Power in Flux
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Thread: Battery Space

              
   
   
  1. #11
    EVangelist electriKAT's Avatar
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    Higher voltage means more speed AND lower current for a given power. Both of these are good things. But higher voltage components cost more. How well 72V works depends on your definition of "well". The main point I want to make is that I think you will be disappointed with the performance of any lead pack if you expect it to be more than a learning experience.

  2. #12
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    I think that I get where you're coming from. Thanks.

    Right now, a learning experience is fine for me. I will definitely want to expand later though. If I gather this correctly, that's not an uncommon experience among builders. I've read a few threads from folks who said "I'D NEVER DO THAT AGAIN!!"

    There will probably be a MK II e-moto. I think that a Ninja frame would work well.

  3. #13
    Junior Member glassblower's Avatar
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    One thing you haven't mentioned (or maybe I missed it) is how far to you drive your Vectrix now? The type of batteries you buy has a HUGE impact on your range. If you are building something to drive say a couple miles to work and back, lead could be an option. I'm not promoting lead, but just as others have said, many ruin their first battery pack no matter the chemistry until they figure it out. Lead is more forgiving and cheaper to learn on, but you have to look at the range you need or are building for. As I said earlier, my first build was a Lead sled in an S10 but I over sized my battery boxes knowing someday when the priced dropped I would replace my "learner pack" with some form of Lithium. Of course it still has lead (Optima Yellow tops) and I quickly lost interest in it since I'm building a faster longer range motorcycle... With Lithium also comes the cost of a good BMS but worth it to protect my investment in LiPo.

  4. #14
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    I've probably neglected that detail. I've been so on and off with this project, it almost certainly slipped my attention to mention this time around. All in all, I've probably been at this thing for 5 or 6 years or so. I've managed my time by building electric bicycles in the meantime.

    I commute about 13 miles to work one way on the Vectrix. If you can believe the battery gauge (and from what I've read, there are very good reasons NOT to) it drains the battery meter just above 50% on a one way commute. It uses 125v 30ah Nimh cells.

    This thing that I've built, whatever you want to call this conversion literally sits in the back of my kitchen. I am somewhat hesitant to have that much energy density sitting idle in my kitchen while I still have a totally functioning, running Vectrix.

  5. #15
    Junior Member glassblower's Avatar
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    Just keep the terminals covered well. I have a spare set of Optima yellow tops (12) with pieces of rubber hose over all of the terminals and then covered with a sheet of plywood. My set of LiPo have all the terminals covered at all times unless I'm working with connections and testing. I freak about the same thing and know you what you mean.

    I don't know much about the Vectrix but I'm sure you can pack more power in a motorcycle frame than one of those.

  6. #16
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    Good point. I was thinking about that too. 48v short across the terminals to the frame... yikes... not good. Gotta watch the bumps.

  7. #17
    Member PaulWay's Avatar
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    The two fundamental equations here are watts = volts * amps and loss is proportional to resistance * amps squared (forgive me for not being technical enough there, I'm trying to keep it simple for non-electricians).

    The lower the voltage, the more current you need to move to get the same amount of power. But the more current, the thicker your wire has to be and the more you lose to resistance.

    So in general it's better to go higher voltage, because you can get away with longer lengths of thinner wire and you lose less in internal resistance.

    I suppose that's my opinion there, but backed up with some kind of engineering

    I think it's better to start by asking how much power you want. Look at motors that will give you that much power, and look at the different voltage ranges. Then decide what voltage you want, and then use my pack calculator (thanks glassblower! ) to work out what cells suit your needs.

    Hope this helps,

    Paul

  8. #18
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    Thanks for the response. I get it a lot better now.

    Right now man, honestly I want to take this thing from scrap metal at the back of my kitchen to something that rolls down the street at the twist of a throttle, faster than the 15 mph I've had it at using smaller, more compact SLA UPS batteries.

    In other words, right now I am cool with it being a glorified go-kart. Eventually I am going to want it to 60mph continuous. But for the sake of next year's electric vehicle competition, I am plenty satisfied with 35mph continuous. That's all I will need.

  9. #19
    Member PaulWay's Avatar
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    Alternative to building a pack all at once.

    One advantage of using smaller cells, like the Headways and other pouch cells, is that you can construct your pack gradually.

    For example, Tony here (SplinterOz) has a 50AH pack at 72V, with five strings of 22 10AH cells in parallel. If he was to get just one string, that'd be 22 cells, and maybe cost him $400. He wouldn't go very far or accelerate very fast, but it'd work. The controller would still work, the charger would still work, and the BMS would still work - it'd be a good proof of concept. Then he could buy more cells as he could afford them. He'd still have to have designed his battery storage to fit all the cells, but with CAD (cardboard or computer) that wouldn't be difficult, and it wouldn't be that much more expensive than fitting a smaller pack.

    The main annoyance would be to have to take the pack apart in order to add the new cells. Now, Tony's battery is designed so five cells are grouped together, connected to the next five cells, and so forth. An alternate way of doing this would be to keep each string separate, so a new string of cells would be added without disturbing the existing strings. It'd mean a more complex battery management system, since effectively each individual cell is being monitored. But it could be done that way.

    And the main tricky bit is that cells do gradually degrade over time, both in terms of physical time elapsed and in number of charge cycles. The limiting factor in each group will be the old cell, because it discharges quicker and charges slower. Adding new cells to an existing battery isn't usually recommended, although it does happen (Tony and I have both done it and I bet a number of other people have too). Adding new strings beside old strings and keeping them separate will avoid some of these problems.

    So that's an option for you if you don't want to spend a lot of money up front and don't want to have to re-engineer everything or throw the cells away.

    Hope this helps,

    Paul

  10. #20
    teddillard
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    I've got to chime in here as well. My experience with lead - AGM - was actually great, for what they are. It got my first bike running, cost very little compared to lithium, and lasted a summer. I learned a lot with them, and am now using them in my lawn tractor conversion.

    Because I had started with the idea of trying to design some sort of universal mount, I could then use other chemistries on that bike, and use that mount, modified a little, when I moved everything to a different frame (the R5). That mount evolved into a removable rack, or module, that allowed me to use several different packs (different capacities as well as chemistries) by simply swapping out the pack. When I got the CALB cells I needed more space, so I simply added and modified a module for the belly. It's not strictly the same, and won't interchange with other modules I have, but still, it uses the same mounting hardware. Here's what that looks like:

    Screen shot 2013-09-10 at 6.43.59 AM.jpg

    My whole system is based on a 40V module. I use one on the dirt bike, one on the tractor, and two on the street bike, in series for 80V. By setting them up in series or in parallel you can easily modify the capacity or the voltage, whatever you want for the application.

    So I learned two lessons. First, not much I've bought for this madness is wasted. (The dirt bike project is code name: Spare Parts.) Even my first batteries are being used in other projects now. Second, it's a work in progress, and if you look at it like that it takes a lot of the pressure off. Sure, you're not going to nail it first shot, but the fun is in refining it. My advice would be to think in terms of the most flexible battery mounting you can work out, and see where it takes you. (It sounds like that's where you're headed anyway... )
    Last edited by teddillard; 10 September 2013 at 1235.

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