Hooking up 2 car battery chargers to 1 battery, in parallel

Discussion in 'The Projects Forum' started by gazzonyx, Mar 6, 2007.

  1. gazzonyx

    gazzonyx Thread Starter New Member

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    OK...Disclaimer:
    • what I'm about to do could be dangerous
    • Don't get any ideas from this unless you know what you're doing (moreso than myself to a high degree)

    OK, that being out of the way, let me provide a bit of background, and the supplies that I have currently, and why I would want to do something so stupid.

    I have made a UPS system much like <a href="http://www.dansdata.com/diyups.htm">
    The DIY (or, if you must, "Ghetto") UPS</a>. Except that I have used an AGM sealed, closed valve self regulated, deep cycle marine battery. I also have 2 chargers, both of which turn off every X amount of hours. They are both sufficient to charge the battery faster than it is being used, before they shutoff after X amount of time. I want to put them on a timer that will shut them off and turn them back on, effectively resetting them, every Y hours. Upon turning on, they only work at 2 amps, for safety's sake. The load on this battery is somewhere around 4 amps, give or take an amp.

    So, I would like to hook these chargers up in parallel on the battery - I need have the collective amperage, but keep the voltage low, or I'll boil my battery, or worse... Another catch; it is possible that both of these chargers will vary in voltage slightly (by somewhere around 2 volts), and I'm worried that it might effectively feed backwards into the charger with the lower voltage.

    How in the world do I go about doing this? Can I just connect the alligator clips from each charger directly to the posts on the battery, or do I need to connect both of the pos. clips to a single wire, and attach that to the pos. terminal, and then do the same for the neg. leads?

    Will different voltages from each charger be a problem? I imagine that the DC will follow the path of least resistance, so, if the battery and the first charger both have higher voltages than the second charger, it all pushes flat back through the second charger and either blows an internal fuse, or worse.

    Any ideas?:confused:
  2. caliusoptimu

    caliusoptimu New Member

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    if you are using typical 'manual' chargers in your setup, they undoubtedly have either full wave or bridge rectification right after the transformer, which would save your stuff from any kind of feedback due to a voltage difference. if they are 'automatic' chargers, the easiest way to be sure you dont fry your stuff would be to add a couple of high current diodes between them and the battery. but as far as i can tell, automatic chargers need to periodically sense voltage from the battery in order to begin and continue charging it properly. if you afraid of overcharging your battery you could add a couple more diodes on anyway, just to drop a volt.

    btw, what you need such uber dc backup for? lol
  3. gazzonyx

    gazzonyx Thread Starter New Member

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    Thanks! Yeah, they're both 3 stage chargers.
    So, you're saying just attach them to the poles (with diodes inline), or chain them to a cable and connect that cable?

    I'm a software development major, so this is for my linux server that serves my source code ... my source code is my life, so I have a rack in the closet with a Slackware server, hardware firewall (BSD based), and a switch. I'm terrified of what would happen if the power dropped and my harddrive was in the middle of a write. And, I don't know that I'd be around when the power went out, so a normal UPS giving me 5 minutes just won't cut it. Basically, it's so I can sleep at night. :D
  4. CaliusOptimus

    CaliusOptimus Well-Known Member

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    wow, yeah i think youve got the right idea building your own UPS. sounds like a fun project!

    i attatched a couple diagrams for ya. if you dont need to drop anymore than a volt or so from each charger, you can ignore D2 and D3, and connect the ground straight through.

    adding these diodes will keep your chargers from 'feeding back' on each other, but it will still allow the charger with a higher voltage to provide a larger share of current. if the difference is under a volt, it shouldnt be a problem. if it is, you may want to add another diode in series (D2, or D3) just to drop another volt. but all that said, it wont matter unless your chargers are tiny. anyways. hey, just run down to radioshack and grab the biggest bridge rectifier you can find. connect each charger to the AC (~) and your battery to the + side. leave the - side unconnected. it'll be easier than finding individual higher current diodes. happy programming!

    Attached Files:

  5. gazzonyx

    gazzonyx Thread Starter New Member

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    Thank you so much! This is exactly what I've been looking for! Rock on, dude! I really appreciate your help.
  6. gazzonyx

    gazzonyx Thread Starter New Member

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    OK, I just wanted to clarify something - I got a bridge rectifier, now, I'm not hooking up ANY of the negative leads? Not the one on the battery, and neither on the chargers? Do I ground any of it, anywhere?
  7. CaliusOptimus

    CaliusOptimus Well-Known Member

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    lol, sry i didnt mention. connect the negative on both chargers straight to the battery. leave the negative on the bridge rectifier unconnected though.
  8. gazzonyx

    gazzonyx Thread Starter New Member

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    Haha! Ok, thanks. I was standing there staring at the battery going:
    I'm sure this will work! But, it has to have ground... Maybe the inverter will ground it. That's just stupid. I'm going to die. I'd better make sure this is right.

    Hehe. Many thanks, once again, friend!
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