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12 volts to 48 volts for an electric bicycle

Discussion in 'Electronic Basics' started by [email protected], Sep 18, 2012.

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  1. Guest

    I got a used 36-volt Crystalyte hub motor and controller from Craigslist. (The guy who sold it to me said they can take 72 volts, but I'm still trying to confirm that from their website, which is a bit skimpy. Looks like I'll have to email them.)

    Motor = 400 X4 (408)

    I tried 48V (4x 12v SLA batteries, 8 A-h each), and the bike worked fine for about 3 miles, but left me to pedal the remaining 9 miles home from work.

    Since the 12V 8 A-h batteries are $33 each from Home Depot, I wondered about returning them and buying a (heavy!) 12V, 85 A-h deep cycle battery from Costco (only $68). If I go that route, I'll need a DC-DC converter, converting 12V to 48V.

    I'm thinking of the LT1339, and apparently the output voltage is set by Vout = 1.25 (1 + R2/R1), from page 9 of the datasheet.

    Is the output current set by what ever the input supply can provide?

    This 555 circuit looks like fun too.

    Since I want the output voltage to be 4x the input voltage, is a duty cycle of 75% correct?


  2. fungus

    fungus Guest

    Huh? If it says 36 Volts then it's 36 Volts.
    72V will make it spin a lot faster and heat
    it up more. Possibly burning it out.
    Sellers tend to say things like that, yes.
    If it was me I'd have confirmed it before
    handing over the money.
  3. (About fifteen minutes of power? (Assuming ~15 mph) so about 32 amps
    of current?
    Ahh that looks like it will almost get you home. But if you crash
    with the deep cycle battery strapped to your bike you may spill acid
    on yourself.
    There is a current sense resistor. You'll need to pick the right
    (But I don't know much about power DC-DC converters.)

    Digikey has 12V/ 33 A-h SLA's... not cheap though.
  4. Guest

    Yes, about 15 minutes, come to think of it. Kind of scary to think I burned through 384 watt-hours in just 15 minutes. Stop-and-go traffic (lots of stop-signs in the neighborhood). The motor was *very* warm to the touch afterwards, too. Loads of fun to bike home with heavy SLAs (and a heavy hub motor to boot). =)

    I bought some more SLAs so I have 6 total (4 new ones, along with 2 old ones I've been charging every month to keep them alive). Since another postermentioned running the motor beyond spec is not advisable, I suppose I could have two banks of batteries in parallel. That's not a bad idea, is it?

    Yeah, good point... even just tipping the bike over would be pretty nasty. And that deep-cycle battery is *heavy*.


    Oh, yay.

    Yeah, I'm aiming for cheap. That was the motivation for looking at an off-the-shelf deep cycle battery.

    I would go for a motorcycle, but (1) they're kind of noisy, and I'd rather save my hearing for enjoying metal music, and (2) in this city of mine the drivers are half-insane anyway and I'm sure I'll die on a motorcycle. At least with a motorized bicycle I can stay in the safer bike lane.
  5. Guest

    Gotcha, thanks.

    From looking it up on the manufacturer's website, the controller looks likeit may take 72V, but you're right, the motor is def. rated at 36V.

    Good point. I was watching the prices on Craigslist for some time though, and this one was offered at somewhat below the average price, enough so that I decided to take a risk and try it out. Sure, the throttle control needs a new spring, but at least the thing works. Or, well, it is dependent onbattery capacity.
  6. Guest


    Hmm, good point! On your advice I found this.

    12V, 28 A-h for $79. 24 A-h (3x batteries @$33) from Home Depot would be $99. But, wow, 21 lbs, huh. Would need three of them... (or, again, a DC-DC converter.)
  7. Biking is good for you... exercise and all that. You could always try
    one of those scooter/ Vespa things. Lots more energy density in

    George H.
  8. Guest

    Looks like even an 85 A-h 12V deep-cycle battery won't be sufficient, huh.

    4x12V @ 8 A-h got me 3 miles. That's 128 W-h/mile. Distance to travel (one-way, assuming I can charge at work and at home) is 12 miles. So I'll need 1536 W-h. That's 3 12V batteries at 43 A-h, or one 12V battery at 128 A-h (assuming a DC-DC converter at 100% efficiency).

    Then again, since I was running a 36V motor at 48V, I was probably losing more energy to heat than I would otherwise... maybe 128 W-h/mile is a bit high...

    More research needed!

    Thanks y'all.
  9. Guest

    On Tuesday, September 18, 2012 12:18:38 PM UTC-7, George Herold wrote:


    Yeah, I know! I can pedal the 12 miles in about 58 minutes. Not every daythough. I thought an electric bike would allow me to do it every day, saving me the gasoline. (Round-trip in a Toyota Camry burns through about a gallon of gas; 24 mpg city is about right.)

    No argument there! I was even thinking of those 2-stroke bicycle engine kits for around $300 or so. Then I remembered when I use the weed-whacker onthe lawn I like to wear hearing protection since those engines are kind ofloud...

    I would totally go for a Honda Trail 90 (they're about $600 on Craigslist),but (1) I'd have to contend with the maniac drivers who apparently are unaware of basic traffic laws, and (2) I'd need to get a motorcycle license.


  10. fungus

    fungus Guest

    And that was only 48V...

    72V will probably fry it.
  11. amdx

    amdx Guest

  12. Umm, 8 AH at 48V, vs 85 AH at 12 volts. (assuming no loss in the DC-
    DC converter)
    A little less than 3 times the energy.

    George H.
  13. P E Schoen

    P E Schoen Guest

    wrote in message
    For SLAs and any Lead-Acid chemistry you must figure in the Peukert factor.
    A battery rated at 8 Ah capacity will provide only about 1/2 that at 8A, or
    1C. An electric bicycle should do OK with about 1/2 HP or 400W, and ordinary
    4 passenger electric cars average typically 250-350 Wh/mile. Since you and
    the bike are probably 1/5 the weight, I would think 50-75 Wh/mile would be
    about right. There are several on-line calculators that can help determine
    what you need.

    My own calculator is:
    It has a Peukert calculator as well as estimate of force and power under
    various conditions.

    Here are others: (This is specifically for e-bikes)

    You can get SLAs for about $22 for 12V 12Ah at

    Here are good prices on LiFePO4 batteries:

    You might check out the DIYelectricCar forum. They have a sub-forum on

    I made a converter from 12 or 24 VDC to 320 VDC to use a three-phase AC
    motor on a small tractor:

    Good luck!

  14. Guest

    On Tuesday, September 18, 2012 8:24:23 PM UTC-7, P E Schoen wrote:


    My numbers were off; my batteries were 7 A-h, not 8 A-hs as I originally thought.

    I'm wondering if my controller may be defective...? Well I was riding the bike pretty hard (stopping at all stop signs, etc. and this was in the city). 110 Wh/mile is what I calculated, and then again I stopped using power when I noticed a drop in power (didn't want to over-drain the batteries). I'm sure my numbers would look better if I only used power on the long flatareas near the airport vs. all the stopping and going near the office at work.

    Wow, $22 for 12 Ah, huh! Thanks!


  15. Guest

    Yup. Thanks for the reminder!
  16. Sure, but the OP proposed replacing four (4) 12V/8AH batteries with
    one (1) 12V/85AH
    George H.
  17. BeeJ

    BeeJ Guest

    explained :
    Get a bike trailer and buy 4 12v Deep Discharge batteries and strap 'em
    on. Go like the wind, but with it.
  18. Guest

    On Wednesday, September 19, 2012 5:39:09 PM UTC-7, BeeJ wrote:


    Yes, that's a good solution too. =)
  19. amdx

    amdx Guest

    I put a suggestion on the DIY electric car forum to mount a
    motor/generator on a trailer that you can tow behind your electric car
    for long distance trips. If it's a short trip say 35 miles just take the
    car, for a longer trip connect the trailer and let it charge your
    batteries while you drive.
    I'll my video again, some of you have seen it and probably tired of
    it, This is an electric gokart my 16 yr old son and I put together
    about 3 years ago. Since then I overheated the original motor, we
    installed 4 deep cycle group 27 batteries and a new motor. The
    overheating was at a connection between one #4 wire and 7 #14 wires.
    The motor windings were ok.

  20. how are you charging these batteries, and are they balanced correctly?

    when they're dead from use with your bike, check the voltages across each
    battery, if they're not close, something is wrong with one or more of
    them, of they're just too chinese to even string up and get a nice 48 volt
    string with. One off battery or even just one cell in one battery will go
    into voltage reversal if it drains before the others, and that will really
    kill performance as a whole as well as that battery.

    I use a gas engine on my bicycle and know from use starting and stopping
    aka "city use" is the real killer with gas use. If I never stopped, way
    over 200miles per gallon, if not more would be easy.
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