The next graph comes from the same Ford-BMW presentation and shows how the performance of a high-quality AGM battery degrades in a micro-hybrid duty cycle. The downward curving blue line near the bottom is the amount of current the battery can accept as it ages. The upward curving black line in the middle is the amount of time required for the battery to regain an optimal state of charge in preparation for the next engine-off opportunity. Once again the core lesson is simple – a micro-hybrid with a new battery can recover from an engine-off event in under a minute, but a micro-hybrid that has 5,000 miles on the battery will need five minutes or more to prepare for the next engine-off event. Micro-hybrids that can't turn the engine off because they're waiting for the battery to recharge can't save fuel or reduce air pollution.
Automakers understand the problem and their current solution is to disable the stop-start system when the battery hasn't returned to an acceptable state of charge. They also know that a short-term patch is not a long-term solution. Once you understand these two graphs, you'll understand why enhanced flooded batteries and even AGM batteries must eventually lose the battle for the micro-hybrid market. They just can't stand the strain.
The most important word in that last paragraph is "eventually." Automakers plan to build about ten million micro-hybrids this year and global production should ramp to 35 million micro-hybrids a year by 2015. There are several new battery technologies that are better suited to the micro-hybrid duty cycle, but they can't be manufactured in big enough volumes to make a difference over the next few years. That means automakers will be forced to settle for batteries they can buy in volume until the newer batteries are available at relevant scale. For the next several years, enhanced flooded batteries and AGM batteries will win the battle for short-term market dominance, even though they can't win the war...
