Forums

Ultra Capacitors

Ultra Capacitors

I was wondering if Tesla can increase range by adding ultracapacitors used in parallel with the batteries, using them for boosting, acceleration and leaving the batteries mostly for constant speeds.

Red Sage ca us | June 19, 2015

Roughly twenty years ago, Elon Musk hypothesized that super capacitors might be the answer toward enabling sustainable electric transportation. A few years later, he met JB Straubel, who explained, and demonstrated, that the theory was fundamentally flawed. Notice the Tesla Model S does not use super capacitors.

Any questions?

grega | June 19, 2015

Yes I'd love to know the arguments that proved to Elon that the theory was fundamentally flawed.

From what I understand, the limitations of batteries is that they can only charge and discharge at the same rate. So while you could design a lower power output battery pack (like the PowerWall uses) by adding super/ultra/hypercapacitors to enable the car's peak acceleration and regen, your battery wouldn't be capable of supercharging.

If you were only ever charging overnight, the argument changes. Tesla doesn't want that approach, so they're inappropriate.

But I'd like to understand other arguments.

Red Sage ca us | June 20, 2015

Using many cylindrical battery cells has advantages over super capacitors:
• Volume
• Energy Density
• Weight
• Durability
• Longevity
• Fine Control

All super capacitors offer in the plus column is quick discharge... As I understand it. But a quick search did turn up this list:

The advantages:

1. Unlimited cycle life; as compared to the
electrochemical battery, they are not subject to the
wear or aging.

2. On-hand charge methods; no full-charge circuit
required.

3. Quick charging times

4. Low impedance; by paralleling it with a battery, it
enhances the pulse current.

5. Cost effective storage; a very high cycle count
compensates the lower density.

The disadvantages:

1. Low energy density; usually holds 1/5 – 1/10 of a
battery.

2. Cannot use the full energy spectrum for some
applications.

3. Low voltage cells; to get higher voltages, serial
connections are required.

4. Voltage balancing needed; when more than 3
supercapacitors are connected in series, the circuit
needs a voltage balancing element.

5. High self-discharge as compared to electrochemical
batteries
Basically, it seems that super capacitors would be better paired with a hybrid ICE.

grega | June 20, 2015

Naturally enough the reason ICEs have big engines is for their peak acceleration. Tesla's big battery and motor allows for its peak acceleration too. But most of the time the engine/motor+battery is not pushed to that peak.

Most of the weak hybrids have a small electric motor and small battery so it simply can't do the peak output power, it needs the big ICE for that. So yes a super capacitor and larger motor could allow for a hybrid with a small battery, small ICE generator, and larger electric motor (and I think that would be a useful car temporarily somewhere in the spectrum).

The Tesla P85D outputs up to 515kW from the 85kWh battery, where Tesla has setup the powerwall to output smaller amounts (7kW peak from 7kWh battery), and I would assume that's because it's much cheaper to have those lower limits.

So rather than just useful for a hybrid, could a cheaper big battery with a super capacitor enable the same peak acceleration at a cheaper price? (given that with a super capacitor supercharging would be impossible).

Anyway interesting if it's useful for hybrids.

Red Sage ca us | June 21, 2015

Yeah. I think the result would be an efficient hybrid capable of 0-60 MPH in ~6.0 to ~7.5 seconds instead of ~9.0 to 10.5 seconds... But while still getting ~40 MPG. This would never be applied to a plugin hybrid variant of course, because the improvement in performance would make them too tempting. And once buyers got any exposure to useful electric drive they would want more fully electric range.

plafor | June 21, 2015

In theory, the main advantage of ultra capacitors in a Model S would be to optimize/increase regen, especially in cold weather. However, I don't think that ultra capacitors can work well in parallel with batteries because of their different voltages for the specific levels of discharge.

That said, for dual motor cars such as the 70D, 85D and P85D, it could be possible, when needed, to disconnect the front motor from the main battery and connect it to an ultra capacitor, leaving the main battery exclusively for the rear motor. This would allow both motors to operate at different voltages.

Brian H | June 22, 2015

A large capacity battery provides plenty of surge. Supercaps with the same ability are too large, and a waste of space.

grega | June 22, 2015

@Brian, supercapacitors are 10 times the volume, do you know the price differential on, say 5kWh?

@Plafor, is that right? You don't want to mix batteries and ultracapacitors? One thing to remember is that you would never need the capacitor to charge the battery, since the regenerated power will be used on the next acceleration.

@RedSage, that's what I'm wondering about plugin hybrid variants. If their performance was brilliant then you usher in the generation of tiny ICEs with small batteries, the natural next step is to grow the batteries... then lose the ICE.

david_edmonton | June 22, 2015

super capacitors make little sense for same reason that adding a steam engine to car to extract more energy from exhaust and/or coolant... they add much more weight and complexity than they help. (the efficiency losses from bigger and heavier car outweigh the gains)

if technology suddenly improves as start up companies have claimed for years but never yet delivered, then super capacitors could one day make sense... what is needed is device that can high volts rather than just 2V per cell combined with some cheap, fancy and reliable dc to dc converters.

On Dc To DC converter end, capacitors jump from zero volts empty to full voltage in linear fashion unlike batteries which tend to stay close to full voltage, so probably need fancy dc to dc conversion equipment. (DC to DC conversion is getting steadily cheaper and better, eg mppt solar battery charger)

david_edmonton | June 22, 2015

Existing super capacitor handles a low voltage, has very low energy density, for years there have been claims like "we have found new materials and way to handle 3000 volts rather than 2 volts per cell" in news, investors have pumped money into startup company, etc. Similar to "lithium air" batteries, the ideas might actually sort of work but be expensive and only last a few cycles

Red Sage ca us | June 23, 2015

grega: And shortly after losing the ICE, the super capacitors would evaporate as battery capacity increased.

greggenge | January 2, 2017

Super Capacitors (Caps) would make sense to "Augment" the existing Lithium Ion battery. The constant use and regen is not ideal for a chemical reaction battery, a capacitor is ideally suited for this and would handle millions of cycles easily while at the same time "saving" the long term health of the Batery. The way it would work is you would charge the battery as usual, the Capacitor would only act as a booster when accelerating making almost unlimited current available. Then with Regen, the Caps would take in all of the current to instantly save the energy for the next acceleration, for long downhills the batteries could still absorb extra energy as it does now. When you park, the control system could use the remaining Capacitor power to charge the battery, as the capacitor would lose the power 100 times quicker than the batteries. This would however require an augmentation of the power management system to include a summing output power regulator to assimilate power from both Caps and Batteries as needed as well as feeding power back into capacitors and batteries. I will have to see what I can find for specs on available capacitors. They could be placed in the lower rear below the rear deck of the hatchback where a gas tank would be.

TeslaTap.com | January 2, 2017

Downsides include significant increase in cost and complexity, and reduced cargo space. There is no gas tank in the Tesla. Better longevity is not a problem at least with Tesla and most other EVs. Batteries will generally last the life of the car (20+ years?).

Frank99 | January 2, 2017

The advantages of supercaps in EV applications are mostly theoretical; the practical implementations of EVs negate most of them. For example, a supercap can charge almost instantly, which might help in regeneration - but as a practical matter, existing battery packs can absorb regeneration power with only minor issues (no regen when cold, for example). A supercap can discharge almost instantly, which might help in acceleration - but in practice any Tesla (and specifically the PxxD models) show that acceleration isn't an issue with current battery packs.

Theoretically, someone might develop a high-density battery pack with low current capability - perhaps it would quadruple available range, but where discharge currents weren't enough to make the Ricky Racer types happy, and where charge currents weren't high enough to effectively capture regen power. In such a scenario, pairing the battery with supercaps to improve regen and acceleration would be great. But it's really not needed in today's EVs.