.  

For the developer, this raises the question of how large the dimensions of the battery should be. If he chooses a battery with a relatively lowcapacity, which is more geared towards average performance, it will have to cope with many currents above its nominal current. This will cause lasting damage to the battery and significantly shorten its service life. A battery with a higher capacity runs more frequently within its specified range and therefore has a longer service life. In return, however, weight, volume and costs increase.  

Figure 2: The patented OR-MOS circuit can predict current thanks to ultra-fast detection of current rise rates and switch between battery and supercap within a few seconds. (Source: Rutronik)

This dilemma can be solved with HESS. The system limits the discharge current of the battery to its nominal current, e.g. to 10A, so that it only works in its optimum operating range. The supercap takes over the peak currents. For this purpose, a sensor measures the rate of current rise (di/dt) within nanoseconds. The measurement data is fed into the digital control circuit, which switches the MOSFET within a few microseconds before the current peak actually occurs. This ultra-fast OR-MOS circuit enables the supercap to take over the current peaks.  

Three examples illustrate how HESS works (Figure 3):  

Figure 3: The OR-MOS circuit regulates the current flow depending on the load's current requirements. (Source: Rutronik)

Detection in nanoseconds for the MOSFET circuit in the microsecond range 

Figure 4 illustratesthe time levels at which these processes take place. The energy status is monitored in the range of seconds. When ‘power sharing’ two energy storage units, as in HESS with battery and supercap, switching between them takes place within a few milliseconds. The MOSFETs themselves are switched in the microsecond range; the switching speed is only limited by the gate propagation time of the logic arrays. 

Figure 4. Ultra-fast detection in the nanosecond range enables the MOSFETs to be switched within a few microseconds. (Source: Rutronik) 

To make this possible, the patented circuit from Rutronik and the university detects the rate at which the current rises within nanoseconds and can therefore predict current peaks. This can also be used for reverse and recuperation currents to charge the supercap. The system also has interlocking logic, i.e. the MOSFET switches block each other to prevent a cross-short circuit in bridge circuits or the parasitic diode in the MOSFET.  

To achieve this, the digital control circuit combines an ultra-fast logic circuit with the fastest possible control algorithms in power electronics, which Rutronik developed together with Prof. Lutz Zacharias and his team at the University of Applied Sciences Zwickau, as well as ultra-fast sensors. Rutronik calls this the Buck OR-MOS Boost topology. With this, HESS achieves an ideal balance between energy and power density, capacity, cost, weight, and volume.  

Together with its research partners, Rutronik has now developed a new reference design that is particularly suitable for the micro-mobility sector (e.g. cargo bikes, last-mile vehicles, intralogistics, etc.) in the 48V supply voltage range. Rutronik also provides customers with the complete bill of materials (BoM). In addition, all components used – from semiconductors to supercaps and lithium-ion cells – can be found in the distributor's product portfolio. 

This article originally appeared in the June'24 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.