[ez64]

Just had some thoughts on how to minimise the energy lost breaking.

Has anyone made a cheap KERS system for our DIY cars (huge flywheel). If so build logs?

I am unsure of how you guys are setting up the current systems in place. I thought of a complicated but cheap setup of a clutch operated set of alternators attached to the rear diff. To activate these the brake peddle would only turn these on 30% down and apply normal brakes after. For this setup I would need an A/C system or a storage capacitor. Are there any light weight super-capacitors I could store a 40 minute journeys worth of braking?

Once stored inside an A/C system I could discharge this slowly to the battery's while parked without a charging point using the on-board charger.

I would rather a mechanical solution as it doesn't require more battery's but outside of racing I think a massive flywheel will be counter productive.

Could be completely wrong here but just trying to think of stuff, any help is appreciated.

[Ziggythewiz]

What part of huge flywheel would you expect to be cheap? Usually electronic switches are much cheaper than mechanical ones, so standard regen as commonly found on AC drive systems is probably the cheapest way to go. I've heard of rewinding an alternator and hooking it to the accessory shaft, but likely even less ROI than standard regen.

There are no light-weight super capacitors that hold 40 min of power, but you wouldn't need that anyway. The capacitors (you'd need lots) would just store it long enough to pump it into the batteries, or to use on the next acceleration. Crazy expensive and crazy heavy anyway.

You don't need more batteries to do regen. The regen fills the ones you already have. Unless you've found some ingenious way to drive for 40 min without using any battery power, you don't need any additional space to put it.

[dougingraham]

The prismatic cells can all accept charge at pretty much the same as the continuous discharge rates so 3 or maybe 4 C. Say you are using 100AH cells this means you can accept a continuous 400 amps regen charge. It isn't likely you will see regen rates that high and certainly not for very long. In a panic stop you would max out the controller but the batteries will not see the controller current unless the motor is at max RPM and that would be for a very brief time only. And in a panic stop situation you only care that you get stopped, not how much energy was wasted. Normal stops the batteries will easily accept all the regen energy you can get out of the motor/controller.

You lose at least 10% of the energy in the motor and probably another 10% in the controller. The batteries accept charge very well and lose about 1/2% at normal charging rates. I dont know how well they do at high rates but I would be surprised if they lost 5% at 4C. So maybe 25% of the regen energy is being wasted as heat.

You don't need anything exotic for regen on a street car. For most people it will give only a very tiny improvement in range. For the additional cost and complexity the benefit is not there. If you take the cash and buy batteries you will almost always get more range than any extension regen will provide.

[PStechPaul]

One major problem with a flywheel is its gyroscopic action which makes it difficult to turn the vehicle in any axis other than the direction of rotation (perpendiculat to the axis). You can use flexible mounting to allow it to "float", but then the mechanical connection to add and extract energy becomes more difficult. There is also the danger of failure and violent explosion, so you need to make it especially well balanced with good high speed bearings and a bulletproof blast shield.

Capacitors may be useful if you have lead-acid cells, and they are also needed to stabilize the DC bus at the controller to minimize ripple and high current surges through the connections to the battery pack. But such capacitors do not store much energy - just enough to keep the voltage fairly constant over one commutation cycle which is typically a couple hundred microseconds or so.

The total energy that can be extracted from braking can be calculated from mass and velocity. Here is a quick calculator:
http://easycalculation.com/physics/c...tic-energy.php

I used 30 m/s and 2000 kG and it is 900,000 Joules (W-Sec) or 0.25 kW-Hr. If you have a 144V system the capacitor needed to store this energy would be 86 F. A 3000F 2.7V capacitor is about $100:
http://www.newark.com/cornell-dubili...ohm/dp/92T9599
You would need 50 of these to get close to 144V and the capacitance would be 60F. Pretty close, but you pay $5000 to recoup 2.5 cents worth of electricity, or to get maybe an extra 1/2 mile of range.

There are some examples of KERS, including flywheel, at: http://en.wikipedia.org/wiki/Regenerative_brake

Please check my calculations, but they seem about right.