**5. Conclusions**

A novel portable DMFC power station has been developed for long-term field IoT applications with modularized semi-active stacks and mechatronic controls. Its configuration and controls are introduced in detail. The architecture of the active DMFC stack system was simplified to solve the problems of low manufacturing yield and high maintenance difficulties of active systems. Meanwhile, the EMS was redesigned to achieve coordinated controls on multi semi-active DMFC stack modules to increase configuration flexibility and power capability while overcoming the instability of the passive systems.

The prototype of the power station was tested for 3600 h operation in an actual outdoor environment throughout winter and summer with the ambient temperature ranging from 10 ◦C to 40 ◦C and a variety of weather conditions to verify its in-field feasibility and reliability. The causes of the 3 faults were discovered and resolved fundamentally. Therefore, the power station is proven to have long-term outdoor weather endurance, stable power generation, high system volume-based energy density, and easy portability. It is also enhanced with Li-ion batteries buffering and an output regulator to provide an intermittent high-power output drive. Therefore, it is feasible and marketable for remote mobile IoT applications.

**Author Contributions:** The project was conceptualized by K.-Y.K. and C.-J.C. under the project leadership of L.-D.T. Subsequent methodology development and validation was carried out by C.-J.C., K.-Y.K., and C.-J.L. on the EMS with mechatronics controls and data curation and analysis by C.-J.C. The academic writing was initially drafted by C.-J.C. and S.-B.J. and was later refurbished by C.-J.C. and T.-L.Y. for publication. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** We thank the Industrial Technology Research Institute of Taiwan for providing the resources, the technology of fuel cells, as well as their supports so that the development and verification was possible.

**Conflicts of Interest:** The authors declare no conflict of interest.
