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First lithium-powered electric car on PriusChat?

Discussion in 'Other Cars' started by daniel, Feb 15, 2008.

  1. mangums

    mangums New Member

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    Thanks for the great information on the lithium packs. I think the cells you are using are these.

    Foxx Specs

    I was considering them for an electric conversion, but I am starting to wonder what the benefit is if you have a very short drive. Your case seems to be an excellent example. Your vehicle is lighter but performance seems to be unchanged at best. You do get a lot more cycles but it is probably not any better than the amount of Lead batteries you could buy to get the same range over time. The only benefit I see is in range or power. You could easily have built a pack that weighs the same as the original lead pack but have double the capacity. Except if the original pack meets your range requirement then why do it. You could also have went to much higher voltage and a similar capacity but you are limited by the existing motor and controller.

    I think I am actually trying to talk myself into lead at this point, so don't take this as a knock on your project.

    Steve
     
  2. daniel

    daniel Cat Lovers Against the Bomb

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    Actually, this is the battery pack I'm using.

    Compared to what I had, performance is reduced, range is about the same, but battery life and reliability should be much better. But compared to a stock Xebra, performance is slightly improved and range is about 2.5 times greater. Lead batteries have not stood up well in the Xebra. The seller is claiming that in the long run the lithium batteries are more economical because of their much longer life. Lead batteries are short lived. And the charging method used in the Xebra (string charging rather than individual) may be contributing to premature failure. Or the application may just be putting too great a strain on the lead batteries.

    Had I been willing to spend $14,000 (instead of 7) for two packs, I'd have gotten better performance

    If I was considering a conversion, there is no way I'd use lead. It's too heavy, too short-lived, and too severely affected by ambient temperature. The lithium batteries are unaffected by temperature, at least within the range we have in Spokane. From 70 degrees to 30 degrees Fahrenheit lead loses half its energy. So if a lead-based car can go 50 miles on a charge in summer, it will go 25 miles at the freezing point. And if they live up to the claims, the lithium batteries will last long enough to be cheaper than lead. It's a no-brainer.

    Unfortunately, there are constraints on the Xebra that you don't have when designing a conversion, and since the Xebra is so limited anyway, I'm not willing to pour unlimited funds into it.
     
  3. mangums

    mangums New Member

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    Daniel,

    What I meant to say was the packs you got from EWI distribution, appear to come from the manufacturer at the link I provided.

    I didn't know the Xebra was so rough on the Lead batteries. It certainly makes sense financially if you are not getting the expected number of cycles out of the Lead Batteries.

    I also don't have to deal with cold weather, so I don't have to worry about the poor performance of Lead in cold weather. I felt like the vehicle performance in my situation would be the same on Lead versus Lithium so the Lithium would need to last at least three times as long. I am not sure I am willing to bet on that just yet.

    By the way, I do use LiFePO4 (Dewalt A123) cells on my daily commuter (bicycle) and they work great. I would never go back to Lead on the bicycle. Unfortunately, scaling these cells up to a pack of the size you have would cost about $10,000 and I would have to build the packs.

    Steve
     
  4. mangums

    mangums New Member

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    Great information on your testing so far. Would you happen to remember what the minimum voltage drop under load was and what the current was at the time?

    This would be really useful is designing my pack.

    Thanks,

    Steve
     
  5. daniel

    daniel Cat Lovers Against the Bomb

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    The voltage drop under load depended on the load. But it also depended on the speed of the vehicle. Or rather, the current draw with the pedal to the floor depended on the speed. The faster it was going, the less current it would draw at maximum throttle. (If the word "throttle" can be used with an EV.)

    But it's hard for me to note those numbers while I am driving. One day soon I will ask Allan to go for another ride with me, and write down the numbers. Then I'll post them here. I suspect they'd be of interest to everyone interested in the technical performance f this battery.

    But first I want to do a test drive for range, and tomorrow I won't be able to drive. (Routine medical exam that involves sedation, so driving not permitted.)
     
  6. mangums

    mangums New Member

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    Thanks Daniel, that would be great.

    Steve
     
  7. daniel

    daniel Cat Lovers Against the Bomb

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    Yesterday I drove 30 miles being very gentle on the pedal, seldom going over 30 mph, though the speed limit is 35 on most of our roads, and not taking more hills than presented themselves along my route. I accelerated slowly, almost never drawing more than 150 amps, and usually keeping it under 100.

    The day before, with a lead foot and a 500-foot-high hill, I made it 29 miles before it dropped to a no-load voltage of 70, which I've been told is essentially empty, and in the next mile and a half to home it dropped to 65 volts. It was atfter 75 volts that it started to drop faster than it had been doing.

    Yesterday after 30 miles the no-load voltage was around 77 volts, so I believe I had at least 5 miles in reserve, and probably a bit more. This means that, although the Xebra with this new battery pack can no longer keep up with the flow of traffic, it can meet my needs for most of my driving. The problem is that this battery pack was not designed with the Xebra in mind. It was imported because it's an improvement over the stock Xebra battery pack. The Xebra is for me a stopgap: It allows me to be driving electric until a real electric car becomes available again.
     
  8. donee

    donee New Member

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    Hi Daniel,

    Congratulations on the new battery. BTW, Voltage only limits a motor RPM. Its current that sets the torque. Voltage limits the RPM because as a motor turns it generates a socalled back-emf. The amount of current that can flow is limited by the difference between the battery potential and the back-emf. This why motors have a torque curve that is flat up to some RPM, and then a straight line from that point to zero.

    So it does not neccassarily follow that the lower voltage battery would result in the car with less umph, just a lower top speed.
     
  9. daniel

    daniel Cat Lovers Against the Bomb

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    Okay. Maybe that's why the 84-volt system I had before got up to 40 mph, while now it only gets up to 35 mph.

    FWIW, the dealer believes, from his observation, that Xebra motors running on 84 volts run cooler than those running on 72 volts.

    I'm more bothered by the difficulty of getting up to 35 now, than I am by the lower top speed, though that, too, is an annoyance.
     
  10. donee

    donee New Member

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    Hi Daniel,

    You could change the gear ratio to get the motor speed down, and this would allow you to stay in the torque flat to a higher speed. Which would help with accelleration, I would think. I am no expert though. But you might also need a motor with bigger windings for the bigger current and you should check the mechanical Max RPM too. There might be some room to maneuver within the rating of the motor. You might want to check out that EV car chat board for more specific experience on your car.
     
  11. mangums

    mangums New Member

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    Daniel,

    It is hard to tell on the website but it looks like they used 24 cells to build the 72V pack in your car, which is good because I have seen many places that rate the LiFePO4's at 3.2V per cell or higher nominal so they would probably have used at least 1 less cell. My experience has been that a very good LiFePO4 cell (like the A123) will hold right at 3V under a 5c load.

    I am sure you don't want to throw money at it, but four more cells would put you back to 84V and your performance should be better than it was with th 84V lead pack. I suspect they could do this for you for under $1000.

    I always recommend using 4 LiFePO4 cells in series for every 12V lead battery you replace. This should guarantee you the same top speed as your lead pack but acceleration should be much better with the lighter weight. I would see how many cells there are in the pack if this is something you can do without too much trouble.

    What is the voltage of the pack fresh off a charge? I think they may not be utilizing the full capacity of the cell which is why they quote such long cycle life. Nothing wrong with this approach, it is exactly what Toyota does with the Prius.

    Steve
     
  12. daniel

    daniel Cat Lovers Against the Bomb

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    I have no idea if lowering the gear ratio of the Xebra is possible, but there's no way I could do it.

    I agree that adding 4 cells would be great, but this pack came as a unit with the charger and BMS. These are designed for 24 cells, and again, there's no way I could retrofit it to charge the extra cells.

    Straight off a charge, the pack voltage was 85 volts, but after sitting overnight the pack voltage is 80 volts.

    The short of it is that an electrical engineer could certainly build a pack with charger and BMS sized optimally for the Xebra, but this pack was imported as is, as a "good enough" fit. It provides better performance than the factory Xebra, and three times the range. Sadly, it was a downgrade from my previous (after-market) pack, and I'll just have to live with it because I'm not throwing any more money at it.

    I'm sure the A-123 batteries are better quality. But the real issue is that I'm using an off-the-shelf pack which is not an ideal fit to the Xebra, but was available, and I was unwilling to continue to buy and destroy lead batteries. It's the price I pay for being unwilling to wait for a "real" EV, and unable to do the engineering to build my own system.
     
  13. daniel

    daniel Cat Lovers Against the Bomb

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    After a few more days of driving The Snail (my new name for Yevette) I've come to the conclusion that if I can alter my mind-set and think of it as a 30-mph car (which gets up to 35 mph if there's a long enough stretch of road without stops) then I can be quite happy with it. Uphills are hard for it, but even on relatively steep hills it seems to be able to stay around 20 mph. It is after all a 100% electric car, with all the social, environmental, and trade benefits of electric (no money sent to Saudi terrorists to fuel it, 100% domestic energy does not contribute to the trade deficit, zero pollution since my electricity is hydroelectric, and even dirty coal-fired plants pollute less than cars). And, like the Prius, it's dead-quiet and dead-still at a stop, but unlike the Prius, it never runs a gas engine. And it will still get me all the way downtown and back.
     
  14. NoMoShocks

    NoMoShocks Electrical Engineer

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    I thought that Doc Willie had the first litium-powered car. Oh, wait, sorry I was thinking of Dilithium power. Never mind.
     
  15. Doc Willie

    Doc Willie Shuttlecraft Commander

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    Well, actually, I do have the first lithium powered Chevy Volt.

    The battery problem has been solved. The wind tunnel thing is still a bit of an issue . . .
     

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  16. Fibb222

    Fibb222 New Member

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    Perhaps you were the first one on Priuschat with a lithium-powered "car" but I may have been the first one with a lithium-powered vehicle!

    Check it out.
     
  17. daniel

    daniel Cat Lovers Against the Bomb

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    Cool! Is it just a battery assist, or can you "drive" it like a motorbike without pedaling? How far can you go if you don't pedal at all? How hard is it to pedal with the motor off?

    I can't ride a bike but I admire those who do.
     
  18. Fibb222

    Fibb222 New Member

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    It is both a battery assist and has a throttle so you don't have to pedal. Of course your range is way bigger if you pedal. It has four levels of assist.

    There is a little computer attached to the handlebars that is a speedometer, odometer and controls the settings.

    In assist mode it automatically senses how hard you are pedaling and provides a certain amount of thrust depending on what level you set it. I use level 4 (the most assistance) on my commute. It really flattens the hills and keeps my speed up on level ground.

    With the heavy battery and motor (plus my panniers) it is a heavy bike, but I can easily get around without having any battery power. My speed is just cut in half when going up hills. It's on a 27-speed hybrid bike so overall, it is easy to pedal.
     
  19. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    congrats on your upgrade!!

    my Zenn only does 36 mph max and it accelerates about the same as yours...its actually pretty fast (since i beat most people off line for first block or so) up to about 30 mph...then drifts to 36)

    and as you know, there are compromises... but that is what EV's are all about. they have been thwarted at every opportunity...

    funny how an hybrid SUV still qualifies for 100 % hybrid tax credit, while pure EV's get ZERO...(tax credit ended in 2006).... and it aint because EV's have sold any particular amount of cars either.

    but daniel, you are right, if we dont start an EV movement, no one will know or care (its hard to care about something you are completely unaware of!!)

    since driving my Zenn, i have had a lot of interest shown with people i run into while tooling around and one guy actually turned me on to option that i would do today if i knew that plug ins were not coming down the pike... once again, its a three wheeler, but much faster with greater range (cost as much as a Prius!@!)

    check it out TWIKE.us


    another thing daniel, i am a member of three different Zenn user groups and there is this guy in the bay area who cracked the Zenn software code and now his Zenn will do up to 45 mph on its standard 25 mph motor and will coast up to 53 mph down hills. (the Zenn is governed so mine for instance will only do 37 mph down hills, iow, no real increase at all) ... check around, see if altering the software will make it run better.
     
  20. Fibb222

    Fibb222 New Member

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    ZENN is a great little Canadian company.