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Current Draw, Charging batteries

Discussion in 'Electronic Design' started by John, May 10, 2007.

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  1. John

    John Guest

    Ohm's law is in here somewhere, but I don't know quite how to apply

    I use a deep cycle battery, ~80AH @ 5hr rate.

    I would like to tote it along on trips, but it needs to be somewhat
    portable, so I am looking to wire up something in the trunk, or back
    of my truck, to juice it up when I am driving.

    Where I get lost is, if I the battery is basically fully discharged,
    how do I know what size wire I need to use, to get it back up to
    snuff? Do I need some sort of control ckt?

    It would be great if I could just plug it into the cigarette lighter,
    but that is fused at 10A, so I am assuming a deeply discharged battery
    of that size, would pop the fuse. Right? Same thing for a 14Awg set of
    connections, they would overload on a dead battery?

    Don't really want to run 8Awg wire to the bed, or the trunk, so how do
    I figure out what the max current is, that a "dead" battery of this
    type, will draw, so I can keep it simple?

    I know there are other issues, like safety, starting the vehicle
    first, and then connecting the battery, and such, but the basic
    charging issue is where I would like to get some advice, please.

    Thanks a lot,

  2. First, it is Really Bad to fully discharge a battery.

    Try here for more info
    This should answer many questions

  3. John

    John Guest

    Actually, it is quite okay to discharge a true deep cycle battery down
    to 20% of capacity, 50% being better, but that wasn't the question
    now, was it?

    I don't need a lesson on batteries, I was asking about a specific
    method of charging from an alternator, for the occasional need.......
  4. Alternators are built to supply uncontrolled, but finite, current to
    shorted battery so the best you can do when traveling connect this
    battery in parrallel with the main one by cables and contactor of rather
    convenctional sizes, just in case. Melting insulation on battery
    connections is not worth even trying.

  5. Rich Grise

    Rich Grise Guest

    If you're going to run a wire anyway, you might as well run a fat
    wire. :) But it wouldn't have to be anywhere near as fat as starter
    cables or jumper cables (which are meant to handle starter current).

    #10 or #8 should be fine to just keep a deep-cycle topped up, as
    long as it's not literally "fully" discharged.

    And the frame should be negative - if not, it needs some bonding
    straps. :)

    Good Luck!
  6. Mook Johnson

    Mook Johnson Guest

    If its not fully discharged and the bettery terminal voltage is above ~9V,
    I'd charge it through a 1 ohm resistor. That will limit the current to
    about 4.5 amps (asuming the alternator voltage is 13.5V) and will not blow
    the cigarette lighter fuse. The resistor will dissipate close ot 25 watts
    at first (hot) so get at least a 40W resistor. Two packs of those 10W 1 ohm
    radioshack resistors should do the trick. Put two in parallel and them put
    the two parallel pair in series with each other to get back to 1 ohm. All
    of $3.50 and some solder and your done.

    As the battery voltage increases, the current will decreasewhich means that
    you'll be getting very little current at hte end of the charge so that last
    few % of capacity will take a LONG time.

    You might want to consider a diode in the path so when you turn off the
    vehicle the deep cycle doesn't keep your electronics lit up. In addition to
    draining the battery it may damage something.

    This a a very crude battery charger and is only used for "special cases" and
    not for regular use.
  7. linnix

    linnix Guest

    Could it much higer than that.
    30 to 50 Watts of wasted power will be coming out of your gas tank
    Battery will peak at 13V but Alternator will not.
    PWM switched MOSFET might be better.
  8. Guest

    I need one of these myself, but haven't got around to designing/
    building one. It can be a simple linear circuit You should not be
    draining your battery to lower than 10V. So the drop across the
    circuit would be a 10A and 4V, or 40 watts. Toasty, but not the end
    of the world. [Mine would be linear because I want to listen to the
    radio and won't put up with the switcher hash. Your mileage may vary.]

    The circuit needs to current limit at whatever is safe for your
    outlet, which is why I picked 10A, though it could be 12A. I wouldn't
    go for the rated load. The circuit would use a PMOS pass device,
    perhaps two PMOS devices so that the body diode of one fet doesn't
    provide a current path should the battery voltage be less than the
    charging voltage. If you had a gel cell, you would want some
    overvoltage protection since some cars put out over 14V.

    Incidentally, if by deep discharge you mean a marine battery that has
    vents on it, I would not charge this in the back of a SUV or car. The
    vapors will cause corrosion. You need a a gel cell.

    You can get black boxes at autostore that allow this two battery
    scheme. I think all they do is prevent the reverse current flow.

    I have done "expeditions" with 5 serious size gel cells. They
    certainly are heavy. ;-) If you are just camping, you might consider a
    very small generator plus one or two of these batteries. You can get
    decent chargers for about $100.

    I use
  9. ehsjr

    ehsjr Guest

    Mook's idea is simplest and easiest to implement.
    Use it to keep the battery topped off. I would
    suggest a modification to the cigarrete lighter
    socket, if it is always live:

    ------ +---[1R]---[Fuse]---+
    ---o-----| | |+ |
    + -----o--^ | | | |+
    Acc --[Relay]--+ |Socket|=[PLUG] [Battery]
    | | | | |
    Gnd -----------+---| | | |
    ------ +-------------------+

    The relay is energized by the accessory line, so the circuit
    to the socket and the deep cycle battery is interrupted when
    the vechicle is off. Use an automotive relay rated for 30 or
    40 amps. If your vehicle kills power to the cigarrette
    lighter when off, no modification is needed, nor is the diode
    Mook mentioned.

    To _charge_ the battery when it is fully discharged (meaning
    to 50% capacity)vs keeping it topped off, you want something
    different. You need to put 40 Ah into the battery - that's
    *well* over 10 hours of driving when the current is limited
    to ~ 4.5 amps and the thing is plugged into the cigarette
    lighter. As the charge builds, the voltage in the battery
    rises and the current drops, so the charge rate tapers off,
    thus > 10 hours for full charge.

    I would use Mook's idea, making darn sure that the resistors
    were not in a place where they could burn anything. If you
    find that insufficient to keep the battery charged, you have
    two choices: either charge it with jumper cables with the
    car running in a parking area, or build/buy a setup for
    dual batteries.

  10. Ross Herbert

    Ross Herbert Guest

    The major problem of connecting different batteries having varying
    states of charge in parallel without any form of isolation is that of
    heavy current flowing from the battery having the higher charge into
    the battery having the lower charge.

    You are best advised to install a battery isolator (intelligent is
    best) thus avoiding any problems.

    Here is an example

    Most 4WD (SUV) shops will have them
  11. Tam/WB2TT

    Tam/WB2TT Guest

    That is about what I would do. If you plan to discharge the battery below 9
    V, consider using a 2 Ohm resistor instead. At any rate, even with 1 Ohm,
    there is no point to using wire bigger than #16 or #18, anything smaller
    gets fragile. I would also put an 8 amp fuse in the line, and consider
    adding a 10 Amp relay to disconnect the battery when the ignition is off. I
    have seen these both ways, but the cigarette lighter in my car does not turn
    off with the key out.

  12. Tam/WB2TT

    Tam/WB2TT Guest

    If you put the battery in the trunk, you should be aware of the dangers of
    possible explosion, and corrosion. To be legal, you will probably have to
    vent it to the outside.

  13. John

    John Guest

    Thanks Rich. *That is pretty well what I was figuring. I appreciate
    you addressing the question I asked, rather than sending me off to
    learn all about batteries, or second guessing what I am trying to do.

    A friend camps with me maybe three times a year, and he uses a cpap
    machine at night, so all I want to do is tote the battery for those
    few times, and, just wanna 'refill' it if we decide to extend the trip
    a few days. Don't need anything more than that.

    I think everyone knows that you don't drain a battery to zero.... I
    sure hate being told stuff like that.....

    Much obliged for the common sense and logic.

  14. Guest

    This is why I suggest a gel cell, though they are not tolerant of over-
    voltage. [The meter on my car peaks at 18V, though it never goes
    there, but Yikes! It is not all that unusual for auto voltage to be
    well above 14V.] Maybe I'll get around to prototyping a circuit. BTW,
    when I did my seat of the pants analysis that the charger might have
    to dissipate 40W (i.e. 10V on the battery, 14V on the car generator,
    and 10A), I didn't consider the battery voltage rising as soon as the
    current is applied. For instance, if it jumps to 12V immediately, then
    the power dissipation is only 20W.
  15. Tam/WB2TT

    Tam/WB2TT Guest

    Also, you can't ignore the resistance of the wires. My previous car had the
    battery under the rear seat. The built in voltmeter on the instrument panel
    often displayed 15.4 V. A digital VOM plugged into the cigarette lighter
    showed the same. I can't believe the battery voltage was actually 15.4,
    since the battery was still good after 4 years, and the 15.4 was probably
    the alternator voltage.

    I used to charge gel cells with an HP lab power supply. It generally took
    about 14.3 V to get a couple of hundred ma charging current into the ~7AH

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