battery power - 986 Forum - The Community for Porsche Boxster & Cayman Owners

O.K. this is a question from an idiot to smart types. Why can't engineering produce a charging system that is driven off the rotation of the wheels to produce a constant electrical current to charge the batteries, like a gas powered welder or generator? Be nice I already declared I'm the idiot!

The wheels spend a considerable amount of time stopped. The engine does not. Plus it's probably easier to mount to the engine.

Hybrids do this, but hybrids have a better system for storing electrical energy. Back in the day, there was work being done to use mechanical flywheels for energy recovery storage - imagine a big carbon-fiber wrapped, magnetically suspended flywheel spinning in a vacuum at thousands of RPM that produces power during acceleration and stores power during deceleration. It's a really cool concept, but 50-100 pounds of exploding carbon fiber maglev flywheel is a big drag. I maintain that the Boxster would make a cool electric conversion platform once 100HP in-hub electric motors become available and affordable.

For any Formula One fans, this is a big topic this year. KERS (Kinetic Energy Recover System) has been developed for exactly this purpose. As the car slows, energy produced from the braking action (through the transmission) is stored in batteries and released when the driver hits a button on the steering wheel to produce an extra burst of speed. Renault is reported to be the first to utilize the system come this weekend at the F1 opener in Melbourne...stay tuned (I can't wait for another F1 season).

Aside from as stated before, the wheels spend a considerable amount of time stopped, there is still the issue of power in = power out. Meaning, that a small amount of horsepower is used for the engine to turn the alternator. The same amount of power would also be used to turn a generator of sorts at the wheels. I think it's the law of kinetic energy or Newton's Laws of Physics...

ahh. F1 finally many changes can't wait to see if 6-10 can compete for the win. so the wheels don't rotate enough or can't produce enough power to keep batteries charged while driving or is the system too large to be efficent. It would seem like in a hwy. driving that there is enough energy to do anything you wanted once you were moving at speed?

Read more about KERS here: http://www.f1technical.net/articles/11805

It takes roughly 1 HP driving a typical alternator to produce 25 amps. Our OEM alternators then cost slightly under 5 HP when charging on full - rare, usually alternator charges about 50-65% most of the time, the rest it's 'freewheeling' and costing next to nothing in terms of parasitic draw on the engine. It wouldn't matter if this HP came from the crank (engine) or the wheels (which are driven by the engine), the parasitic loss would be the same. If you wanted to capture the momentum in decelleration and convert it to electricity, you still wouldn't be saving or gaining anything because the engine is still turning and any energy it produces is now lost, or more to the point wasted or unused. Electric or Hybrid cars gain an advantage here because they can easily shut off their drive motors on decelleration. I suppose you could invent a system to start/stop the engine in an IC car, but anything you'd gain in preserving crank HP or reducing fuel consumption (about 0.1 MPG at 100% charging) would be more than offset by additional wear or diminished service life to the motor (remember 80% of engine wear occurs at startup). More efficient alternators are the key. Current alternators are about 75% efficient which is really quite good in a mechanical system - an engine is usually only about 25% efficient - Yep that's right...fill-up your 16.9 gal. fuel tank, and only about 4.2 gal. worth of energy ends up actually driving the rear wheels - the rest is lost to mechanical innefficiencies like friction and heat. Most of the energy lost through an alternator comes from mechanical innefficiencies of the belt/pulley system and air resistance to the cooling fins, the rest to heat. Watercooling, as several GM/AC-Delco alternators now use helps some, but mainly extends alternator life as opposed to creating more usable power from the same energy. So in short, to answer your question: "Why can't engineering produce a charging system that is driven off the rotation of the wheels to produce a constant electrical current to charge the batteries, like a gas powered welder or generator?" It can... there's just no advantage in doing so.

I'd be very surprised if there's any real gain in such a system. In order for it to work, there'd have to be a pretty sizable electric motor(s) somewhere in the drivetrain (to act when the driver "hits a button..."). Such a motor would, especially in racing terms, be pretty weighty and the added weight (and subsequent parasitic draw) would probably more than offset any gain. It would possibly aid in acceleration for short periods only - an advantage in certain circumstances, only if the competition wasn't armed with the same technology, but at the expense of lower fuel mileage. It'll be interesting to see if it gets fully adopted or retained longterm.