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charging a fully discharged car lead acid battery

Discussion in 'Electronic Repair' started by Veggie, Oct 12, 2005.

  1. Veggie

    Veggie Guest

    I have a car with a battery that is completely discharged (accessory
    left on for over 24 hours). Read 0 volts.

    What is the best way to remedy this?

    a) jump start - it seems to not be a good option as it dumps high
    current into the dead battery. If you're on the road somewhere, sure,
    you need to get going but jump starting seems to be undesirable.

    b) put it on a battery charger, one of those 5/10/25 amp ones. If so,
    which rate is the best for a completely flat battery?

    There seems to be a lot said about sulfate build up on batteries. It
    seems to be an unsettled subject on the Net, as many say one thing but
    an equal number refute it. One claim in interesting to me- that leaving
    a lead acid battery in flat condition for a long time caused sulfate
    build up. Is this true, and what is a "long time"? Are we talking
    days, weeks, or months?
  2. Frank S.

    Frank S. Guest

    Usually when a battery is pulled all the way down, They don't come back.
    you should try slow recharge. It might work. I have found that when car
    batteries are pulled down to about 10v 3 times, they become worthless.
    Good Luck
  3. RonKZ650

    RonKZ650 Guest

    If you're just talking 24 hours of drain and the battery is discharged,
    it should just charge right up using any method you have, slow charge,
    50A boost then start, jump start then drive around a few miles,
    anything should work. Car batteries are pretty tough. I have a 1993
    Ford van that sat 6 yrs, battery sitting in a dead condition for
    probably 5.5 of those in temps ranging from 110 degrees to -25. I
    figured the Van dead including the battery with only 6000 miles on it.
    Charged the battery, fired right up with 6 yr old gas and all. This was
    2 yrs ago and battery and van still running fine. Nice to be back from
    that 6 yr "vacation".
  4. Guest

    About the only option you do not want to do is to jump start the car.
    The excessive current demands on the charging system of the car might
    cause alternator or regulator damage. Some car charging circuits have
    protection for excessive battery current draw during charge, most until
    recent model years do not.

    The best option is to use the lowest current charger until you can
    verify 10 volts on the battery unloaded after it sits a few minutes off
    the charger. Then any higher current chrage should do fine. This is
    only to provide a margin a safety on the excessive current draw the
    battery is going to attempt to pull when completely dead. No sense
    opening up the thermal fuse inside the transformer in your el-cheapo
    battery charger.

    I am suprised it actually reads zero volts, unless the battery has an
    internal excessive discharge protect cutoff device. Disconnect the
    battery and read the unloaded voltage.

    FYI, Were it my car with winter coming up, I would not even worry about
    it, replace the darn thing. Even the best batteries are less than
    $100. It might even be new enough to get a pro-rated warranty
    exchange!!!! Having been out in the cold single digits with a bad
    battery ONCE, I never chance a weak battery on an upcoming winter.
  5. Asimov

    Asimov Guest

    "Veggie" bravely wrote to "All" (11 Oct 05 23:19:43)
    --- on the heady topic of "charging a fully discharged car lead acid battery"

    Ve> From: Veggie <>
    Ve> Xref: core-easynews

    Ve> I have a car with a battery that is completely discharged (accessory
    Ve> left on for over 24 hours). Read 0 volts.

    Ve> What is the best way to remedy this?

    Ve> a) jump start - it seems to not be a good option as it dumps high
    Ve> current into the dead battery. If you're on the road somewhere, sure,
    Ve> you need to get going but jump starting seems to be undesirable.

    Ve> b) put it on a battery charger, one of those 5/10/25 amp ones. If so,
    Ve> which rate is the best for a completely flat battery?

    Ve> There seems to be a lot said about sulfate build up on batteries. It
    Ve> seems to be an unsettled subject on the Net, as many say one thing but
    Ve> an equal number refute it. One claim in interesting to me- that
    Ve> leaving a lead acid battery in flat condition for a long time caused
    Ve> sulfate build up. Is this true, and what is a "long time"? Are we
    Ve> talking days, weeks, or months?

    Charge it at a current rate as high as possible but that doesn't cause
    rapid dissociation of the electrolyte (hydrogen outgassing). One might
    start at only a mere 1 ampere and work up from there as the terminal
    voltage reaches a nominal minimum 1.75V per cell (depending on

    Mind that if the battery measures zero volts under load that it may
    still exibit 12 volts open circuit and not accept any charge at all.
    In this case one might try raising the charging voltage until it
    begins to accept a charge. Then continue the charging process in the
    usual contolled manner. If it won't accept a charge then chuck it.


    .... Real techs don't lick nine-volt batteries!
  6. James Sweet

    James Sweet Guest

    Do either, the battery will never be the same again, but if you just stick
    it on the charger on the normal setting or jump start the car and drive it
    for a half hour or so then it might work for a bit.

    Sulfate will build up very quickly, hours to days, after months it'll be
    very heavily sulfated.
  7. Bob  AZ

    Bob AZ Guest

    Jumper a small 12v bulb like a stop light bulb in series with a good
    battery. I will be at 10 or more volts in a few hours if not less. Then
    use a headlamp bulb to complete the charge.
    Bob AZ
  8. Veggie

    Veggie Guest

    I ended up borrowing a lab power supply and am charging it at 1 amp. I
    figured it can't be healthy for the battery to take a high current
    charge. From my experience in nickel batteries, fast charging shortened
    battery life significantly. Not sure if this translates to lead acid.
    You're right, I measured 0.0 at the cigarette light with the key in
    accessory position. I guess they run a relay now to power accessories,
    and there wasn't enough voltage to energize. The battery actually read
    about 8.5 at the terminal, no load. I gave it 2 hours of 1 amp charge,
    then disconnected it for the night. After sitting overnight, it reads
    11.0 volts.

    It's back on 1 amp charge again.
  9. What voltage was needed to pass 1 amp? This tends to be the clue. A
    heavily sulphated lead acid can have a very high internal resistance so
    most modern chargers can't pass enough current. Same as trying to charge
    it with the car alternator after a jump start.

    Leave the lab supply set at one amp on for about 3 days.
  10. Think every car alternator ever made will limit the current to a safe
    value for it. Indeed, even early ones had voltage limiting which meant
    they simply wouldn't recharge a sulphated battery with a high internal
  11. zantafio

    zantafio Guest

    Got the battery of my bimmer flat after 10 months of storage without
    disconnecting it. 0 millivolts !
    Put a batt charger, less than 0.5 Amps for hours at the beginning. Then the
    current slowly increased. Left it for 12 hrs during the day only for 2 days.
    The battery wasn't filled yet when and I felt too much impatient thinking it
    was dead. Turned the key, the starter ran slow, black smoke then the engine
    started. Got a 3 or 4 km drive then left it for 1 hour in idle. The voltage
    climbed close to 11 V with the lights On, engine stopped. Today, 3 months
    later the voltage is correct and I've no trouble starting after a week
    sleeping in the garage (the car, not me!) .

    Shall I add that the battery is a low cost _all black_ battery, 4-y old ?
  12. Veggie

    Veggie Guest

    I'll check that when I disconnect the charger for the night later
    today. It can't be much more than 12v, because the max of this supply
    is 12v. Which reminds me, as the battery comes up to voltage, the
    charger won't do the trick anymore. I'll have to get a real car battery
    It seemed to be common sense to trickle charge it for awhile, instead of
    jolting it with a jump from a charged battery. Beyond common sense, is
    there a reason for a low 1 amp charge for several days? Is it to
    prevent hydrogen formation?
  13. Jim Adney

    Jim Adney Guest

    A healthy lead acid battery can take a huge charging current with no
    difficulty. 5/10/25 A are all fine. In fact, you can put as much
    current as you're able, as long as your wires can take it, and the
    actual battery voltage does not exceed 14.2 V (for a nominal 12 V
    Jumping does not hurt a healthy battery. If it really only shows 0 V,
    however, this is not a good sign. That generally means that the
    battery is shorted inside. Honestly, I've never seen one that had ALL
    the cells shorted, so you usually see 6, 8 or 10 volts.

    If your battery is actually completely shorted, you could damage the
    donor charging system by jumping this shorted battery.

    To be prudent, you could put it on a trickle charger and watch the
    battery voltage while you do this. The voltage should come up to 12V
    rather quickly. Once it does, you know that you have no shorted cells
    and you can proceed to actually charge it.

    If your car is stuck somewhere remote, you can use an automobile
    headlight, in series with the jumper cables, to limit the donor
    current to something that you know will be safe for the donor. Do this
    just long enough to see if you can get your battery up close to 12 V,
    then jump directly, without the headlight.
    Any of these is fine for your healty battery. Once the battery comes
    up to 14.2 V you should reduce the charging current to keep the
    voltage below that number.
    Sulfation is a very slowly developing process. It starts as soon as
    the battery begins to be discharged, but it takes months in this state
    to get to a point where the battery won't take a charge.

    Sulfation can be reversed. There are many ways to do it, but if you
    want to restore your battery to good health, you must not try to rush
    it. Put it in a slow charge, as always, keeping the voltage below 14.2
    V. A badly sulfated battery will only draw 50-100 mA in this state,
    but there is a chance that it will recover if you are patient. It can
    take a week or 2.

    For a battery that has sat a day, or a month, you'll never notice it,
    and it will recover as you use the car, as long as your charging
    system is working properly.

  14. Jim Adney

    Jim Adney Guest

    A CV?CC lab supply is a perfect battery charger. Set the voltage limit
    to 14 V and the current to whatever your the supply and your wires can
    take. A lead acid battery in good condition can take charge at any
    rate up to what it can deliver. Most of these can deliver 200+ amps,
    so most of us are not likely going to be able to come up with chargers
    bigger than that.
    This proves that it's healthy.
    Any charging current, up to the point where you get 14.2 V across the
    battery terminals will be fine. (Above 14.2V, you start to dissociate
    water into hydrogen and oxygen. If this happens vigorously, it can
    damage the porous sintered plates, plus you get an explosive gas.)

  15. Jim Adney

    Jim Adney Guest

    At only 12 V, you really won't have much stored energy in that
    battery. Yes, you'll need a different charger, or a jump.
    The trickly charge is always safe, and no problem at all if you aren't
    in a hurry.

    There's no hydrogen problem as long as you keep the battery voltage
    below 14.2 V.

  16. Jim Adney

    Jim Adney Guest

    I think you're right, but I'm not sure what one would do if run into a
    dead short. There's probably some minimum safe impedance that they
    A sulfated cell does appear to have a high internal resistance, but
    that's not the whole story. The reason it looks this way is that the
    lead plate, in the process of discharging, has been turned to lead
    sulfate, taking the sulfate ions out of the sulfuric acid solution.

    The lead sulfate (a solid) is deposited in some particular state, but
    slowly changes crystalline form over time. Unfortunatley, the latter
    crystal form is not nearly as soluble as the one originally deposited,
    so when the current is reversed, you can only remove the sulfate ions,
    taking them back into solution, as fast as the new crystal state will
    let go of them.

    Unfortunately, that's slow.

    You can push the voltage up, but that doesn't increase the solubility.

    In the end, increasing the voltage just quickly puts you above the
    point where hydrolysis occurs, and this just turns out to be a waste
    of energy, because it does nothing toward recharging the battery. High
    voltage, and hydrolysis, also tend to damage the porous sintered
    battery plates.

    If you want to reverse sulfation, you just have to set the voltage at
    about 2.35 V per cell and wait for the ions to come back into
    solution. If you're patient, it usually works.

    I rather regularly recover sulfated batterys and it generally takes
    about a week. I had one which took 2.

  17. Veggie

    Veggie Guest

    Sorry for the dumb question, but how many cells are in a 12 volt car
    battery? Must be six at around 2 volts each. If this is right, then
    your advice is to set the charge voltage at 14.1 volts. If I recall
    right, the car's charging system runs at 13.8 volts so the 14.1 is right
    in that ballpark.
    There is all manner of hubbub about pulsed desulfators, etc. For the
    average person, taking a week or two with off the shelf equipment is
    much desired over special desulfators.
  18. Guest

    A dead battery will damage an alternator as will a battery with a bad
    It is a matter of time and chance when and how hot the alternator,
    regulator, or rectifier get and when they will fail.

    "An alternator was never designed to charge a completely dead battery"

    A discharged, but not dead, battery is ok to jump start and allow the
    alternator to charge it up. A discharged battery will read around
    11-12 volts unloaded or minimal load, 100mA. 8 volts on the battery
    indicated that it was in a deep discharge state, not good.

    Unless it was a nearly new battery, I would do one of two things if it
    does charge and start the car:
    1. Get it load tested on the coldest day in the near future, replace if
    it is marginal.
    2. Simply replace the battery with a new one for the peace of mind.
  19. Says nothing about damaging the alternator, though.

    It's true an alternator won't charge a *totally* flat battery. I had this
    once after leaving the car at an airport long stay carpark while on
    holiday. Still don't know what caused it. Got a jump start and drove the
    50 or so miles home. At the end of the journey the battery was still
    *totally* dead. Put it on trickle charge for a week using an ancient non
    regulated charger. After it was fully charged I checked the capacity by
    discharging into a load and timing it. Only a rough test. And it would
    start the car ok. Dunno if this shortened the life as it was not new and
    I'd bought a replacement. ;-)
  20. Dave-

    I've heard that the reason a totally flat battery won't charge is that its
    chemistry has changed because of sulphation. To get it to take a charge,
    a much higher voltage is required, which your alternator may not be able
    to deliver. Perhaps your trickle charger has a sufficiently high
    open-circuit voltage to overcome the problem. After the trickle current
    had flowed for a sufficient amount of time, the chemistry returned to

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