use gear-less transmission (IVT/CVT)
an automatic transmission with {variable ratio on the engine side} would save the energy and the battery last longer. the electric engine output (main shaft) could be connected to a gear with variable radius, while the wheels' receiving gear could stay of the same diameter; then, the more driver pushes to the accelerator (the more RPM rises), the more the engine-output-gear is squeezed into a smaller diameter, tansmitting more power on uphill. "chain excess consumption" could work as on bicycles; (I don't mean toyota's CVT here, please contact me for the technical drawings / mockup)