Question about EV motors.

[omgwtfbyobbq]

I've been dicking around with the idea of a diesel/electric bunny, assuming I manage to get the Cd around to an acceptable number (~.25-.3) and run LRR tires. If I can do this, then assuming 90% motor efficiency, I should only need 6 t-105 batteries to cruise ~45miles@45mph, assuming the one hour rate is something like ~150ah compared to 225ah@20hr and 185ah@5hr. This would allow for short range EV mode, and longer range diesel mode, with the only downside being a ~500-600lb weight penalty. Could I use something like a 400amp controller/36V ~10-15hp golf cart motor connected to one of the rear wheels via chain drive? Any ideas?

[vegasjetskier]

If you can get your power consumption down to 300 Whr/mi. (a reasonable number for a Rabbit, but you very well might do a lot better), you will need a: 300 Whr/mi. X 45 mi. = 13500 Whr battery pack to go 45 miles. The pack you propose to use will give you: 150Ahr X 36V = 5400 Whr, so you will only be able to go about 1/3 the distance you want to.

Your controller will provide: 400A X 36V = 14400w. 14400W/746w/hp = 19.3 HP. That should be plenty to propel your car @ 45 mph. Just make sure that the controller is rated for continuous duty at the amperage you expect to run - some list the peak output that is good for a couple minutes, then it's downgraded for continuous use.

Blue skies,

Patrick

[omgwtfbyobbq]

300 Whr/mi is almost spot on for a stock Rabbit cruising at 45mph, how'd you know? Ideally I'd like to get the car around to 80-100 Whr/mi cruising at 45mph, but I'm not sure if that's possible, so we'll see. In any event it seems that the transmission will be toughest to rig... Thanks for the advice, this seems doable!

[vegasjetskier]

Just a SWAG on my part. I think if you want to go 45 miles, though, you're going to need a bigger pack or a lot more efficient car. I think one guy got his Karmann Ghia down to about 100 Wh/mi, but it's a lot more aerodynamic than the Rabbit. There was a site with a guy adding an additional wheel to his car just for battery power. I'll see if I can find it and post the link.

Originally Posted by omgwtfbyobbq

300 Whr/mi is almost spot on for a stock Rabbit cruising at 45mph, how'd you know? Ideally I'd like to get the car around to 80-100 Whr/mi cruising at 45mph, but I'm not sure if that's possible, so we'll see. In any event it seems that the transmission will be toughest to rig... Thanks for the advice, this seems doable!

[omgwtfbyobbq]

That is f'ing brilliant! It could even be retractable like landing gear, with a spring or strut on it so it'll stick to the ground well, but still be able to move with the suspension. As for the energy requirements, I plotted curves using "stock" Cd and Crr, compared to an efficient version of the same car (belly pan, wheel covers, maybe airtabs if they work or a boattail, etc...), and if I can cut my Cd in half, I figure less than 100Whr/mi should be doable. Ultimately after all is said and done, I should be able to compare my before/after diesel cruising mileage to figure out how much I increased efficiency and base a specific battery pack/motor/controller off of the range I'd like to travel in EV mode.

[MetroMPG]

The Honda Insight e-5th wheel inventor used to have a page documenting its development on his web site, but it's been taken down since HybridFest. I'm not sure why - but guess it may be to protect his design in advance of selling it commercially.

Anyway, you can see a pic of it here, tucked under the back of his car in the "deployed" position:



Discussion can be found on his site, and on insightcentral.net

http://99mpg.com/theofficialmimadis/
http://www.insightcentral.net/forum/viewtopic.php?t=4391&postdays=0&postorder=asc&star t=150

Apparently it works well. He got 120+ mpg in a recent efficiency rally.

[The Toecutter]

Ideally I'd like to get the car around to 80-100 Whr/mi cruising at 45mph, but I'm not sure if that's possible, so we'll see.

At a steady 45 mph, that 45 mile range and 80-100 Wh/mile efficiency is possible with really good aero, LRR tires, synthetic transmission oil, 0/0/0 alignment, machined brakes, and other efficiency mods. Getting it in stop and go? Not so much.

One thing you must keep in mind is that Peukert's effect will dramatically reduce your available capacity. Only 6 trojan T105s is a 36V pack. You'll be using at least 400 amps to accelerate the car to 45 mph, and that will cut heavily into your available capacity, especially if you have a lot of stops and accelerations. Using such high amps routinely will lower your available capacity to about 1/3 of what you expect.

If I were to do such a setup, I'd use at least 48V, would be comfortable with 60V.

Given that your range demand and capaity size isn't much, NiCd batteries become an attractive proposition as well. SAFT's large EV-size NiCds last I checked were going for about $750/kWh. So 8 STM-100s would give a 48V system, cost $3,600, have 4,800 Wh, and weigh 230 pounds. They are unaffected by Peukert's effect, and will deliver all 100 AH. But you don't want to draw more than 250A max from them. Their shelf life is in decades, and they last thousands of cycles to full discharge.

[omgwtfbyobbq]

Yikes, using the expression T=(C(C/R)^n-1)/I^n, where T is time in hours, R is the number of hours the rating is at, C is the capacity in amp hours, and I is the current, at 400 amps I only get ~12 minutes of run time, and that's discharging the pack completely! 200 amps is a bit nicer at 28 minutes, while ~90 amps when cruising at ~45mph is great at an hour and fifteen minutes. I wonder how long it'd take to accelerate to ~35mph using ~5hp? If it's around a minute, then ~eight accelerations per trip should allow still allow for ~30-40 miles at ~45mph based on the ratio of 20min/28min=.71(1.25hr)=.89hr@45mph=40 miles. Since the majority of local trips I run are less than ~20miles RT, and have at most, maybe 10 starts, this should be doable, assuming that expression is accurate.
Otoh, now that you mention it, NiCDs sound very tempting, especially considering the entire system might be ~350lbs, have no penalty for pulling 10hp, and from what you've mentioned on another board (small Google?) long life at over 30 years. The only reservation I have would be dropping that much cash on NiCDs only to have a new lead acid that performs as well come out a few months later.

P.s. That Insight is awesome, I'm totally stealing that idea!

[The Toecutter]

Is your area mostly flat, or are there hills?

Even a small 1-2 degree gradient can increase your current draw over 50% at low speeds.

[omgwtfbyobbq]

One big one, so that'll pull a lot of juice too... Good catch!
One more thing, is it in any way possible to have some kind of regen system, so I could set the wheel down and have it spin the motor in reverse to recharge the batteries.

[The Toecutter]

One more thing, is it in any way possible to have some kind of regen system, so I could set the wheel down and have it spin the motor in reverse to recharge the batteries.

Yes, but not with a series wound DC motor.

You'll need either an AC setup(inverters are expensive) or a seperately excited DC motor(affordable home-brew control systems are complicated).

Most EV enthusiasts don't even bother. Regen only extends range by 10-15% anyway.

[omgwtfbyobbq]

In an all electric maybe, but I was thinking about using it for deceleration when using the diesel too. So I could grab all that energy that would've gone to the brakes and recharge the batteries with it. Do you have any idea what the efficiency of a seperately excited DC motor is?

[MetroMPG]

You could also stick an alternator or generator on the tail shaft of a normal series wound DC. I've seen that done once or twice.

[omgwtfbyobbq]

Hmmm... I wonder if the output is clean enough to charge a NiCD battery? In any event it's something to tinker with. Thanks!

[JanGeo]

reconfigure an alternator for high voltage output to recharge at 36-48 volts and figure on recharging lead bats right after use - nicads not so often but they have a greater charge/discharge energy loss.