racecars, onboard battery, and booster batterys

Discussion in 'Automotive Electronics' started by bksmith, Jun 21, 2015.

  1. bksmith

    Thread Starter New Member

    Jun 21, 2015
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    Hello all! I'm new and ready to learn.

    I have a small open wheeled formula car that has a 12v battery on-board (260 Cold Cranking Amps, ~18ish Amp Hours) to run the starter(best guess, less than 30A), two electric fans (6A each), and one electric water pump (1.5A).

    I also have a large car battery that I use when the car is off to run the on-board fans and water pump. This plugs into the car though the same charging port for the on-board battery (so the batteries in parallel). When I race, the repeated starting and running the fans + pump will cause the battery to drain before I'm done. It gets put onto a charger before every race. Race day usually find me running the fans and pumps for about 2 hours total and 6 to 12 starts.

    here is the question: If i switch to a lithium battery, how do i keep the big booster battery (a regular lead acid car battery) from trying to charge the lithium battery? (I know that Lithium batteries have to be charged at a certain rate.) Do I not even need to worry about it? Is there a diode big enough to keep the battery from charging? Educate me! (a higher capacity on board battery is kind of an option - it add weight (bad) and there really isn't one big enough in my price range to run everything for as long as i need.)

    Brad.
     
  2. Kermit2

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    Feb 5, 2010
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    not enough data for use to give useful advice. General knowledge you can get anywhere on the web.
     
  3. Kermit2

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    what part number is the lithium pack. do you have the data sheet?
    The number of starts and power drain each causes is variable. Better to use off board power and never even tax the on board battery for this type of variable use before a race
     
  4. bksmith

    Thread Starter New Member

    Jun 21, 2015
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    The lithium battery has the same stats at the onboard lead acid battery (cca and Ah). I chose it because it's 2.2 lbs vs 14 lbs.

    The reason I use the booster battery is to help with those starter cycles. The car doesn't have an alternator so there is no onboard charging. Any power to drive the fans and pump have to come from battery.
     
  5. Kermit2

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    If all you want is to prevent current flowing in one particular direction then a power diode will do what you want, but I think you know that already. The problem with Lithium is the need for special charging supervision circuits. just dumping amps into them like you can with lead acid work horses doesn't work. And when something goes wrong you don't get the spectacular lithium battery scenarios the you tubes are full of :)
     
  6. bksmith

    Thread Starter New Member

    Jun 21, 2015
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    Yes, exactly what I was thinking. I just don't know how to calculate the size or type of diode. I just know I need one...
     
  7. Kermit2

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    If you are willing to take the extra risks for that little bit of weight trade off, and the specs are matched then a power diode is the simplest way to go. Do the manufacturers of the lithium pack have any special recommendations you might have failed to mention here?
     
  8. Kermit2

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    If ALL starts are done by off board batteries then the power needs will be minimal, other wise you will need some kind heatsinking mount for the diode(adding to weight again). And all this could be avoided if you just use fusing or switching to keep your racing lithium battery out of circuit when using the testing/standby lead acid config. and only ever switch over to that system when the flag is about to go up on the race.
     
  9. Kermit2

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    I not saying a diode wont work but power dissipation, mounting issues, voltage drops.
    Old school says just disconnect the two systems somewhere, like a fuse box. bring out a wire for keeping it charged and have another point for off board batteries to power the system. less muss less fuss and fewer places for problems to creep in. just keep your connections clean and tight and never sweat the silicon layer stresses of the diode scene.
     
  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Why not use one lithium battery for the pump and fans (13.5 Ampere-Hours) and another for the starter. You will still be 9 lbs lighter with two batteries.
     
    Last edited: Jun 21, 2015
  11. bksmith

    Thread Starter New Member

    Jun 21, 2015
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    True but if I had a stall I wouldn't be able to fire the starter on course.

    The belly pan is made of .090 aluminum or I could mount the diode to the frame (steel) to work as a heatsink. Even a dedicated aluminum heat sink wouldn't be a problem if it saved me from frying $150 battery ;)
     
  12. #12

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    Nov 30, 2010
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    I think you missed a sentence.
     
  13. Kermit2

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    You cant use a switch that will let you lithium battery fire the starter? :) come on now! I had a few beers 'for I started this convo but seriously?
    If you are dead set on going with the diode, then, a few points first. Aluminum is recommended over steel for heat sinking.
    First problem is heat sinks are usually on the grounded side of things and diodes need ISOLATION from this electrically and this is where quite a few problems can creep into a power system over time. The isolation is easy to provide BUT insulators are not good conductors of electricity OR heat. so they must be kept very very thin and provided with creavice filling greases to aid in the heat transfer. All problems that have been solved in industry, but you are not industry. you are just average one of us. Your installation will work just as good as mine would for maybe just as long, maybe even longer if you had fewer beers in your system along the install pathway ...)\\
    point being it will work GREAT up until it doesn't... A battery has its positive terminal isolated from direct path to its negative terminal by a very thin piece of material and it is exposed to air flow and dirt almost by neccesity for cooling needs. When it fails it has the potential to be a dangerous event or you might blow a fuse and sit on the track. We'll know it by watching for lithium failures on youtube I guess :)
    knowing all this now if you still want to proceed, the operation is an easy one and we will have you up and walking in just a few. :)
     
  14. bksmith

    Thread Starter New Member

    Jun 21, 2015
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    Not missed just both posted at the same time ;)

    It's a really good idea - kind of expensive but doable. Wouldn't that be the same as just wiring them in parallel? Just increasing the Ah total? If absolutely necessary I could just plug the booster in to fire the starter and unplug
     
  15. #12

    Expert

    Nov 30, 2010
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    No, two batteries that are wired to completely different circuits are not in parallel.
    One should be wired to the starter and one should be wired to everything else. The only reason they connect to each other anywhere is that the frame is made of metal.

    What's going on here? Don't you have a starter switch???
    A Ford starter solenoid weighs less than 1 pound.
     
  16. bksmith

    Thread Starter New Member

    Jun 21, 2015
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    Did not know about having to isolate the heatsink. That's good info ;)


    If we boil down the issue, I have a set amount of electrical need (fans, pumps, starts) and a set amount of storage (battery)

    I was trying to find a way to temporarily increase storage by adding a battery that's removable while on the starting line. The other suggestion is to just add more batteries or get a bigger single battery which is a really expensive option.

    Just trying to explain why I was asking about diodes because that's got to be less expensive than another battery and figuring out the additional battery mount.

    B.
     
  17. Kermit2

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    glad to see you here #12! I'm approaching my cutoff at the bar.

    I would recommend some small easy to handle 50 or slightly higher amperage units with very straight forward mounting requirements that come with all the needed hardware and insulators and using them in a parallel fashion for power needs of the starter. I think I remember reading 200+ cold cranking amps so three in parallel minimum?
     
  18. bksmith

    Thread Starter New Member

    Jun 21, 2015
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    The battery has 260 cold cranking amps, the starter only draws 30amps. (It's tiny, the engine is a 500cc snowmobile 2 stroke engine)

    B.
     
  19. Kermit2

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    ??? you could just skip the heat sinking if you use precautions , provide ample cool down time for the silicon device like say 5% or less duty cycle. Hell, I was picturing things needing STARTER motors. :) These could almost be started by pull cords...
     
  20. Kermit2

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    Thirty amp diodes in a single package not needing isolation or heatsinking(within reason) can just be soldered into line like a fuse these days.
     
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