Battery Pack replacement

[Kobushi]

I was thinking today that it wuld be possible to scrap the whole charging system, and the NiHM pack... and replace it with an array of lightweight capacitors. Any thoughts? Good idea? Bad Idea?
(perhaps no scrap the charge control all together, but fool it so that it allows 100% charge and drain of the caps since they can handle it)

Thanks in advance

[Mike Dabrowski 2000]

The IMA stops working when the pack drops to 120v, the pack is full and accepts no further charge at ~180V(BCM controlls this)
The discharge curve of the batteries is pretty flat, so the batteries stay within that voltage window. Caps drop voltage quickly as they discharge, since they store charge electrostatically, so the caps would drop to 120V in seconds.
A complete new system would be needed to run off only caps. I believe Cap based systems have a voltage converter that allows the cap voltage to drop while keeping the output voltage constant.
So bad Idea with our present electronics.

[clett]

Honda initially wanted to use ultracaps in the Insight, but switched to NiMH batteries at the last minute. Ultracaps are still used to buffer the Honda Fuel Cell Vehicle and the Nissan diesel hybrid truck is ultracap powered too (Japan market only).

BMW also favour them, as their philosophy is that with batteries you can't use 100% of the available storage - only about 40%, so the rest is just being carried around as dead weight. Also, caps can be charged 0-100% and back to 0% again with huge power outputs and minimal losses, while also being good for millions of cycles, so in these respects are better than batteries.

You can buy ultracaps from Maxwell, but the main issue is the low energy density - currently about 3 - 5 watt-hours per kilogram. Compare that with the 46 Wh/kg of the Insight battery and you can see how heavy the ultracap pack would have to be to provide the same charge buffer (about 100kg before adding the DCC converter etc?).

[chrs]

Interesting that the Insight was originally going to have ultracaps.

Originally Posted by clett

You can buy ultracaps from Maxwell, but the main issue is the low energy density - currently about 3 - 5 watt-hours per kilogram. Compare that with the 46 Wh/kg of the Insight battery and you can see how heavy the ultracap pack would have to be to provide the same charge buffer (about 100kg before adding the DC:DC converter etc?).

Yeah, but the IMA works most effectively just to store regen energy and use to it accelerate again--25 kg of ultracaps could store the full energy to accelerate from 0 to 60, and you need less than that since you can't accelerate on IMA alone. So I think it would work very well if interfaced properly.

Charlie

[b1shmu63]

Even Maxwell does not recomend their caps for 100 percent of energy storage but suggests that they be run in parrallel with a tradditional battery. Ultra caps have the down side that they can produce almost infinite current if shorted. This makes them dangerous and serves no practical function as the motor and associated electronics can only draw a limited amount of current. Ultra caps would probably fail as often as Nmh batteries as the primary causes for failure are manufacturing defects and evaporating electrolyte (ambient heat related). Capacitors can not be used fully as the conversion electronics becomes increasingly inneficient as the source voltage decreases.

Ultra caps are an excellent idea for the future, but the control electronics and long term reliability remain to be proven.

[Foxpaw]

The current from the Ultracaps is easily limited by a resistor, removing most of the danger. Ultracaps are also not very prone to failure as they contain no electrolyte, nor are they "rolled" from discrete materials like electrolytics, so there are no pieces that can come apart if it is improperly manufactured. They are virtually failure-proof.

[b1shmu63]

Ok I was wrong. A quick check of Maxwell?s site shows that they are making a paper available describing a practical method of using only Ultracapacitors in a hybrid vehicle. The paper did note however that due to the low power density of existing Ultracapacitors, the system was only suitable at present for mild hybrid use for acceleration and braking.

The Maxell Ultracapacitor is constructed using a metal foil bonded to an activated carbon mat. The activated carbon is separated by a glass paper and layered or rolled into a container. They use Quaternary salt (tetraethylammonium tetrafluoroborate) and Organic solvent (acetonitrile) as the electrolyte. The electrolyte is flammable and toxic. The activated carbon consists of a sponge like matrix structure of microscopic scale. An aluminum backing is used to lower resistance and provide an electrical path. Ultra capacitors have an expected life of 10 years according to Maxwell. Life expectancy is generally limited by the integrity of the insulated seal on the capacitor. A better seal gives you a lower power density. It is a tradeoff.

If you put a resistor in series with an Ultracapacitor efficiency suffers.

[clett]

Re the whole safety and reliability issue - it's worth pointing out that ultracaps are out there on the roads already. The Nissan diesel hybrid truck as I mentioned but also Toyota's Prius. It uses them as an emergency energy store for the brakes. I think if Toyota are happy with the reliability and durability of ultracaps over the life of a car to look after something as major as the braking system, then we can be relatively confident they are quite reliable!

Originally Posted by b1shmu63

due to the low power density of existing Ultracapacitors, the system was only suitable at present for mild hybrid use for acceleration and braking.

Surely you mean energy density? Maxwell caps are at about 3-5 Wh/kg, which compares with about 46 Wh/kg for the Insight battery and >220 Wh/kg for lithium-ion.

The power density of ultracapacitors is spectacular - several kilowatts per kilogram is no problem.

This group are working on the next generation of ultracap - 10-20 fold improvement in energy density and 100 kilowatts per kg power density!

A capacitor like this weighing only 540 grammes could produce the same amount of power (73 bhp) as the entire Insight engine! (Although only for 2 seconds before running out of juice.... )

[IamIan]

the site you listed shows...

"Our analysis shows that the utilization of a matrix of vertically aligned CNTs ( Carbon Nano Tubes ) as electrode structure, can lead to an ultracapacitor characterized by a power density greater than 100kW/kg (three orders of magnitude higher than batteries), a lifetime longer than 300,000 cycles, and an energy density higher than 60Wh/kg."

Energy Density is what will determine how long it will run for ... so the 46Wh/kg of the current Insight NiMH Battery pack that is more than 7 year old technology has just about the same ( ~77% ) as the possible future "can lead to" theory number of 60 Wh/kg... the benifit would be in rapid use Acceleration and Rapid Charging Regenerative Braking.

Of course power density is higher in a cap... it had better be that and cycle life are 'almost' the only reason to use caps for anything... these theory numbers they are showing which are still all theory with no even protypes having been built...

Curent Li Batteries in production can beat the crap out of these theory Capacitors for Energy Density.... Heck Current 12 Ah D Cell NiMH batteries which are far from the best NiMH Batteries Have greater Energy Density than these Theory Caps.

Sorry Dude.... My Money is on NiMH and Li Batteries... Caps are good to make electronics.... Ultra Caps can be useful for 2 or 3 seconds worth of hard acceleration or braking.... and that is about it for Caps.

My 2 Bits

[Foxpaw]

I hate to rain on MIT's parade, but JEOL has already completed ultracapacitors packing up to 75 Wh/kg. In fact, they have already partnered with Nissan Diesel to bring the technology to market on hybrids. Won't they be disappointed when they finish their research and find that the technology is already obsolete.