Why the heck don't any Hybrids / EVs use Ultra-Capacitors?

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TimeHorse

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May 13, 2010
Messages
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Why the heck don't any Hybrids or EVs use Ultra Capacitors for regenerative breaking? Surely these dense storage houses would be great for the short-term electrical input of the breaking system, non? I just don't get it, can someone explain?
 
TimeHorse said:
Why the heck don't any Hybrids or EVs use Ultra Capacitors for regenerative breaking? Surely these dense storage houses would be great for the short-term electrical input of the breaking system, non? I just don't get it, can someone explain?

I don't know anything about that, but there is a bus system that will use Ultra Capacitors. The capacitors themselves only give the bus a few miles of range (4 or 5 IIRC), but there is inductive charging at every bus stop and the capacitors recharge in the time it takes for the bus to board and disembark passengers.
 
TimeHorse said:
Why the heck don't any Hybrids or EVs use Ultra Capacitors for regenerative breaking? Surely these dense storage houses would be great for the short-term electrical input of the breaking system, non? I just don't get it, can someone explain?

They are not "dense storage houses". They have very good power rating but very low energy density. EEStor is all about that - they promise both high power of capacitors and high ED of batteries. Alas they seem to contradict some basic physics ...
 
evnow said:
TimeHorse said:
Why the heck don't any Hybrids or EVs use Ultra Capacitors for regenerative breaking? Surely these dense storage houses would be great for the short-term electrical input of the breaking system, non? I just don't get it, can someone explain?

They are not "dense storage houses". They have very good power rating but very low energy density. EEStor is all about that - they promise both high power of capacitors and high ED of batteries. Alas they seem to contradict some basic physics ...


Please don;t mention that name here, it could get ugly:)
 
For hybrids it makes sense - the short high-power bursts from regenerative braking can be absorbed by supercapacitors much more effectively than by any battery. For EVs, the battery pack is generally large enough that it doesn't have too much trouble absorbing the charge, but I think supercapacitors in tandem with the batteries would still help in absorbing the rapid power influx and then slowly transfer it to the batteries. Yeah, it makes sense - I don't know why they're not used more commonly.
 
johnr said:
Yeah, it makes sense - I don't know why they're not used more commonly.

I guess they don't yet make economic sense ...

http://www.ultracapacitors.org/index.php?option=com_fireboard&Itemid=99&catid=15&func=view&id=252
 
johnr said:
Yeah, it makes sense - I don't know why they're not used more commonly.

One reason is that current lithium cells are capable of handling the surges. Caps made more sense (well, the promise of caps made more sense...) when the best batteries we had were lead-acid and NiMh.

Current retail lithium polymer has a 45C constant output, a 90C burst, and a 10C charge capability. A123 LiFePO4 round cells are about 30C discharge and 4C charge. Even older 4C charge-capable LiFePO4 delivers more than 8000 hybrid cycles with little capacity loss or internal resistance gain.

Maybe the cap folks could have some market share if they'd gotten products on the streets 5 years ago, but I think they missed the boat.
 
evnow said:
They are not "dense storage houses". They have very good power rating but very low energy density. EEStor is all about that - they promise both high power of capacitors and high ED of batteries. Alas they seem to contradict some basic physics ...
I don't know much of anything about this stuff, but trying to put this in layman's terms it sounds like "low energy density" means that to have a 24kWh capacitor system you would need something "as big as a house." Well, not literally, but something a lot bigger than the Leaf battery. Is that true? Any rough idea how much bigger? (I have this hilarious mental image of a Leaf jacked up three feet on huge tires to make room for the capacitors underneath.)
 
Here's a guy in the DC area turning an EV1 into a series hybrid with supercaps: http://www.endless-sphere.com/forums/viewtopic.php?f=34&t=15794

These supercap modules reportedly hold about 250Wh usable (about 450wH total).
DSCF1415.jpg


DSCF1508.jpg



A quote from a physicist and battery pro: "You can easily put more energy storage into the pockets of your pants, and not need a belt to hold them up still by using [lithium polymer]."

Andy
 
Actually, the idea was keep the battery but since it can only load at, what, 90W or so, if the regenerative breaking produced more power than that, it would be wasted as heat. Of course, I will admit that if the LEAF has a 80kW motor and I assume for the LEAF the regenerative component would simply be turning the electric motor into a generator, I guess there's no way to recover more energy anyway, at least here. But if a higher-capacity generator was used for Regenerative Breaking, then Ultra Caps would definitely be wonderful at capturing the excess charge and releasing it back into the battery more smoothly. Anyway, that would be the idea. As someone said, this may be a better issue for the hybrids, depending on how their configured. It's really a question of how the Regenerative Breaking is configured.
 
What is the max regenerative power that can be captured - considering typical stopping deceleration, 3000 lb car and initial 50 mph speed ?
 
A Tesla Roadster will generate 80-90amps (DC) back into the battery pack during full regen. I believe this amount could be higher, but is limited because it would otherwise cause trouble with the Traction Control (imagine doing this on a slippery uneven surface). I have heard of current in the triple digits (low 100's) in the case of Toyota RAV4EV, but can't confirm. (Under full acceleration the Tesla will easily pull 600-700amps (DC) out of the battery (which is discussed as being 375V nominal, but that of course varies with SOC).)

And the Roadster will slow from 55 to 15 in about 700 feet (2,735 lbs curb weight) under full regen (that 80amp number).

Hope these data points help you.
 
It appears that the Ranger EV can move 200A into and out of the pack. That's well within the range of lead acid and NiMh - and certainly within the range of lithium.
 
80kW is 80,000 watts, a lot over "90W".

80kW is moving about 240 amps out of (or into) a 333v battery, ignoring losses.

The 240 amps in would be essentially a full charge (20kWh) in 15 minutes.

But, likely the regen rate is limited to some fraction of that. A 20kW (1C = one hour) regen recharge rate would be reasonable, but Nissan could have chosen to use a much smaller maximum regen rate.
 
LEAFer said:
A Tesla Roadster will generate 80-90amps (DC) back into the battery pack during full regen. I believe this amount could be higher, but is limited because it would otherwise cause trouble with the Traction Control (imagine doing this on a slippery uneven surface). I have heard of current in the triple digits (low 100's) in the case of Toyota RAV4EV, but can't confirm. (Under full acceleration the Tesla will easily pull 600-700amps (DC) out of the battery (which is discussed as being 375V nominal, but that of course varies with SOC).)

And the Roadster will slow from 55 to 15 in about 700 feet (2,735 lbs curb weight) under full regen (that 80amp number).

Hope these data points help you.


I have seen at least 300 amps on the ACP drive with full off-pedal regen (set on high). I was going down a large hill with 3 passengers and came to a stop on a dry surface as fast as traditional brakes could. The regen released at about 3 mph and the then I hit the brake pedal. I never touched the brake pedal until the last second or two. This was FWD so there was plenty of traction for the regen.
 
Wow, Thanks!
That is an example of really useful, usable amount of Regen.

There is no good reason that this amount of Regen could not exist (gradually increasing through the first half-inch of the brake pedal travel) before the mechanical brakes engage.
 
EVDRIVER said:
This was FWD so there was plenty of traction for the regen.
That's probably a key difference. I think the Tesla could do even more regen, but I think Tesla has decided that it's "strong enough" compared to conventional cars, and being RWD with Traction Control they are also being conservative about it. The fact that 2/3 of weight is in the rear may also be a factor arguing for less regen than what the drive train is capable of (basically a "reworked" ACP version).

(Keep in mind, Tesla's regen works purely off the "go pedal". The brake pedal -- if and when you need it rarely -- is completely friction brakes.)
 
I thought the issue with Ultracaps was that they have high power density (charge/discharge rate) but low energy density (storage). Wouldn't UltraCaps be the perfect instant power boost in an electric sports car?!? ;) What would today be like Nitrous for sports cars and sport bikes? Ya know, little red button on the steering wheel called "boost"? Hint, hint Nissan...? For a sports car...! HINT!!!
 
TimeHorse said:
Of course, I will admit that if the LEAF has a 80kW motor and I assume for the LEAF the regenerative component would simply be turning the electric motor into a generator, I guess there's no way to recover more energy anyway, at least here.
Leaf regen is limited to 30 kW max (about 1C) - presumably to preserve battery life. And as documented the amount of regen when the SOC is above 80% starts tapering off until there is none at 100% charge. So presumably, a series super-cap should allow one to increase regen up to 80 kW (max of the motor). If the super-cap had a capacity of 0.5 kWh, that would take about 20 seconds to charge to full from empty - more than enough to capture all the energy required to stop from any reasonable speed at a regen rate of 80 kW.

TimeHorse said:
As someone said, this may be a better issue for the hybrids, depending on how their configured. It's really a question of how the Regenerative Breaking is configured.
Definitely - combined with the typically low capacity of the hybrid pack (a couple kWh at most) which also limits max charge/discharge rates, even a small super-cap of a couple hundred Wh should significantly improve efficiency.

The big question is: at what cost/weight/complexity penalty? I suspect that in most cases it's more cost effective to simply increase the size of the pack a bit which will allow you to increase the power rating of the pack itself while not increasing the load on individual cells.

BTW - a lot of the previous discussion of maximum regen amps is not too informative unless we also know the voltage and capacity of the cells in question...
 
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