Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Dead Car Battery Load when car off

  1. May 9, 2012 #1


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    My car has a problem that there is some sort of load on the battery even when the engine is off. So the battery slowly drains with time. If left for more than a couple of weeks, the battery will be too drained to start the car.

    I had this happen to me, and so I bought a mobile booster pack (i.e. external battery with built in jumper cables and ability to connect to mains AC to charge) and boosted it. I drove it for a while and then had to leave the car parked for another long period of time*. Since I knew I would have to do this, I disconnected the battery until such time as I was going to start using it.

    After reconnecting the battery three days ago, the car started on the first attempt, and so I took it out and drove it for an errand, gassed it up, and brought it back.

    But today when I tried starting it, it wouldn't start at all. There wasn't even enough charge to light up the dash instrumentation. The battery had only been reconnected for 3 days and was already completely drained. This tells me that the battery is incapable of retaining a charge and must be completely dead.

    I took a voltmeter to it and measured 0.04 V across the terminals! I had to check it twice, because I couldn't believe it. I also looked at the resistance and got a value of some 330 ohms. I'm not sure if this is normal as an internal resistance for a lead acid battery. EDIT: never mind: these numbers were measured with the terminals connected, so the voltage does't represent the open circuit voltage, nor does the resistance represent solely the internal impedance.

    It seems clear to me that the battery is toast, and I have no choice but to use my booster pack to start the car and drive it non-stop to somewhere where I can get a replacement battery installed.

    Has anyone ever experienced something like this before? How can I avoid this happening in the future, short of driving the car regularly, which is not always possible? What could possibly be draining the battery when nothing is turned on? Why is the battery (which is not that old) already at the stage where it cannot retain a charge at all? Is it just that if you let lead acid batteries drain too low, they become irreversibly damaged?

    EDIT: I do see from Wikipedia that deep discharging is a really bad thing to do to a lead acid battery designed to deliver starting current, and that, in fact, even when in storage, such batteries should be charged every couple of weeks.

    *Please don't ask me why I have a car if I'm going to leave it parked for long periods of time: it's none of your business.
    Last edited: May 9, 2012
  2. jcsd
  3. May 9, 2012 #2
    Putting an ohmmeter across a car battery is very dangerous, you are lucky the battery was totally dead.
    If you don't know why you may be better off not messing with this. That said, if you suspect a slow drain you can confirm this by measuring the battery current while the car is off. But this has to be done *carefully*. If you use an ammeter in series with the (+) lead, do not attempt to start the car! The safe approach (what they would do in a shop) is use a "hall effect" current probe clipped around the (+) lead. Once a drain is seen on the ammeter, start removing fuses.
  4. May 9, 2012 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It was a stupid thing to do and it should have occurred to me. And yes, I know why: an ohmmeter is meant to be connected to a passive resistive load, not to a *voltage supply* with a low output impedance that can source tremendous current (like a car battery). Fortuanately, nothing bad happened, and my multimeter even seems to be working still.

    My pocket digital multimeter is fairly cheap and doesn't have an ammeter function built in. I've read that you can buy specialized battery testers for this purpose, but they seem kind of expensive ($50). Still, I do suspect a slow drain, and this may be the only way to get to the bottom of the problem.
  5. May 10, 2012 #4
    If all you have is a voltmeter there is still a test that may help find an unwanted load.
    Charge the battery.
    Disconnect the (-) lead.
    Measure voltage between the (-) battery terminal and the (-) cable lead.
    If you see voltage here start removing fuses until it goes away.
    Note, even very small loads, such as your clock, will cause you to see voltage. All you can tell from this is that there is a load on the battery, but not how much.
  6. May 10, 2012 #5

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Have you checked whether your under-hood and in-trunk lights really turn off when you close hood and trunk?

    Radio gets power to its station memory and clock while key is off.
    Engine control computer gets power while key is off.
    So do interior lights, door locks, radio antenna raise-lower motor, security system, etc etc.
    Alternator has rectifier diodes inside that can get ruined by wrong polarity during a jump-start and they'll run the battery down through altenator windings possibly wrecking them too.

    Normal current draw is on the order of 1/20 amp for radio memory and the sleeping computers. When you open door or trunk and the light in there comes on, of course current is higher. If something is malfunctioning, current will be higher.

    could be as simple as dirt on top of battery between + and - posts that's moist and conductive. Clean it with detergent to cut grease then baking soda to neutralize acid.

    Can you make a test light from a lamp socket and bulb? Auto parts stores sell sockets.
    Get a 1156 lamp and socket to fit, and two alligator clips. Attach clips to the pigtail wires on your socket.
    Disconnect battery negative terminal.
    Connect the test light between battery negative and chassis.
    If the lamp lights it means something in the car is passing enough current to light the lamp. It takes more that 1/20 amp to light that 1156 lamp, its full current is 2.1 amps.

    So make that test hookup and disconnect things in the car until you find the culprit.
    I'd start by lifting the big wire at rear of alternator. Then pull fuses one at a time, as emiguy suggested..
  7. May 10, 2012 #6
    How old is 'not that old'? If the battery is still under warranty you may be able to recoup some or all of the expense of a new battery in replacement of a defective one.

    While you may assume something is draining the battery, one can also assume the battery had an internal short in one or more of the cells. Lead sulfate cystals form on the plates from each discharge/charge cycle. A discharged battery, not even fully discharged, exasperates this process. As the cystals grow they will eventually contact a nearby plate and a slow dsicharge will begin. Over more charge/discharges cycles the health of the battery will deteriate until complete failure in being able to hold a charge.

    You can buy a unit called a hydrometer ( http://www.ehow.com/how_4964575_use-battery-hydrometer.html ) to test the charge on each cell.

    Since you state that the battery has been through several discharges, and finally will now not hold a charge I suspect it has been the battery all along and there is nothing out of the ordinary in your car electrics draining the battery.

    If you purchase a new battery it should be able to start the car after several months of being left idle.
  8. May 10, 2012 #7


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    car batteries seem to only last ~ 3-4 years these days unlike up to 10 years in days of old.
    when was the last time you checked the electrolyte level in the battery ?
    have you ever checked it ? should be done every few months.
    if the battery electrolyte level has dropped well below the top of the plates then it is going to loose its ability to be charged fully and be able to hold a charge

  9. May 10, 2012 #8


    User Avatar

    Staff: Mentor

    When a car battery dies, it's *POOF* -- dead.

    My last dead car battery was just barely noticeably slow in starting my car to go to work in the morning and when I tried to go out for lunch 4 hours later, it had nothing at all.

    If it was more than 3 years old, it was probably dead on its own; if it was less, you may have killed it. I killed a car battery this way while on a 5 month deployment with the Navy.
  10. May 10, 2012 #9

    I like Serena

    User Avatar
    Homework Helper

    I had a car battery that I drained completely by accident.
    Apparently they don't take that well.
    It was not new anymore either.
    I could still recharge it and my volt meter gave off 12 V, but it wouldn't start the car anymore.

    I have a smart car battery charger (standard equipment from the store) that conditions the battery when you leave it on.
    It also shows whether a battery is still healthy (after about an hour).
    Not surprisingly, it showed that the battery was no good anymore.
  11. May 10, 2012 #10


    User Avatar
    Science Advisor
    Homework Helper

    They don't need to handle that in normal operation. Unless you leave the lights on when parked, the only time a car battery delivers a significant amount of power is when cranking the engine to start it.

    In fact car batteries are designated as "cranking batteries" or "starting batteries", compared with "deep cycle batteries" which ARE designed for sitations where normal use involves full discharge and full recharge. But a deep cycle battery will have a lower maximum cranking current than a similar sized car battery (and will probably cost more as well) so don't be tempted to swap one type for the other!
  12. May 10, 2012 #11

    I like Serena

    User Avatar
    Homework Helper

    Before, when I had left the lights on, afterward the battery would still show some life.
    When I turned the key, I would at least hear a clicking noise.
    And it would recharge with my charger showing it was still healthy.

    In this case, I had drained the battery until it was completely dead.
  13. May 10, 2012 #12
    Sometimes when your alternator can put a load on a battery. Usually a sign that it is going out. Also have you tried checking the water and electrolyte levels on the battery. Usually topping it off and applying a trickle charge can get a little more life out of them (if the battery is the issue)

    Either way, how about a discussion about how car batteries work! That would be fun! :)
    Do car batteries use a salt bridge? And if so how do they actually work?
  14. May 11, 2012 #13


    User Avatar
    Science Advisor

    This is mainly about RE batteries but the basic facts are the same for car batteries. http://www.eecis.udel.edu/~honsberg/Eleg620/Storage%20chapter-v6-printout-for-class.pdf [Broken]
    Last edited by a moderator: May 6, 2017
  15. May 11, 2012 #14


    User Avatar
    Science Advisor

    I saw a study where a PHD student collected dozens of dead batteries and cut them open to see why they failed.

    90% of them failed because of faulty connections between the cells and in the connections to the terminals.
    The Chemistry of the batteries was still working OK when the batteries were discarded.

    So, this may be why your battery went open circuit. Inside a battery is a very hostile place and corrosion eventually causes most connections to fail.

    Unfortunately modern batteries are not designed to be repaired so they have to be thrown out.
  16. May 11, 2012 #15

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    that's my experience.
    "In days of old" (which i remember) car batteries had those connections exposed, on the top of the battery. Nowadays they're inside where you can't see them.

    The el-cheapo batteries from discount auto parts stores have not given me good service, the internal cell connectors break after a year or two. When that happens the battery fails suddenly. It may run the radio or some lights by conducting through the sulfuric acid that wets the broken connection but it won't handle the starter. One actually ignited the hydrogen gas and blew its top off when i tried to start the car. That revealed the flimsy cell connector that had cracked. It was my last 'bargain battery' .
    A battery that's dying of old age does it gradually. Starter cranks audibly slower and it won't hold a charge for even a week. I replace them then rather than wait till stranded. Usually that's about a month before the pro-rated warranty runs out so you get like half a dollar off the next battery.. just bite the bullet.

    I buy high end batteries now even though it's a few extra bucks. It saves a lot of aggravation because they go usually five or six years.

    my two cents . Been at this since days of 6 volt cars. '49 Buick taught me generators, '51 Chrysler taught me voltage regulators, '53 Ford taught me dashboard instruments.
    '88 Ford and '91 Dodge taught me about today's 'bargain' car batteries.

    old jim
  17. May 11, 2012 #16


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    As an update, this battery lasted 4 yrs.

    I got a replacement for it. It has a 4 year free replacement period and a 10 year lifetime warranty. I installed it without incident. The car starts fine. I don't have time try to track down the slow drain for the next two weeks, but after that I want to try. I'll check back with you guys here when I do. Thanks for all the info and suggestions so far.
  18. May 11, 2012 #17
    While you are at it gob a liberal amount of Vaseline on the battery terminals and cable terminal clamps before reattaching. This will provide a moisture barrier and prevent corrosion.
  19. May 12, 2012 #18
    Like I said before, chances are it was the old battery draining itself with an internal short all the time when you thought there was an external slow drain through the car circuitry.
  20. May 13, 2012 #19
    That makes sense too lol
  21. May 13, 2012 #20
    I think the OP was suspecting that the battery was draining via an external path because he charged the battery, disconnected it, then after leaving it for a "long period of time" it started fine. Then 3 days later it was dead, with the only difference that it was connected up. He makes a good point, if it were an internal battery issue disconnecting it would not have helped it maintain a charge.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Dead Car Battery Load when car off