Electric AC Cobra (looking to reduce high energy consumption - 489 Wh/mi)

[Galane]

Originally Posted by hamsterpower

A little late to the thread here. I know you said you had an alignment, but was it aligned for handling or efficiency? There is a big difference. I also really think you need some narrower, harder tires for the rear. Drag radials are way to sticky.

Most efficient is zero rolling toe-in, but it can make the steering twitchy. Getting to that can involve adjusting the static toe-in several times.

[ev99saturn]

Good question. I would assume that it was for handling. The alignment specs are from the kit manufacturer and show 1/16" for toe-in. The car handles very nice after alignment..... my eyeball-based alignment (when assembling the car) was just a bit off, at 1.5" toe-in!

I may have the worst possible tires for efficiency, and am considering a second set for this very reason. But for the moment, it will be interesting to see how much gain I can get while still running them. The calculator on this site really confirms the tire efficiency issue.

For the tire rolling resistance, there seem to be three components; 1) width of tire (how much air hits it), 2) inflation pressure, and 3) stickiness. Can anyone comment on how much of each of these contributes to the problem?

I'm thinking that a front air dam will reduce the air velocity at the front tires by quite a bit, but not affect numbers 2 & 3 above. So it would be interesting to understand the relative influence of each factor, and also, did I overlook anything regarding tire rolling resistance?

Originally Posted by hamsterpower

A little late to the thread here. I know you said you had an alignment, but was it aligned for handling or efficiency? There is a big difference. I also really think you need some narrower, harder tires for the rear. Drag radials are way to sticky.

[hamsterpower]

I'm not sure but I would think your drag radials would be off the charts on all three. Even if you over inflated them they still would be soft and sticky, puting a huge flat contact patch on the ground.
1/16" toe in does not sound to bad. What about camber and is the rear adjustable too?

[RE_Farmer]

ev99, I know a guy who had similar high Wh/mi on his BMW Z3. The Warp motors were originally developed for drag racing and the brushes that were usually installed were made of an H49 compound for HIGH CURRENT applications. Your Cobra is so light I doubt it's capable of drawing the current these brushes require.

When Tim switched to Helwig's "Redtop" brushes during a motor rework, he experienced a 15% improvement in Wh/mi. Here's his Blog:

evz3.blogspot.com/search/label/Brushes

The Redtop brush is a split or double brush made with a harder H60 compound. The split brush design maintains better contact with the commutator lowering resistance and consequently energy use & commutator temperature.

Tim's results were confirmed in testing by EVTV's Jack Rickard:

blog.evtv.me/2012/05/escaladus-interruptus-and-the-catellier-effect

You can tell which brushes are in your motor by looking at the number of wires to each brush - the Redtops have two wires (one for each half of the brush) while the stock H49 only has one. Jack has the brushes for sale on his site or you can get them from Warp motors. They're not cheap at ~$200/set, but they should save you money in the long run in energy (fuel) cost and, being harder, last longer.

It's more work but if you want to occasionally race your Cobra, you could switch to narrower Bridgestone "Ecopia", or a similar LLR tire and the Helwig Redtop for daily use; then swap to your current wide tires for shows and the H49 brushes for the occasional race.

[Ryland]

Originally Posted by ev99saturn

For the tire rolling resistance, there seem to be three components; 1) width of tire (how much air hits it), 2) inflation pressure, and 3) stickiness. Can anyone comment on how much of each of these contributes to the problem?

I'm thinking that a front air dam will reduce the air velocity at the front tires by quite a bit, but not affect numbers 2 & 3 above. So it would be interesting to understand the relative influence of each factor, and also, did I overlook anything regarding tire rolling resistance?

Low rolling resistance tires often have a silica rubber compound instead of a carbon rubber, as I understand it, this rubber has more texture to it while being hard so it's more like gripping with sand paper, there is also side wall flex and tread block shape.
Width affect load capacity, an example of this is that I am to big of a guy to ride a bicycle with racing tires on it because even at max pressure they create a longer contact patch then a wider tire does, because it's not just about contact patch size but how long it is, as the tire is always trying to fight that flat spot to roll, at some point of course you reach the widest tire that is practical for the weight of your vehicle and anything wider is more weight and aero drag.

[ev99saturn]

My WarP 11 motor does not have the Helwig red-top brushes. The Helwig red-tops were added shortly after my motor was built. They offer two brush types, H49 and H60, one for racing and one for street driving. The racing brush is softer, seats more quickly and is better at handling very high current. The street brush seats more slowly but lasts a lot longer. The brushes I have sit in between these two. I had a good conversation with George Hamstra about this a few weeks ago.

It is a bit surprising to see how much difference the Z3 owner observed with the change in brushes. I was not planning to change them, but given this information, I will have to consider that too.

Gee whiz.... all of these "round 2" changes to make and I haven't really finished with "round 1" of completing the initial car build!

Originally Posted by RE_Farmer

ev99, I know a guy who had similar high Wh/mi on his BMW Z3. The Warp motors were originally developed for drag racing and the brushes that were usually installed were made of an H49 compound for HIGH CURRENT applications. Your Cobra is so light I doubt it's capable of drawing the current these brushes require.

When Tim switched to Helwig's "Redtop" brushes during a motor rework, he experienced a 15% improvement in Wh/mi. Here's his Blog:

evz3.blogspot.com/search/label/Brushes

The Redtop brush is a split or double brush made with a harder H60 compound. The split brush design maintains better contact with the commutator lowering resistance and consequently energy use & commutator temperature.

Tim's results were confirmed in testing by EVTV's Jack Rickard:

blog.evtv.me/2012/05/escaladus-interruptus-and-the-catellier-effect

You can tell which brushes are in your motor by looking at the number of wires to each brush - the Redtops have two wires (one for each half of the brush) while the stock H49 only has one. Jack has the brushes for sale on his site or you can get them from Warp motors. They're not cheap at ~$200/set, but they should save you money in the long run in energy (fuel) cost and, being harder, last longer.

It's more work but if you want to occasionally race your Cobra, you could switch to narrower Bridgestone "Ecopia", or a similar LLR tire and the Helwig Redtop for daily use; then swap to your current wide tires for shows and the H49 brushes for the occasional race.

[RE_Farmer]

ev99, Just FYI, Jack also did an EV Cobra and did a comparison test between two sets (widths/brands) of tires for the Cobra.

blog.evtv.me/2011/10/lrr-tires-worth-it

Additionally, Ron Adamowicz's "Warp Factor III", an EV "camaro" dragster, uses A123 pouch cell pack. At last month's EVCCON, Ron experienced really deep voltage sag during a 1/4 mi. run (~80V sag on a 200V pack if I recall correctly, don't quote me on that). The speculation is all the individual connections between the pouches may be causing a high "internal pack resistance" resulting in large sag during high current runs.

[ev99saturn]

Thanks for the link to Jack's Cobra data..... hadn't seen it yet.

Ron's Camaro is not using A123 pouches, he is using cells made by Haiyin. They are a similar cell style and shape but use a different chemistry and have a lower per-cell capacity. I think they are 5 or 6 Ah compared to the 19.6 Ah for the A123 pouches, so more cells in parallel are needed. I'm running 4 cells in parallel, and I think Ron is running 10.... and he is now thinking about increasing that.

He is trying to squeeze everything he can out of all of his power train because the one and only goal is 1/4 mile runs. I'm interested in a few drag races eventually, but my primary goal is to have some fun with the car and keep it for a long time.

The A123 is nanophosphate lithium ion, the Haiyin is lithium polymer. The lithium polymer typically does provide a higher C rate for discharging, but is also has some safety issues that the A123 cells do not. The A123 cells are still no slouches when it comes to C rate. I have tested to 19C and the cells are rated to 30C discharge. This is quite good compared to the more popular prismatic lithium cells like Thunder Sky and Calb, which can do 3 or 4 C discharge rates..... and very briefly higher. The A123 cells can discharge at the full rate until they are drained.

Ron is also trying to pull something like 4,000 amps off his pack, which needless to say, is a huge amount of current. Most of the electric drag racers expect their sag under full load to be as deep or deeper than that, up to 50% sag or so. So I am not surprised to see Ron's level of voltage sag.

In my cell testing, and at the maximum planned battery pack load of 1,400 amps, my cells only sag about 22%. But I have left headroom in the design so as to not push anything to the breaking point or premature failure.

The interconnection between cells, groups of cells and packs has to be sized properly, and our homegrown battery packs probably leave room for improvement.

One thing I am watching closely is the internal resistance of my packs. Cell set #4 in my second pack is a little higher IR than I'd like to see, especially compared to all of the other cells. So it's possible that this too is part of my range-challenged car, where the voltage might crater earlier than the other cells and in turn, the BMS reports a lower State of Charge.

Originally Posted by RE_Farmer

ev99, Just FYI, Jack also did an EV Cobra and did a comparison test between two sets (widths/brands) of tires for the Cobra.

blog.evtv.me/2011/10/lrr-tires-worth-it

Additionally, Ron Adamowicz's "Warp Factor III", an EV "camaro" dragster, uses A123 pouch cell pack. At last month's EVCCON, Ron experienced really deep voltage sag during a 1/4 mi. run (~80V sag on a 200V pack if I recall correctly, don't quote me on that). The speculation is all the individual connections between the pouches may be causing a high "internal pack resistance" resulting in large sag during high current runs.

[RE_Farmer]

Looks like you've done your homework. I only looked at Ron's car and spoke with him briefly. Since many people have assembled A123 pouch packs that looked like Ron's, I thought his were too.

Not that you're looking for other cells, but you might look into CALiBpower's new CA cell. I've installed it in my MG and Jack has tested them at 10C discharging with ~15% sag +or-. I'm using CA100 cells, which are similar in size and wt. to SE100, but are much "stiffer" in the sag department. Unfortunately, I was a few parts short and didn't bring my car to EVCCON so I don't a data to offer you at this time. However, a guy called Damien from Ireland has tested the CA180 by dead-shorting a cell with a spanner. Check out his youtube video - search: CALB Grey Cell Test 01 : Half impulse. Warning: his method is not for the faint of heart - spanner turned white hot @ 2000A.

[ev99saturn]

10C in a large format prismatic is pretty nice! I will have a closer look at them. Your MG must run great with that pack.

Gotta go find that Damien short circuit video. During my cell testing, I made a white hot piece of 12 ga wire. Didn't take very long to change color! The a123 pouch didn't have any problem filling the 'load' of 18" of 12 ga wire.

Originally Posted by RE_Farmer

Looks like you've done your homework. I only looked at Ron's car and spoke with him briefly. Since many people have assembled A123 pouch packs that looked like Ron's, I thought his were too.

Not that you're looking for other cells, but you might look into CALiBpower's new CA cell. I've installed it in my MG and Jack has tested them at 10C discharging with ~15% sag +or-. I'm using CA100 cells, which are similar in size and wt. to SE100, but are much "stiffer" in the sag department. Unfortunately, I was a few parts short and didn't bring my car to EVCCON so I don't a data to offer you at this time. However, a guy called Damien from Ireland has tested the CA180 by dead-shorting a cell with a spanner. Check out his youtube video - search: CALB Grey Cell Test 01 : Half impulse. Warning: his method is not for the faint of heart - spanner turned white hot @ 2000A.