Improved Battery Charging in an Ultracapacitor - Lead Acid Battery Hybrid Energy Storage System for Mild Hybrid Electric Vehicles

Recent announcements ^[1] of competitive pricing in the ultracapacitor industry indicate that the automotive industry should consider the use of these devices in conjunction with lead acid batteries for Hybrid Electric Vehicles (HEVs). Although ultracapacitors are becoming cost competitive with NiMH batteries, the technology is not yet capable of replacing the battery alone. However, the combination of an inexpensive lead acid battery and an ultracapacitor may indeed become competitive with the NiMH in performance and cost. Although lead acids have a lower energy density, their main problem for HEV usage has been the issue of sulfation during partial-state-of-charge operation. However, in this new system the lead acid battery can be maintained close to 100% state of charge to reduce sulfation and thus lengthen the lifetime of the battery. Two different methods for charging the battery are also shown along with experimental results.