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  1. #1
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    Exclamation Need help to figure out the battery

    Im a student in nz and want to build an electric supermoto. I think I'm going to go for a 20kw motor 72volt 330amps.the problem is that i don't know how to calculated how much 18650cell would i need(first is it the good battery to use).I would like to have a max speed at 100Km/h and ride for about 100Km .
    could anyone help me
    THANKS
    Elian
    Last edited by electric.supermoto; 19 March 2018 at 1915.

  2. #2
    Senior Member T Rush's Avatar
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    I can not tell you my exact calculations...but I played with some of them to try and figure out how many 18650 cells I would need
    the basic formulas are simple:

    voltage per cell X number of Cells in Series = Volts

    ah rating per cell X number of Cells in Parallel = Amp hours


    next:

    Volts X Amp hours = Watt hours

    ...as I understand it: "Volts" is how quickly the electricity can work(with a battery, the voltage will vary with the state of charge; a normal 3.7v cell might be fully charged as high as 4.2v, and drained at 3v) fast(high voltage) or slow(low voltage) frequency, no matter how much its actually doing at once...kinda like spinning away crazy if without any amps
    then:
    "Amp hours" is the amp level a battery can output if it was drained during one full hour of time; with battery storage, 'time' is the key factor, and 'amp hours' are different than a single momentary 'amp' load, and you could use it faster than a hour and get more amps all at once(so like it could be 3amp load for an hour, or 6amp load for a ½hour, or 12amps load 15min. etc) so how much the electricity can actually do no matter how fast or slow it does it, all at once or spread out over a longer time...kinda like sitting there doing nothing if without volts
    so:
    "Watt Hours" is the total capacity of the battery pack...it can be high amp hours with low volts, or low amp hours with high volts and still be the same(so like digging slow with a shovel, or digging fast with a spoon...but still digging the same size hole in the same time)
    ...there is also "current" which might be how fast and how much a battery pack can flow its power; like high current to a motor could suck a battery dry quickly and give you lots of motion power to move mass far and fast, but low current flow and you are barely moving anything or anywhere slowly with long battery life; the job of the 'controller' between the battery and motor
    and:
    "Series" battery cell connection is when they are lined up end to end with the + on one going to the - of the next
    "Parallel" connection is when the battery cells are all next to each other, with all the + ends together but separated from the - ends wired together
    finally:
    number of Cells in Series X number of Cells in Parallel = total number of Cells in the battery pack




    ...but when I was looking to do 100kph for 100km, the 18650 cells seemed to be the only battery pack that would do that with keeping the weight and size of the bike down
    (I was trying for around a low 100kg motorcycle weight, with 100v X 100ah = 10kWh battery pack)


    you should be able to build an el.motorcycle that would reach a max speed of 100kph
    -or-
    an el.motorcycle that can go 100km distance

    ...either of those might be pretty easy...but building one that can do both at the same time(run for one hour @ 100kph for 100km) and you will need to have everything in 'perfect balance'! [weight, size, aero, power, battery]
    so there might be other sacrifices(like you might need something more aerodynamic than a supermotard, you might not have lower speed performance, and you might have to use a smaller motor with lower weight)

    good luck
    ...and I will be watching to see if other forum members know how to do it! and help you
    Last edited by T Rush; 20 March 2018 at 0541. Reason: please correct me if any of the above is totally wrong

  3. #3
    Senior Member T Rush's Avatar
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    let me see if I can get this right(as I'm still learning)...as I was curious to what I'd figured out before, and if my math was correct


    it is important to decide the capacity of the battery you will need...too small and you don't get the range, too big and it weighs extra and slows you down

    I was looking at what the big name brand manufactures get for range at highway speeds, and what level of battery they use to do it
    ...like the Zero Motorcycles http://www.motorcycle.com/mini-featu...cle-range.html
    looking at that info, lets say you need a 10kWh battery to go 62miles at 62mph with a lighter aerodynamic motorcycle
    (so that would be my 100km @ 100kph, with the 100volt X 100ah = 10kWh battery for a 100kg bike)
    ...and with your 72volt system you are designing...then you need a 139ah battery
    (72volts X 139ah = 10kWh)

    using the 3.7V 18650 cells, you need to have about 20 'groups' of them in series to get 72volts
    (3.7volts X 20 = 74volts)
    ...but each one of those 'groups' needs to be made up of a block of cells in parallel to get the amp hours

    you might get 18650 cells rated at only 2600mAh
    (so many fake ones out there with different ratings...I wouldn't know which ones to get to actually achieve the best 3ah or even the 2.6ah I'll use here)
    ....so each one of those 'groups' would have 54 cells in parallel to get up to 139ah
    (2600mAh X 54 = 140ah)...so this is where the total number of cells needed for a 72volt battery pack multiplies way up

    so by my calculations; to build close to a 10kWh battery you would need about a thousand 18650 cells
    (54 cells in parallel for each group X 20 cell groups in series = 1,080 cells for 74volts 140ah)
    ...a thousand 18650 cells also can work out the same for different voltages and amp hours designs
    it could be ... 27 X 3.7v cells series = 100volts, 39 X 2.6ah cells parallel = 101ah ... for 1,053 cells total
    (27 cells series X 39 cells parallel = 1,053 cells and 100volts X 101ah = 10kWh battery pack)

    18650 cells weigh about 45g each, so a thousand together would weigh 45kg(about 100lbs) so that is a pretty huge battery
    ...now keep in mind that battery would give you maybe twice that range at slower speeds in town, so riding at 50kph and you might be able to go 200km or even further
    ...so you might not really need that big of battery....and I was just guessing that a 10kWh battery is necessary for 100km @ 100kph
    (you might find that you need a larger battery pack, or maybe a smaller one depending on motorcycle and rider: weight, size, aero, and power efficacy)

    you will still be able to hit a max speed of 100kph with a smaller battery pack using fewer 18650 cells...say you only buy 400 cells
    still you would have 20 groups of 3.7v cells in series for 74volts.. then just 20 in parallel for each group at 52ah(the 'amps' will just get used up faster when riding at 100kph, and wont last an hour; maybe 20min run time at that fastest speed)... however, that smaller 3.8kWh battery might still last 100km if you only ride at 50kph(so then the bike would run for two hours at only a slow 50kph)....and with the lower weight; it will be quicker(more fun) to get up to 100kph, and stop faster
    Last edited by T Rush; 20 March 2018 at 0711. Reason: again; 10kWh is just a guess at what it would take to run full speed for an hour stright

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    ...



    Thanks for you answer ...that what i calculated so i was pretty right but it seems that is is a lot of 18650 to go in the frame of the motorbike (that's why i thought it was wrong but in fact it is not)do you think if i get 800 of them it would be enough to ride in the city??

  5. #5
    Senior Member T Rush's Avatar
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    800 is still a lot, and will cost much... you also will have to figure out how big a 800 cell battery pack will be, and if you can fit it in your motorcycle frame

    you could even start with 500 or 600....and then later add more, but to add more you will have to take the whole battery pack apart to add cells into the parallel cell groups to increase amp hours for run time
    ...if you just added a new 'group'(or they call it a 'module') in series, then you increase voltage

  6. #6
    Senior Member T Rush's Avatar
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    you might consider a higher voltage system for such a large battery pack using so many cells

    other forum members hopefully know way more about this than me...as I am new here

    I don't fully understand the voltage vs amp hours thing yet, which is better to have more of....but I think most of the big company EV manufactures use higher voltage systems with many more modules(groups) in series, with fewer cells in parallel in each module..and I don't know exactly why, but it might be some simple obvious answer for someone who has more experience with electric than me

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    Senior Member Spaceweasel's Avatar
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    When starting out I found it helpful to think in terms of stacks of voltage. Figure out how much voltage you want to run (likely limited by your motor and controller choices), then figure out how many cells it takes to get to that voltage in series. Think of that stack as one unit. It will run your bike at the power/speed you want, but only for a limited time. Then you can add additional stacks for range until you run out of space/money/weight capacity. Later on, you could go back and add additional units in parallel to increase range without really changing the system.

    For instance, my first build was 2 sets of 24 cells. Each cell was 3.2v and 30ah. So each set of 24 cells in series made 76.8 volts and 30ah. I had space/money for two sets of these, so I wired the two sets in parallel to make a 76.8v 60ah pack. If I had wanted to spend the extra money I had considered adding a third set (probably in a saddlebag arraignment) that would have made the whole pack 90ah.

    Each set was theoretically 2.3kWh (76.8x30), so my pack was 4.6kWh. If I was running a larger motor/controller combination that could handle higher voltage (like my new build) I could have wired them into one stack of 153.6 volts, but it would then only have 30ah. On the other hand, such a pack would have an equal amount of energy at 4.6kWh.

    Controllers have a voltage rating, and most of them will allow a bit more than that number but exceeding your controller's voltage will usually cause them to shut off for safety. Look at the manufacturer specs to determine what this highest number is and then design your pack to come in under that at full charge. For instance, my Alltrax 7245 is rated for 72 volts, but will allow as high as 90 before it faults. My batteries were 3.65v at full charge. When hot off the charger the pack would therefore sit at 87.6v. Even one more cell in each set would have pushed me over the 90v limit.

    Motors have voltage and amperage limits, as well. These ratings can be exceeded, too, but at the risk of something melting/breaking. Figure out what the hard limits are, and then pair with the appropriate controller. My motor in the first build was rated for 72v/450amps, which matched nicely with the Alltrax controller. It likely could handle a little more power, but not a whole lot more.

  8. #8
    Senior Member Spaceweasel's Avatar
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    I've never seen this analogy before. It's a fun way to think about batteries.

    Skier Analogy:
    Imagine your electrical system as a mountain with chair lifts and some skiers on it.

    The voltage of an electrical system is comparable to the height of the chairlifts, i.e. the amount of potential energy per unit (skier or electrons). If you have ten small chair lifts (12V) you can either put them all in a row up the hill which adds the height of each getting your skiers pretty high up the hill (giving 120V) or can have them all next to each other allowing more skiers to go up the mountain but not as high (12V).

    Amps are the number of skiers travelling on our little mountain circuit (from the bottom up the chair lift then back down the slope to the bottom). In the example above stacking the chairlifts up the mountain (series connection) gives a lot of height (10x voltage) but limits the amount of skiers (1x amps) that can go up the mountain. Having the chair lifts all next to each other (parallel connection) allows lots of skiers (10x amps) to go through the circuit but they won't go as high (1x voltage).

    You can think of power sources (like batteries) as chair lifts i.e. they add energy to the system by taking the skiers up the hill. Power consumers (motors, resistors etc) are like the downward slope of the hill, the energy that was given to the skier by the chairlift is used when they go down the runs on the slope. The cables that join everything is sort of like the skier cutting across the mountain without going down very much. The skier can get to the slope (eg. motor) where he wants to go without losing much of his height (cables have a small resistance but generally don't drop much voltage across them).

    Power is like an instantaneous (not influenced by time) measure of how much fun people are having in your resort. You get enjoyment happening when people travel down the runs on your hill. Having a few people (small amps) on very long runs (high volts) is the same amount of overall fun (watts) as having lots of people (high amps) on a small run (low volts). When you have too many people (amps) on the same small run they start to melt the snow away (melt wires, burn out motor etc.) but if your skiers have too much height and speed (volts) they might break the more fragile bits of the circuits as the go past them (i.e. motor brushes).

    http://www.fveaa.org/forums/index.php?topic=302.0

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