A digital hybrid energy-storage system

Rutronik has developed a revolutionary new hybrid energy storage system (HESS) in collaboration with the University of Applied Sciences Zwickau (WHZ). The combination of battery and ultracapacitor improves the peak current characteristics of the energy storage system, minimizes battery degradation significantly, and ensures greater flexibility thanks to digital control. In a wide variety of applications, this technology additionally contributes to extremely high levels of reliability while enhancing the capacity of development effort.

A viable solution

The aim of the project for Rutronik was to prove that every battery system can be combined with ultracapacitors in a real working environment. It all boils down to the fact that this technology facilitates ideal distribution of the workload: While the battery works as a continuous supplier of energy to meet the constant power delivery demands, the ultracapacitor deals with the temporary peak currents and voltages. The discharge current of the battery is limited to its nominal value, thus ensuring it never exceeds its rated operating range. This "gentle operation" helps to increase system lifetime by up to 100%. Furthermore, there is less or no heat generated inside the battery, which inevitably extends the operational lifespan.

A battery-ultracapacitor pack can be recharged at any time, irrespective of the charge status, without damaging the battery cells, and delivers full power performance during its entire lifetime. A charged battery-ultracapacitor pack is immediately ready for use even after months of idleness, as ultracapacitors have an extremely low self-discharge rate. Discharged ultracapacitors can be fully recharged within a matter of seconds. Moreover, they have a very robust design and offer great performance even at temperatures below 0°C.

This increases system reliability considerably. This kind of hybrid energy storage system (HESS) is, therefore, also of interest for safety-critical applications, e.g. medical devices such as defibrillators. The design is additionally ideal for leasing or rental equipment that needs to offer a guaranteed service life. This includes all types of consumer power tools, from cordless screwdrivers to circular saws.

Ultracapacitors: robust and durable

This performance is influenced by the characteristics of the ultracapacitors: They charge and discharge extremely high rates of energy within just a few seconds. In stark contrast to batteries, they achieve a lifespan of up to 10 years and feature an ultra-high cycle life (at least 500,000 cycles). Furthermore, their extensive working temperature range from -40°C to 70°C means they are less temperature sensitive than batteries. One feature ultracapacitors lack is the ability to store large amounts of energy (energy density). Where these double layer capacitors do stand out from the crowd though is in their ability to cope with frequent deep discharging: Conventional Li-ion batteries have a depth of discharge (DOD) of around 25%, whereas ultracapacitors supply about 75% DOD. Unlike batteries, a DOD drop below this value will not have a detrimental effect on the ultracapacitor's long-term performance, as only the number of charge cycles is reduced (see Figure 1).

Click image to enlarge

Fig.1: Comparison of ultracapacitor – battery

In order to combine the best characteristics of both energy storage elements in just one system, the charge and currents of the battery and the ultracapacitor have to be measured and balanced by a hybrid buck-boost converter. This is based on the definition of threshold values for both energy storage devices while taking the respective characteristic curve into consideration.

Various basic topologies already exist for this type of system design, e.g. with a parallel battery/ultracapacitor configuration, with a bidirectional converter and the ultracapacitor on the primary side and the battery on the secondary side, or the combination of a unidirectional and a bidirectional converter. One thing all the topologies have in common though is that they are relatively complex and thus highly time-consuming and cost-intensive.

Averaging concept with boost converter

Both research partners decided the topology of a unidirectional DC-DC converter was the best way to minimize this level of complexity. This enables a comparatively compact and efficient circuit technology structure.  The development time and costs as well as the number of required components are reduced accordingly. In many respects, the system is far quicker and easier to customize due to the digital solution.

Other key advantages include the fact that the voltage at the inverter can vary within a very wide definable range. If necessary, the ultracapacitor can also be coupled directly and dynamically to the inverter in order for it to support peak currents. The DC-DC converter is only limited by the necessity of providing the peak current through a controlled diode (MOSFET). To guarantee optimum voltage adaptation, the higher voltage in the intermediate circuit can be set to a ratio of 2:1, i.e. the voltage at the ultracapacitor is twice as high as at the battery. In other words, the energy of the ultracapacitor is utilized to the full; it can supply 75% of its available energy at 50% of the voltage.

Topology of the demonstrator

Manufacturers of high-quality cordless power tools are always looking for ways to guarantee or increase battery life. Strongly encouraged by industrial support, the design engineers defined the platform for developing a demonstrator – a cordless screwdriver.

The demonstrator's topology is based on a combined buck OR MOS boost structure which had never before been applied within this context. It additionally incorporates fully digital power management, corresponding controllers, and end-to-end software configurable parameters. The higher impedance battery system therefore delivers lower impedance performance. The result:

-           Longer battery lifespan

-           Adjustable current limitation

-           Excellent high power characteristics

-           Battery's lifespan and state of health (SOH) can be estimated

Besides the ultracapacitors and the Li-ion battery that is connected to a primary power supply, the innovative power switching regulator forms the centerpiece of this topology. It is complemented by an ultra-fast current direction logic which engages as soon as energy starts to flow from the ultracapacitor. In addition, the analog current and voltage signals of the Li-ion battery and the ultracapacitor are monitored to ensure ideal power conditioning for optimum energy utilization.

The microcontroller defines the signal specifications and accordingly generates the PWM signals for the power MOSFETs and thus for the switched power supply. A special switch guides the current straight from the Li-ion battery to the motor when peak currents are not required. Suitably dimensioned, the ultracapacitor can be recharged by the battery at any time during breaks in operation (see Figure 2).

Click image to enlarge

Fig. 2: Simplified presentation of the topology

Control technology

Development of appropriate control algorithms was carried out by Prof. Dr.-Ing. Lutz Zacharias, Dipl. Ing. (FH) Ringo Lehmann, and Dipl. Ing. (FH) Sven Slawinski from the University of Applied Sciences Zwickau (WHZ). After in-depth system analysis and determined controller synthesis, accompanied by simulation-based preliminary inspections, the required time-discrete algorithms were implemented to meet target hardware constraints.

The latest methods of model-based software design were used to create the control software. As a result, the entire power management was modeled in VHDL-AMS. Using this standardized model description language, control systems can be modeled and simulated in line with the physical hardware and, once automated via Auto-Coding, transferred to the target hardware.

An additional, ultra-fast logic circuitry is necessary to ensure safe operation at all times. Since the safety and real-time requirements that are essential to delivering a compliant, viable solution cannot even be met by high-performance, fast microprocessors. A decision was therefore made to invest in hardware components, for example in the utilization of ultra-fast comparators.

The greatest challenge during the modeling and simulation stage was to describe and map the real characteristics of the controller, the battery, the ultracapacitor, and the power stages as exactly as possible.

Lean, affordable, and intelligent

A further simulation showed that the ultracapacitors would, except for a few specific situations, not actually require balancing in this application and that it is therefore not really appropriate. This obviously helps to reduce the complexity of the circuit – making it lean, affordable, and intelligent.

Once modeling had been completed, the entire system was simulated and mathematically analyzed before being accepted and implemented.

After setup, the demonstrator was subjected to a thermal analysis. The result: Even without a heatsink the temperature never exceeded 50°C. This indicates that both the hardware and the control parameters had been defined correctly without any discernible switching losses. Operation within the safe working temperature range, without any thermal stress, has an additional positive effect on the system's lifespan. This is only possible when using the developed buck OR MOS boost topology (see Figure 3).

Click image to enlarge

Fig. 3: The thermal image of the demonstrator indicates a slight temperature increase for the active components when operating at peak load.

The overarching aim of the research project was to show that the hybrid energy storage system actually works in real life conditions – and this was fully achieved through reliable operation of the cordless screwdriver.