The reliability and lifetime of batteries remains an issue even though we have witnessed a good improvement in these departments in recent years. Aging of batteries is not an issue for consumers if only boost and regenerative braking is considered. The Prius was developed some years ago and at that time the Double Layer Capacitor (DLC) was too expensive for cars. Maybe therefore a Nickel Metal Hydrate (NiMH) battery was used. The next car generation may have DLC’s.