|Valence U-Charge U24-12XP.|
The hybrid school bus features Enova’s parallel-architecture, post-transmission 80kW hybrid drive system, and is expected to offer double the fuel efficiency of a conventional school bus. The current version of the hybrid bus offers a choice of two battery systems—NiMH and lead-acid—as well as a plug-in configuration that uses a NiMH pack.
Enova is evaluating the Valence U-Charge XP Power System, announced earlier this year. (Earlier post.) The U-Charge XP Power System U24 model is a 12.8-volt nominal battery designed in the BCI Group 24 size that operates at standard 12-volt lead-acid voltage ranges. It offers a gravimetric density of 82 Wh/kg and volumetric density of 126 Wh/l.
The Valence batteries use a phosphate-based cathode material rather than the cobalt-oxide used in traditional Li-ion cells. Phosphates are stable in overcharge or short circuit conditions and have the ability to withstand high temperatures without decomposing. When abuse does occur, phosphates are not prone to thermal runaway and will not burn.
The U24 battery can deliver 1,500 watts continuous through a full discharge cycle and can handle peak loads up to 3,800 watts. The U-Charge XP Power System can charge in less than two hours and power is stored at a high charge efficiency.
Each U24 model includes integrated battery monitoring electronics and is used in conjunction with a U-BMS or application specific battery management system. As many as 30 of the U-Charge XP Power System batteries can be connected in a series for high voltage applications such as pure electric vehicles, hybrid electric vehicles, marine applications, and power back-up applications.
We selected Valence’s Saphion phosphate batteries for their inherent safety, which addresses the limitations of traditional lithium-ion chemistries particularly in large batteries like those needed for hybrid vehicles. In addition to safety, Saphion technology delivers the power and energy that these buses demand.—John Dexter, Director of Operations and Planning, Enova Systems