Regenerative braking capacitors!

[Xaero]

What Brammo needs (and other e-bike companies) is to use this idea that Mazda is doing.  Turn braking power into take off power instantly.

http://www.motorauthority.com/news/1069983_mazdas-regenerative-braking-system-switches-batteries-for-capacitors

[FreepZ]

There are a few ways to recover energy when breaking.

Here's one that uses a flywheel

The trick is coming up with a way to efficiently convert that energy into something useful. Ideally the transfer would be so good that all of the momentum of the vehicle would be saved and there would be no need to use standard breaks (which convert the momentum into heat), but I don't see that happening any time soon.

I would love it if we could replace batteries with capacitors (they have super quick charge times and have a long lifespan). Sadly there are a few issues at the moment that prevent them from being a viable option: besides not being able to hold a charge for long periods of time (as mentioned in the article), capacitors have a lower energy density than chemical batteries. Also, chemical batteries maintain their voltage as they discharge and only drop when they are depleated. Capacitors drop voltage fairly quickly. Perhaps in the future some smart scientists can figure that out and make it work.

Using capacitors to store the energy from breaking is a good idea (no issues with the gyroscopic effect of a flywheel), but it may not be feasable for a motorcycle with limited weight or volume to work with.
 
As for what Brammo needs to do; I think the most important thing right now is to deliver a usable bike! (Where's my Empulse?!) Save the improvements for next year.

[Richard230]

Amen to that FreepZ.   

[Xaero]

Doesn't taking off take more energy than maintaining a constant speed?  If you discharged whatever energy was gained from braking instantly when you take off wouldn't that save some battery life?  Caps may not hold charges long or hold as much as a battery but they are very light and discharge quickly.  It would seem that they could be a great thing to assist batteries but not replace them.

[FreepZ]

Doesn't taking off take more energy than maintaining a constant speed? ...  It would seem that they could be a great thing to assist batteries but not replace them.

You are right! It's very encouraging to see that there are some sure-fire wins out there. I think that there are still a bunch of technical hurdles that need to be overcome, but in the future, adding a few capacitors to collect breaking energy to be used for reaccelleration is going to be ubiquitous for EVs.

From the article, it looks like the Mazda isn't an electric vehicle and is only using that stored energy to power things like the A/C and stereo when the engine is off (which happens any time the car is stopped at a traffic light, or something). That makes sense since there's no need to keep the engine running when the car isn't moving. And using capacitors makes sense since the poor lead-acid battery isn't going to do too well if it has to run all those electric devices for any length of time.

[860]

The problem with capacitors is that they don't have constant voltage.  A 12 volt battery stays within a relatively narrow range of voltage throughout it's usable charge range.  So a battery at 50% usable charge will be at nearly the same voltage as a battery at 100% charge.  But a capacitor at 50% charge will have roughly half the voltage of a fully charged capacitor.

If you want to power an electric motor that expects 72 Volts constant directly on a capacitor, you would need to convert that dropping capacitor voltage into constant 72 volt power. 

If the battery can accept charge fast enough, it's easier to just sent the regenerative braking energy straight into the battery.  With modern fast-charge batteries, this is a significant amount of energy.  Capacitors can capture the excess energy between the maximum charge the battery can accept, and the total amount of regenerative energy the braking system can produce.  In a racing application, where there is repeated hard braking, this could be a significant amount of energy.  But on a day-to-day commuter run, this probably isn't enough total energy to justify the added complexity of adding capacitors.  It might be easier to just work on faster charging batteries.

[protomech]

I agree. On something like the Empulse, you're probably getting around 20kw at most into the storage device. High charge-rate batteries should be fine with a short duration 2-3C charge if partially depleted.

Capacitors make more sense as an assist for hybrid vehicles with smaller batteries, or as in mazda's i-eloop, a conventional gas vehicle with an alternator. A car at highway speeds has around 200 wh of kinetic energy, you can probably recover a bit more than 100 wh into a storage device. If you charge a capacitor from half voltage, you can store about 75% of the rated energy - so 133 wh total storage. Maxwell's best ultracaps are 3 wh/kg, so you need a 45 kg (100 lbs) ultracap pack to store the energy.

The 1.3 kwh NiMH pack in the 3rd gen prius weighs about 30 kg. Much more storage, and a bit lighter.. but the ultracap vehicle would probably be more fun to drive. Consider the following:

3rd gen Prius has a 98 hp (73 kw) gas engine and a 80 hp (60 kw) electric motor (MG2). The power output of the motor is limited by the battery pack output (27 kw = 20C), giving a total system power of 134 hp (100 kw). An effectively 100 wh ultracap pack could power the MG2 motor at full power for 5-6 s alone, or 10-11 s assisting the battery. Total system power is now 177 hp (133 kw), so maybe 2 s faster 0-60 mph. F1-style push-to-pass would be pretty fun in a street car : )

[860]

Very good points.  They could be like installing nitrous in a gasser.  Tons of fun!

It also helps when the front wheel can be used to collect the regen energy, because weight shifts to the front wheel allow more resistance for regen before the tire loses traction.  That gives FWD EV cars an instant advantage in regen over a RWD motorcycle. 

When I'm at full braking on my SV650, the rear wheel is just barely holding it's own weight to the ground, and would be worthless for collecting regen energy.  I don't even dare touching the back brake or downshifting until I can ease up on the front brake.  Sure, you could add front wheel regen along with capacitors, but the cost/return gets questionable for any situation other than trying to squeeze out every last watt of charge on track days or race days.  When just commuting or riding curves at 9/10ths pace, rear wheel regen directly back to the battery would probably capture the majority of the regen energy.