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Originally Posted by redpoint5
But it hasn't been shown that there is no fuel savings.
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There is no real data that says it has saved any. That is why its here
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Various sources suggest that lead acid batteries have about a 50% charge efficiency. There is an inverse relationship between charge efficiency and state of charge. At about 50% state of charge, the charge efficiency is over 90%. However, lead acid batteries are normally operated at nearly full charge, which has a charge acceptance efficiency of 50%.
Capacitors have almost a linear charge efficiency of 95+ percent. This should translate into fuel savings, albeit very small.
I'll also note that LiPoFe4 batteries are more efficient than lead at accepting a charge, especially near the full state of charge. I believe they are somewhere around 80% efficient.
No unicorns here, just science.
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Capacitors have an exponential discharge characteristic which leads to I^2 * R losses as they discharge. Batteries have a more linear voltage drop as they discharge. This characteristic becomes very prevalent on starter motor operation. Motor efficiency drops drastically in an undervolt condition (which is why thy tend to burn out when this occurs). In undervolt conditions motors draw significantly more current and I^2 * R losses go through the roof. Also most capacitors are not rated for extreme current discharges required for starting (i.e. you can kiss that 95% efficiency goodbye).
On your typical car trip, the energy used in starting the engine is negligible.
The Mazda system uses the capacitor system in an intelligent manner. It doesn't even try to use it for starting. It charges the supercap under braking which reduces energy wasted on the brake pads and rotors and stores it in the cap. It may also charge the cap up at idle taking advantage to try and boost the break thermal efficiency of the engine. It then releases it reducing alternator load on the engine and thus putting the power to the wheels saving fuel. It is thus a light duty hybrid.
I'm not saying that replacing a battery with a cap might not reduce fuel consumption in some infinitesimal amount, but you will never be able to show it.