Battery Tender

Discussion in 'The Projects Forum' started by tracecom, Mar 24, 2011.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    What I want to do is build a 12V lead acid battery maintainer to keep the charge topped off while I intermittently draw up to 5A from the battery. For about $71, I can buy a battery tender that will meet for my needs.

    http://www.google.com/products/cata...a=X&ei=u4WLTfe3NMiitgeGu5WDDg&ved=0CCgQ8wIwAw#

    However, I already have a suitable transformer, 5A diodes, regulators (7812 and 317), a good assortment of caps and resistors, and a couple of 2N3055 transistors with heat sinks. Can someone supply a circuit for me?

    Thanks.
     
  2. PackratKing

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  3. tracecom

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    Thanks for the link. Looks like this one is set to limit current to 2 amps. I guess I could put in a second 2N3055, but I would really like to have a design that I can just build and not have to tinker with.

    Anyone have a "tried and true" 4 to 5 amp tender? Thanks.
     
  4. SgtWookie

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    Jul 17, 2007
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    Did you see Beenthere's "Battery Charger Regulator" project?
    http://forum.allaboutcircuits.com/showthread.php?t=6099

    The output will have 120Hz noise on it, but it'll probably work better than trying to make one from the things you have on hand.

    Trying to use linear regulators for things like this is initially attractive, until you figure out how high the power dissipation in the regulators will be. It'll make a great room heater. You would need large heat sinks on the transistors.

    You said:
    but didn't give any specifications on it.

    National Semiconductor has, in the datasheet for the LM117/LM317, on top of page 17, a schematic for a "High Current Adjustable Regulator". However, it would need modifications to be used as a battery charger/tender.
     
  5. jerseyguy1996

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  6. tracecom

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  7. SgtWookie

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    The "PICAXE-controlled battery charger" project?

    Well, I see that it goes to sleep for an hour. Apparently, it was designed to simply maintain a battery at between 13.1v and 14.1v. If you put a load on it while the picaxe is sleeping, the battery may wind up being discharged quite a bit by the time it wakes up.
     
  8. tracecom

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    That's the one.

    I can shorten that to whatever time seems appropriate...maybe 5 minutes. I also don't like using a relay.

    What I want to do is power a device that draws up to 5 amps intermittently (25 to 50% duty cycle) from a deep cycle lead acid battery (about 33 Ah) and have the charger keep the battery continually topped off without overcharging it. Do you think this circuit will do that (assuming that the transformer, full wave bridge, and relay will provide 4 amps or better)?

    I also have never seen an LM7812 run off AC; is that ok?

    Thanks.
     
    Last edited: Mar 30, 2011
  9. SgtWookie

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    OK.
    I really don't know why there is such a long delay in there to begin with. If the battery goes bad, the charger will cycle on/off very frequently; maybe that's why they did it.

    I don't like the high limit being 14.1v. That's OK for the very first cycle, or if the battery has been discharged below 12.5v, but not for general "float" maintenance. It should stop at around 13.7v, and start back up around 13.3v - this is at 25°C, or 77°F.

    You've stated that before.

    It might. It's supposed to be used with a standard "dumb" battery charger.

    It's not AC by the time it gets to the 7812. There's a diode and 220uF cap before the regulator.

    Neither 7812 has an 0.1uF/100nF cap across the OUT and GND terminals as close as possible to the regulator; this is required per the datasheets.
    Also, there should be 0.33uF/330nF caps across the IN and GND terminals as close as possible to the regulators; also required.

    Without these caps, the regulators may oscillate at high frequencies (MHz range) and the output will be much lower than expected.
     
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  10. tracecom

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    Attached is the schematic for the PICAXE-08M battery charger. I have redrawn it and added some small caps on the inputs and outputs of the voltage regulators. If anyone cares to look it over for me, I would be grateful.

    I plan on using 1N4001 diodes instead of 1N4004 as shown on the original project. In addition, I wonder if the 2.2uF cap from pin 1 of U3 to ground is sufficient or would 220uF be more appropriate? I have all the components except the BC338; would a 2N2222A do?

    Thanks for any input or corrections.
     
  11. SgtWookie

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    Yes, the schematic you drew looks OK.
     
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