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Car Battery Rapidly Losing Voltage Difference

  1. Feb 1, 2010 #1
    I'm attempting to figure out why my car isn't starting. I've reseated all the grounds and made sure the ground connections were clean and bare. I've cleaned the battery terminals and clamps with a wire brush as there was some corrosion on the terminals.

    I charged the battery two days ago due to abundant corrosion and a lack of voltage on the battery. It was reading 11.8V. I charged it up to 12.8V with a battery charger that plugs into the house. I didn't attempt to restart the car at that point.

    Today, two days later, the battery is at 11.8V again and will not turn the starter. I measure the amperage drain on the battery and it's .06A, which for the Porsche 944, is within tolerance. It should be around .05.-07 Amps. Obviously, a 60mA drain over 2 days shouldn't drain a full Volt off the battery.

    I charged the battery up again today to 12.3V. However, when I turn on the headlights and the blower motor, (not engaging the starter), the voltage on the battery plummeted to 8-9V within 10-15 seconds of doing so. From what I've gathered online, shouldn't the battery only be seeing a .1-.2 voltage drop when the headlights are on?

    Why is the battery acting this way? What could be going on?

  2. jcsd
  3. Feb 1, 2010 #2
    Check the battery voltage directly on the terminals with the headlights on. If it is also 8-9 V, replace battery. Can you check the electrolyte level in all 6 cells?
  4. Feb 1, 2010 #3
    Sounds like you need a new battery.
  5. Feb 1, 2010 #4


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    Sounds like it's dead. But you should expect more than 12.3V on it when it's charging and near fully charged. You could see as much as 15V during charging.
    But if it dies that quickly with only headlamps connected, it's probably bust; you can run them for an hour with the headlamps on, when they're in good shape.
  6. Feb 1, 2010 #5
    Unfortunately, it's a closed battery with no electrolyte access, as far as I can tell. I have no way of seeing the level.

    The battery was only charged to 12.3 because I was in the middle of the charge when the idea was given to me to test with the headlights on. It'll charge to 12.8V from the home charger, and reads as high as 14.1V when the alternator is charging it.

    In order to test with just the headlights on, I'll have to recharge the battery, which could take hours. I won't know what's going on with that instantly.

    Any other thoughts? I've never seen a battery drop its voltage so fast. How does this occur? What are the conditions for a battery to do so?
  7. Feb 1, 2010 #6


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    The question is whether your battery is actually low or if there is still a poor connection somewhere. So, by testing the voltage at the actual terminals you will eliminate the possibility of a poor connection. If it is a poor connection, your battery is probably not even low to start with. I don't think you need to wait hours to charge it up again. I think you could tell what you need to if the battery is low.
  8. Feb 1, 2010 #7
    Well, I took the battery out of the car, and it's reading 12.1V now. It's too dark and too cold and I don't have a garage or anything to work on it. I also don't have another car.
  9. Feb 1, 2010 #8


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    It is possible to have a battery that reads above 12V, but that has a weak cell that limits your cranking amperage. I just replaced the battery in my wife's Legacy for that reason. If you have a sealed battery, it's not possible to do a cell-by-cell test with a multimeter, so your options as a shade-tree mechanic are limited.
  10. Feb 1, 2010 #9


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    If you haven't already done it, you might need to clean the inside of the battery connectors with a circular file. This is the part of the connector that connects to the battery terminals.

    I have seen a tough ceramic-like substance form here, and it eventually stops all conduction.

    If you can get a meter right on the battery terminals and they are at a higher voltage than the connector, then there might be a drop as mentioned above.
  11. Feb 1, 2010 #10
    I thought of that too. But I took a circular wire brush to the insides of the terminals as well as the terminals themselves. The voltage readings don't fluctuate, as far as I can tell, from terminals to other points that are at ground voltage.

    After class tomorrow, I'll attempt again to make sure the connections are solid.
  12. Feb 1, 2010 #11


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    Are you in a cold region? If you've had a cold snap lately (as in -20 C or so), and your battery was low to begin with, your battery may have frozen up (though the last time that happened with my mom's car, I was reading around 3 V, not 12ish).

    I think most auto parts stores (Canadian Tire, UAP/NAPA, PartSource, Costco, etc.) will check out your car battery for you for free (if you fish it out and bring it in). If you do have a Porsche 911 (how does an Undergrad get one of these?!), your battery might have a non-standard form factor (i.e. it'll be pricey, but would you expect any less?)

    EDIT: Even if it's not fully frozen, the cranking amps will be significantly reduced at low temperatures. If you're in a really cold region, you may want to invest in a battery blanket. See Cold Cranking Amps:
  13. Feb 1, 2010 #12
    Well, it's been cold, but not THAT cold. it's been averaging about -6C (20F) for the past few weeks. I did take it in last time this happened (around October/November) and they said the battery was perfect and that it was the terminal corrosion causing the lack of charging. Cleaned it all up, charged it, and it was fine until now.

    And it's a 1986 944, not a 911 :) It's bluebook value is about $4000, cheaper than most cars on the road today I'd guess. It's pretty though and fun to drive [WHEN IT WORKS!!]. BIG 2.5 liter 4cyl engine. Also, I'm a 32 year old undergraduate going back to school. I saved up for nearly five years to restore this ebay purchase (without an engine).


    The battery used is not non-standard, but yes, more pricey due to its form factor. It's a BCI Group 41 battery (11&3/16" L x 6&7/8" W x 6&7/8" H). Most places only carry one brand, if any. Costs are usually ~$100 for it.
  14. Feb 1, 2010 #13
    i've had a car before that ate alternators. when they go bad, you can end up with just poor charging that may slowly drain the life out of your system.

    i would start by cleaning the battery posts and clamps with the proper round brushes, so as not to grind it down unevenly. then, coat these shiny uncorroded lead surfaces with grease (axle grease is fine) and re-clamp. it will still conduct fine, but inhibit corrosion (which doesn't conduct fine at all).

    check that your alternator's belt is tensioned properly and not slipping. tighten/replace belts and tensioners as necessary.

    if that doesn't work, replace battery. if that doesn't work, replace alternator.

    yes, it can add up. but what i did learn years ago is that i could replace all that stuff for less than the garage mechanic wanted to just replace the alternator. plus, everything's new, which makes it more reliable.
  15. Feb 2, 2010 #14
    If the problem is ground points or cables, the fix can be had for under $10. Alternators are $200+ for the car, and batteries over $100. It can get pricey on this car to just replace stuff willy nilly. I'd rather try to fix it for a few dollars first :)
  16. Feb 2, 2010 #15


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    Take your battery in for a load test. Then, once they tell you it's dead, buy a new one.

  17. Feb 2, 2010 #16
    I took the battery in for the simple test and it returned "Battery good - but needs recharge".

    Now that's totally funny because it's been on the charger all morning. I charged it up to 12.8V outside of the car on the front patio (no extension cords etc). I took it off the charger and put the voltmeter up to the terminals. I read 12.81V. 12.80V. 12.79V. 12.78, 77, 76, 75, 74, 73, 72, 71, 70,.... I took it down to Advance Auto (about 3 miles away) and on the tester it read 12.3V.

    They popped the top on the battery (apparently, you CAN pop the top off to see the distilled water levels, which is nowhere indicated on the battery itself) and 2 of the cells are a little low, maybe 1/4" below the refill line. They put it on the charger for a full charge and are going to call me back in a few hours when it's ready. I asked them to give it a test 20-30 minutes after the full charge is done to see if any of the voltage trickled off.

    (Because the 944 has reverse terminals, there is only one battery in stock for $89 that would fit if I had to get a new one.)
  18. Feb 2, 2010 #17


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    But, whilst charging, it should read up to 15V. Is your charger working properly?
  19. Feb 2, 2010 #18
    Yeah. It charges somewhere around 14.1V. But it shuts off when the battery reaches somewhere around 13V.
  20. Feb 2, 2010 #19


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    What do mean when you say "It charges somewhere around 14.1V."? Do you mean that is its unloaded voltage?
  21. Feb 2, 2010 #20
    When it's charging off the battery charger, there's 14.1V at the terminals. But, when the charger is removed, the battery holds a charge of 12.8V. The alternator on the car also delivers 13-14V while running. But the battery only charges to 12.8 or so.
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