A thermal storage device for batteries is provided. In some examples, the thermal storage device is provided for a battery pack that includes one or more rechargeable battery cells. In some examples, the lithium ion battery cells are used. The thermal storage device is in thermal contact with the battery cell. The thermal storage device is made of a material that absorbs heat that is given off by battery cells during discharge. In some examples, the thermal storage device is made of a plastic material. The material has a relatively low thermal conductivity but a relatively high specific heat capacity, allowing heat energy to be stored in the thermal storage device. The thermal storage device prevents the battery pack from overheating during use. The thermal storage device defines one or more cell receiving volumes. In some examples, an axially extending relief volume is provided.