Help needed with battery charger multiple outputs

Discussion in 'The Projects Forum' started by uncletufty, Aug 1, 2013.

  1. uncletufty

    Thread Starter New Member

    Aug 1, 2013
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    Hello to everyone,
    Right to the clux of the matter, has anyone out there built a 12v battery charger with multiple outputs ?.
    I have a EAPC ( Electric Assisted Pedal Cycle ) looks like a moped/scooter that uses 6 x 12v 21ah batteries, problem being that using a standard 72v charger the batteries are not charged evenly,what i need is a charger with auto cutoff once charge has been reached for each battery that will charge 6 x 12v 21ah sla batteries seperately but built into one package that can be installed into the bike.
    I can read and understand circuit diagrams and use a soldering iron, but when it comes to designing a circuit i have less than no idea.
    So any help in achieving my goal would be great.
    Cheer's.
     
  2. #12

    Expert

    Nov 30, 2010
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    Without looking anything up for you, I decided to do a float charger for my SLA battery. It has been working well for 15 months, but that is not long enough to prove anything about how wonderful my choice was. One point I remember is, "Do not Outgas the SLA". You can't treat them like a car battery and watch for the bubbles at the fill ports.

    So, I read the label, chose the middle of the float voltage range recommended and used a 78L05 jacked up to 14.4 volts to provide both a current limit and a voltage limit on a 7AH SLA.

    I'm sure there are more sophisticated methods, and you wouldn't want a current limit of 1/10th of an amp, but that's how I did it, and it only required a wall wart and a cubic inch for circuitry.
     
  3. wayneh

    Expert

    Sep 9, 2010
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    First, it seems that bicycle modification fall under the ban on automotive modifications on this forum, so don't be surprised if/when this thread gets closed.

    Are you hoping to charge these batteries while they are still connected in series mode? You could do this with separate chargers but it's obviously trickier in a single charger.
     
  4. #12

    Expert

    Nov 30, 2010
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    Charging 6 batteries that are connected to each other requires a separate transformer winding for each battery to keep the grounds isolated from each other. It is essentially 6 completely separate chargers. A schematic can not be produced without knowing your current and voltage choices.
     
  5. uncletufty

    Thread Starter New Member

    Aug 1, 2013
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    Its not a modification, charging the batteries in series causes uneven charging to the point that 2 of the batteries are now getting to the point of no return, what i'm hoping is that by charging the batteries one by one but at the same time this will not happen again, 6 new batteries are ready to install but i do not want to install them until i can do so without creating the same effect.
    I have seen chargers made for marine use that achieve what i need but the costs are over the top, so i was hoping that someone would be able to point me in the right direction.
     
  6. uncletufty

    Thread Starter New Member

    Aug 1, 2013
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    Just to clarify, what i need is a charger that has a 240v input ( UK voltage ) and 6 separate outputs each capable of charging a 12v 21ah battery all built into one box that can be installed into the bike to replace the 72v charger.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Wouldn't it be possible to have 6 different constant-current loops all powered by the same source? I don't have any details in mind, just a sense that it might be possible. The OP already has a 72V charger. Seems to me you could do it with 6 loops. Not completely separate, but of course you're right that this is essentially 6 chargers.
     
  8. wayneh

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    Sep 9, 2010
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    I'm not the judge of such things. Just sayin'
     
  9. #12

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    I suppose one could make 6 KI chargers in series and have a comparator lock out each stage as its battery becomes full. It's going to require some skill to design it.

    The supply voltage will be at least 87 volts and the regulators will have to be able to protect themselves in case of a shorted battery, an open battery, or the whole set of batteries unplugged. At 3 amps, this gets really large. Big transistors, big heat sinks, that kind of stuff. This is so much simpler with 6 transformer secondary windings!
     
    Last edited: Aug 1, 2013
  10. wayneh

    Expert

    Sep 9, 2010
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    These float chargers are often on sale at Harbor Freight for ~$5. If they could be "stacked", it would be a fairly cheap solution even with 6 of them. Not too good for onboard a bike, though, and not enough juice for anything other than maintenance.
     
  11. #12

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    Those are labeled 15 volts max, 1/2 amp. No information on what voltage the controller shuts off at. Demonstrates my approach: Wall wart, controller, battery clips.

    Still praying for a current requirement or a time requirement or the labeled float voltage to be stated. My lawn mower has a week to charge a 7 AH battery at less than 100 ma. I expect a bicycle would be needed more often.

    Research at batteryuniversity.com comes up with C/7 (3 amps) until 2.40-2.45V/cell, then drop to 2.25V/cell float voltage.
    Six isolated circuits at a couple of cubic inches each is a piece of cake. Trying to get each circuit to survive about 90 volts in case of a failure makes it really complicated and large.
     
    Last edited: Aug 1, 2013
  12. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You might want to buy six cheap regulated float chargers and run them separately making sure their chassis don't touch each other. You can buy them pretty cheap like $10 each.

    http://www.harborfreight.com/battery-float-charger-automatic-69594.html
     
  13. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Check out battery tender company
    4-Bank International Charger - 12V @ 1.25A Each Bank - USA & Western Hemisphere$255.95


    [​IMG]

    10 Bank Charger 12V @ 2A

    $835.95


    [​IMG]
     
  14. #12

    Expert

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    $800:eek:

    Those things are why hobbyists stop by the surplus store, buy half a dozen transformers, and build their own.
     
  15. uncletufty

    Thread Starter New Member

    Aug 1, 2013
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    Thanks for that, i would have said it first but was to late :D, as i already said i've seen marine multi bank chargers that would fit the bill but the cost would be more than i payed for the bike.
     
  16. wayneh

    Expert

    Sep 9, 2010
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    The "manual" says the maximum voltage is 13.2VDC. It also clearly shows that it uses an old-fashioned isolation transformer. Wall wart.

    So I really think they can be stacked. Total cost for 6, ~$30.

    But this doesn't solve the problem of how to bulk charge and it's not practical for a mobile solution. Maybe I've misunderstood: Does the solution have to be ON the bike?

    I think it can be done for less than $800 !!

    Thinking out loud: Use the existing 72V charger. Power a quad comparator (one for each battery) from the battery to be charged. (This requires the battery to be maintained so that it's never dead.) Have one comparator watch a current shunt resistor to limit maximum current during bulk charge. Another comparator can watch battery voltage to control trickle. A third could maybe detect a voltage on the leads and keep power to the leads disconnected unless a battery is sensed in the right polarity. Charging currents controlled by MOSFETs.

    This is far from a working prototype but I don't see any hurdle except design time.
     
  17. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Then why don't you use the ones I posted that cost $10.
     
  18. uncletufty

    Thread Starter New Member

    Aug 1, 2013
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