John F. Christensen

Modeling Side Reactions and Nonisothermal Effects in Nickel Metal-Hydride Batteries

Previous modeling of the NiMH chemistry has not comprehensively covered the phenomena occurring in NiMH batteries, and model validation has focused on the low rates characteristic of electric vehicles. We have expanded a previously developed Fortran battery model to treat the NiMH system rigorously. The model now includes material balances (in the superimposed solid, liquid, and gas phases), Ohms law (in the solid and liquid phases), kinetic expressions (for the main insertion and side reactions), and a charge balance. Other features include a rigorous energy balance, appropriate temperature-dependent properties, and the ability to treat a variable solid-phase diffusion coefficient. The model can now simulate the following experimentally observed features of the NiMH system: overcharge protection, self-discharge, and pressure-voltage-temperature coupling. Model validation has been carried out at a variety of rates (charge/discharge times from 6 min to 20 h) on a battery module removed from a 2005 Toyota Prius. We have constructed an optimized Ragone plot that allows comparison of the performance of the NiMH chemistry to lithium-ion chemistries and shows the NiMH chemistry to be a capable high-power system. Our simulations also show how aging of the NiMH system may lead to significant generation and venting of hydrogen gas.