Isolating 3X series batteries for parallel charge?

Discussion in 'The Projects Forum' started by rudyauction8, Mar 26, 2014.

  1. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I have an electric scooter running on 36v, and I need to isolate each battery for charging from a 12v power source. I'm normally good at figuring these things out but this one has got me stuck. Can I simply use high current diodes as shown in the drawing of my setup to isolate each battery, or will the batteries short out?
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    That would short out the generator.

    The only way to do it using one generator is to disconnect the batteries so they are no longer in series, then connect the generator to them in parallel.

    Note, you only need to cut two wires to stop the 3 batteries being in series. That can be done with any standard DPDT switch.

    Then to attach the generator you could use a 6 pin plug, which can be part of the scooter. It will only parallel the batteries when the other side of the 6 pin plug is plugged in (the generator side of the plug).
     
  3. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    I knew there was something I was missing. I can't disconnect the batteries to charge, the generator will be running while I'm riding. Is there any easy way to do this?
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Easiest way is to charge one battery at a time.
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    What is running the generator. If you say it's the scooter, then think it through. You can't draw power out of the batteries to run the generator that is trying to put power back into the batteries. This violates one of the laws of thermodynamics. Since no system is 100% efficient, you never can charge up something when the only energy source is itself.

    ak
     
  6. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    It's an electric scooter, and I'm using a small gas engine to spin the generator. It needs to run occasionally while I'm riding to keep the batteries charged. I'd prefer to not have to stop and wait for each battery to charge every 15 minutes.
     
  7. RamaD

    Active Member

    Dec 4, 2009
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    What is the capacity of the generator and of the batteries?
    Connect the generator to a boost converter which gives the float voltage for all 3 batteries. Connect the output of the boost converter to the end connections of the batteries. Sorry, can't think of something simpler.
     
  8. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    Generator is 3hp, still working on the electric side. I have a car alternator which would work if I can isolate the batteries, but I'm looking for a 36v motor to use. The batteries are 12v12ah sla batteries and only hold just under half of their original capacity.

    How much do boost converters cost? I have a very low budget of $20 to finish the electric side of the generator. I do have a 120v inverter but I'd prefer to not have 3 10+ amp solid state chargers mounted to the scooter.
     
  9. Plasmahunt3r

    New Member

    Feb 6, 2014
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  10. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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  11. Plasmahunt3r

    New Member

    Feb 6, 2014
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    OK, I get it. You need more Amps. Your Alternator could probably produce over 50 Amps and you can use a custom (low budget) regulator for 36V.

    Look at the following site. It shows a regulator using a zener, voltage divider, and Op-Amp as a comparator. His homebrew regulator is set for 13.8v, but you can modify the parameters to produce 38-42v.

    http://www.qsl.net/ns8o/chrysler.html

    The following site explains the regulator operation and has a good definition of how the voltage divider and zener operates as a comparator:

    http://autonopedia.org/renewable-energy/generators/alternator-secrets/
     
  12. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    well, I solved my problem, one of the scooter motors puts out 36 volts at 15 amps.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    What alternator are you planning on using?
    Most automotive generators are alternators nowadays, and they have internal regulators. Some of the newer regulators are digital, the older ones are analog. What you want to do is to change the regulator, so that the alternator outputs 40.5v to 43.5v instead of 13.5v to 14.5v. This may result in your rectifier bridge blowing up, but you appear to be building this thing on a shoestring, and so you're going to have to use mostly what you have already, with few additional parts.

    If your batteries are already down to half of their capacity, they aren't long for this world; and subjecting them to the stress of operating a scooter will likely see them dead very shortly. When they fail, they just might short out your alternator.
    [edit]
    Whoops, didn't see that the thread had gone to page 2.

    You will need a regulated output; 36v won't do - you really need to get higher than normal battery voltage to charge/float charge them.

    Plasmahunt3r Plasmahunt3r's reply in post #11 is what you need to do.
     
  14. burger2227

    Member

    Feb 3, 2014
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    For one thing, the regulator was already designed in another thread, but we thought that the generator was already working with the batteries in series.

    What I would do is use the 36 volt generator when riding and use switching to charge the batteries in parallel during the night.

    That solves the budget and battery charging problems.

    We'd still like to find out how the regulator works and how often the generator kicks on while riding. I would meter the batteries to see how things will work out before taking it far from home.
     
  15. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    252
    2
    First of all, by 36v I meant it had enough voltage to charge 36v batteries. I'm sure it tops out at around 50v at redline.

    I took it for a ride today, the generator supplies about 90% of the power used at full throttle. I figure I'll run it whenever I'm going up hills or fast and turn it off for level ground. If I add a tensioner pulley to the belt drive, I'll be able to get quite a bit more from it. I need to install an amp meter so I don't burn up the scooter motor used in the generator.
     
  16. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Congrats! :)

    And definitely put an ammeter on that thing, you can even get bidirectional DC ammeters. The needle sits in the centre when there is no current, and will indicate current flow in either direction (charge or discharge).
     
  17. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    252
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    Thanks! since the max current I should be pulling from the scooter motor generator is 10 amps, I'm probably going to use a cheap 10a multimeter. I have one that indicates positive or negative draw. I'll have another multimeter to check voltage so I don't overcharge the batteries, or modify an old 10v voltmeter gauge with a resistive divider to show 1/4 or 1/5 of the actual voltage, and use a pen to mark the max. and min. values that should be shown.
     
  18. burger2227

    Member

    Feb 3, 2014
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    You could make another window comparator or something like this to monitor the charging:

    [​IMG]
     
  19. RamaD

    Active Member

    Dec 4, 2009
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    That will be close to 14A of charge current, which appears too high for a 12AH battery. I would prefer a charging current of not over C/2.5, that is about 5A max, which fits with 180W.
     
  20. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    252
    2
    Remember the 350w drain at the same time, that leaves 150 watts to charge the battery.
     
    RamaD likes this.
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