DC/DC converter in lithium-ion energy storage for robots at ETH Zurich
Constant Current DC/DC Battery Charge Converter with Constant Voltage Input
In the context of a master thesis at the Robotic Systems Lab (RSL) at ETH Zurich, a customer-specific DC/DC converter (Demke DC Transformer) was developed and tested for use in a novel energy storage system.
Due to the high performance and strict weight requirements in dynamic walking robots, power density becomes a key parameter. In this project, this applies in particular to the galvanically non-isolated DC/DC buck-boost charging converter and the high-power battery.
The hybrid energy storage system was specifically optimized for the needs of the autonomous four-legged robot ANYmal C from the ETH spin-off ANYbotics. This HESS combines the high power density of lithium-polymer batteries with the high energy density of state-of-the-art type 18650 lithium-ion cells.
A high-power battery from Swaytronic supplies the load peaks, which occur during dynamic movements of four-legged robot. This enables discharge currents of up to 400 amperes. The high-performance battery is charged using the specially developed Demke DC Transformer with 48V output voltage at 11A and up to 520 Watt from the highly energy-dense lithium-ion battery modules. This enables more than five times higher peak performance compared to conventional Li-ion batteries at similar weight and only slightly reduced capacity.
The DC-Transformer 258E1-27.5-On (Buck-Boost DC/DC converter with 27.5V output, specially designed for stabilizing 24V systems) could be easily upgraded with a modified input voltage range from 30 to 58 VDC and an additional doubling of the power from 220 Watt to the required output power of min. 500W at 48V. Due to the low minimum input voltage, this new charging converter DC-Transformer 258E-50 can effectively use the energy from supercaps.
The new input voltage range of 30 to 58 VDC is also ideal for series connection of twelve lithium-ion cells.