The Shocking Truth About Start-Stop Systems | The Truth About Cars
Not having looked into the technology in any appreciable depth, Pacific Crests research was eye-opening. It turns out that the first generation of systems actually used variations of lead-acid battery technology known as absorbed gas mat (AGM) and enhanced flooded batteries rather than the newer NiMh or Li-ion chemistries. Though these technologies clearly offer advantages over standard lead-acid batteries (see comparison from Johnson Controls, above), they are still far from perfect. Pacific Crests research notes:
Gen1 AGM and enhanced flood batteries perform poorly, leaving future market share in doubt.
The start-stop battery cranks the engine 10x more than a traditional battery, and the lead-acid chemistry is simply unsuited for this workload. Current AGM and EFBs degrade rapidly, with AGM batteries losing half of the charge acceptance within two weeks after first use (i.e., it loses half of its fuel-efficiency gains). They are not good at holding steady voltage during a stopping event (e.g., car stereos/windshield wipers may not work when the car engine turns off).
Yes, you just read that right: the current generation of stop-start systems lose half of their benefits after two weeks. Which means theyre great for juicing up scores on Europes urban-efficiency test, but they are as good as useless for the vast majority of the life of the vehicle (we already knew they were less-effective in cold weather). In an industry that typically validates equipment for hundreds of thousands of miles of useable life, this is nothing short of shocking. Especially when you jump ahead and find out the OEM response to these concerns
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Thats right, the OEMs can live with it. Because they literally dont have to live with it: consumers do. How this issue has not erupted into a semi-serious scandal in Europe is beyond us. After all, if this is accurate and verifiable, it means manufacturers are building cars that may not actually comply with emissions standards in the real world for most of their lives. Whether consumers will live with this is one question
whether governments will is a very different proposition. But, as the paragraph above implies, manufacturers arent likely to abandon advanced lead-acid designs for some time. Luckily, though, the next generation of AGM batteries should be much improved
and they had better be!
Though ultracapacitors sound sexy, if the next-generation of AGM batteries improve to the point where stop-start systems are offering full benefits for longer than two freaking weeks, then we say bring em on. Meanwhile, every auto media outlet with stop-start-equipped long-term loaners (and possibly government emissions-control agencies) should be running regular tests to verify or disprove these troubling claims. A two-week lifespan for full-function on any automotive system is nothing short of misleading.