A heat-flow calorimeter was designed and built. Its measurements are 26cm×26cm×26cm (17.6dm3). 18,796 thermocouples are used to measure the heat-flow from the vessel walls to the outer walls, for which temperature is controlled within 0.01°C from 0 to 100°C by a refrigerating/heating circulator. Homogeneity of temperature in the measuring vessel is improved by a fan with constant power. The calibration was performed with an electric heater, with input power up to 50W. The device constant is 5.8954±0.0025WV−1; the time constant is 501.7±3.2s; the uncertainty is 0.6% at 2W to 0.06% at 50W. Nonlinearity of this calorimeter is analyzed. The calorimeter can be utilized for studies of kinetics and thermodynamics of physical, chemical and biological systems of decimeter-size. A Sony 26650 Li-ion rechargeable battery was tested with this device during charging and discharging, and an electrolytic cell with Pd-D2O was also tested with this device.