High current DC voltage booster circuit, small space

Discussion in 'General Electronics Chat' started by MrSoftware, Oct 21, 2015.

  1. MrSoftware

    Thread Starter Member

    Oct 29, 2013
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    I'm looking for feedback on how bad this circuit idea is, before I go any further than daydream status with it. ;)

    The goal is a circuit to boost DC voltage from 12v to approximately 24v for the starter on a motorcycle, while adding less size and weight than a second battery. My idea is to use a few capacitors with a timer, a few logic bits and some MOSFETs. The caps would be repeatedly charged to 12v in parallel, then discharged in series to get the voltage boost. The OEM battery can deliver 80A @12v, so assume the starter motor draws 80A @12v. This battery will be replaced with a LiPo motorcycle battery that can deliver 200A @12V, so I think there will be enough supply power. I've got about 4" x 4" x 6" space to work with, so this will limit the capacitor size. I started doing the math on how quickly the caps can charge and discharge, and realized I'm missing a lot of data (and knowledge) and decided I would ask the general question before going further.

    Is it possible this type of circuit would work, or am I way off the mark? Is there a better way to get a voltage boost with this much current from a physically small package?

    The use case is for racing. The races are dead-engine start; when the whistle blows, the riders run to their bikes, start them and take off. Trimming a second or two off the starting time can significantly improve track position into the first corner.
     
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    There is no easy way of boosting 12V to 24V at the currents you require and capacitors won't do the job. A better battery and/or starter motor is the most practical way to go.

    But, are you sure that it is the starter motor that is delaying the engine start and not the ignition or fuel systems? Will 24V start the engine quicker or will it just lead to premature death of the starter motor?
     
    Last edited: Oct 21, 2015
  3. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Why do you need to run your starter on 24 volts? What exactly are you expecting to gain or solve by doing so?
     
  4. GS3

    Senior Member

    Sep 21, 2007
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    I think it may be the case that the races are dead-engine start; when the whistle blows, the riders run to their bikes, start them and take off. Trimming a second or two off the starting time can significantly improve track position into the first corner. I could be wrong though.
     
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  5. ian field

    Distinguished Member

    Oct 27, 2012
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    There was a blog some time ago by some bloke had picked up a crate of large supercaps real cheap.

    He'd rigged a box of them approximately the same size as a car battery and was able to run his car like that - including cranking the engine.

    At a guess - a motorcycle starter draws maybe 1/3 to 1/2 what a car starter does, so it should be easier. The supercap bank can sit in parallel with the battery to charge, and only be switched to series for starting.

    Using a switch mode unit to double the battery voltage would more than double the starting current draw from the battery - that would probably finish off a good one! It won't do the starter motor much good either.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    more voltage = faster starter = faster starter... literally.. geesh guys :D

    What ever happened to driver skill winning races... kids these days.. meow
     
  7. GS3

    Senior Member

    Sep 21, 2007
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    My guess is that the fraction of a second you might gain is so small it is not worth it. If you could manually pre-engage the bendix drive you might gain a similar fraction of a second.
     
  8. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    There's a very easy way to find out whether 24V will bring any benefit and the TS should try this before pursuing the idea further.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    I have been known to use an arc welding set to turn the starter on motorcycles with a clapped out engine - but the starter doesn't last long doing that.

    The older type with a field winding could be operated by AC, but one motorcycle had a PM starter motor that would only work with a rectifier between it and the welder.

    On one small motorcycle, I was having trouble keeping the battery charged. A couple of big fat electrolytics added to the bridge rectifier after the alternator, converted it to a voltage doubler.

    It seemed to be doing the trick - until I turned the main beam off, then a cloud of smoke came from under the tank. The indicator relay had burned up.
     
  10. ronv

    AAC Fanatic!

    Nov 12, 2008
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  11. MrSoftware

    Thread Starter Member

    Oct 29, 2013
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    The supercap idea is great. I have a simple circuit that puts 2 batteries in parallel for staring, I should replace the second battery with enough supercap capacity for a couple seconds of boost, plus a small charging circuit, and see what happens.

    To elaborate; there are many different bike models in the same race. The 2-strokes spin over super easy and start instantly with the kick starter (faster than the electric starter). The 4-strokes require a lot more energy to spin over, especially the big ones, and they just take longer to start. LiPo starting batteries already give a good boost over lead acid batteries since they can deliver a ton more current, boosting the voltage would help even more. It is understood that the starter life will likely be negatively affected. ;)

    Races go through the woods, and if you get into the tight woods immediately after the start, and get stuck behind a slow person, it can really screw your result.

    Here's an example, I ended up winning my class, but you can see my 4-stroke bike took a while to start and you can clearly hear a 2-stroke out front first. A starter that could get the motor up to speed faster would definitely help.

     
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