A hybrid vehicle includes a traction battery, an internal combustion engine, an electric machine configured to provide torque assistance to the engine, and a controller or a powertrain control system having a controller. The controller is programmed to respond to a percentage of state of charge (SOC) allocated for torque assistance. An allocation for torque assistance is a change in SOC of the battery attributed to a current flow to the electric machine for torque assistance. When the change in SOC attributed to the current flow is greater than a predetermined change, the controller will halt the current flow to the electric machine to cease torque assistance.