Micro-hybrid Energy Storage: One or Two Battery Solution?

Last week’s Advanced Automotive Battery Conference (AABC) in Pasadena, CA covered a range of 12V battery solutions for micro-hybrid systems. Micro-hybrid applications are getting more attention because they offer meaningful fuel economy gains for modest incremental cost. While the development of early generation micro-hybrids (or start-stop systems) focused on smooth engine restarts, next generation systems are looking to recover braking energy as a path to even greater fuel economy. Unfortunately existing lead acid battery technology introduces some design constraints because it can’t be charged very quickly and most of the vehicle’s braking energy is still lost. Energy storage alternatives for micro-hybrids were well discussed at AABC including direct one-battery replacement of today’s lead acid technology as well as hybrid approaches with two batteries in parallel. 

 Advanced lead acid   There are advanced versions of lead acid 12V batteries that support today’s start-stop vehicles, but they still fall short in dynamic charge acceptance capability.  Charge acceptance is important because it enables regenerative engine braking or “recuperation”, leading to better fuel economy.  The fact that there were no presentations at AABC about advanced lead acid batteries was a strong message that this solution, although low in cost, will not survive long term due to limited capability to contribute to the aggressive CAFE standard increases over the next decade.

 Lead acid + small lithium-ion   A low-cost traditional lead acid battery coupled with a small, high power lithium ion battery allows each battery to play to its strengths in the system.  The lead acid battery is relied on for lower cost energy storage to support long term parking requirements.  The more expensive lithium ion battery is reduced in size to allow for enough charge power for improved recuperation and discharge power for cold cranking.  A DC-DC converter is not required to regulate voltage between the two batteries if the lithium-ion battery is of lithium iron phosphate or lithium-titanate chemistries, which are able to be configured such that they match the lead acid voltage range.  However the integration complexity and additional hardware required is perhaps a weakness as compared to a single battery solution.  Additional vehicle weight and packaging is a concern as well since the vehicle must accommodate two batteries instead of one.

Lithium iron phosphate   The best performing single battery 12V solution available is lithium iron phosphate (LFP) in both the durability/life and charge acceptance categories.  It also provides a net weight savings to the vehicle which is an incremental benefit to fuel economy.  Two such solutions were presented by A123 Systems and GS Yuasa at AABC.  Significant strides have been made on cold cranking performance, a perceived weakness of LFP starter batteries.  From a technical prospective LFP is sound, but there are challenges with the cost of manufacturing LFP battery cells.  Dr. Menahem Anderman, AAB President, argued that a system’s incremental cost premium for advanced micro-hybrid solutions must be less than 200€/$270US in order to challenge the position of existing solutions.  Lithium ion batteries are on track to meet this target within the coming years.

At the end of the day, most vehicle manufacturers will prefer the least complex solution with proven technology and value.  Two-battery solutions come at the price of additional weight and real estate for a second battery, as well as complex algorithms to integrate the two batteries in the start-stop system.  Lead acid and lithium ion LFP batteries are the only single battery solutions on the market, and although lead acid is a lower cost solution, it is tapped out with regard to dynamic charge acceptance and contribution to fuel economy improvements.  Micro-hybrid vehicles with start-stop are forecasted to become main-stream by 2017 and thus LFP 12V starter batteries are positioned to capture a share of this market.  With a first generation battery in production and a second generation battery set to launch this spring, A123 can provide proven technology for a single battery solution with less mass, less package space, and less complexity than other competing solutions.