Nickel metal-hydride (NiMH) cells have been of considerable interest for the powering of unmanned underwater vehicles (UUV), primarily for the outstanding safety of their water-based electrolyte compared to the flammable organic solvents used in Li-ion cells. High energy and power densities, rechargeability, commercial availability, and low cost in the AA form further strengthen the NiMH case for integration into UUVs. While these factors are motivating, secondary cells generate heat during charge, which in excessive amounts is damaging to both the cell and nearby electronics. Our objective is to model the temperature (T) rise of NiMH cells caused by waste heat (Q) accumulation during battery charging according to the equation following this text.

Heat generation is a function of charge current (I), time (t), and the internal resistance of the cell (R). The galvanostatic intermittent titration technique (GITT)¹ is being used to investigate resistance as a function of the current , temperature, and state of charge. Calorimetric measurements are ongoing to investigate specific heat of the cells (c). The change in cell temperature (dT) can then be modeled for various discharge currents and masses (m) of cells². A Thermal model of NiMH cells configured at the battery level may be used to determine the safety limits for charge rate with respect to heat production during charging in UUVs.

1.Weppner, W. & Huggins, R. A. determination of the Kinetic Parameters of Mixed-Conducting Electrodes and Application to the System Li₃Sb. Journal of the Electrochemical Society: Solid State Science and Technology, 124(1997),1569-1578

2.Maleki, Hossein et al. (1999)Thermal Properties of Lithium-Ion Battery and Components. Journal of the Electrochemical Society, 146(3), 947-954