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Is there a way to determine if a booster pack (battery) is still good?

  1. Feb 18, 2013 #1
    I have a booster pack that I think has a 12V battery inside. One of the typical ones you get at a hardware store with air compressor, booster cables, light etc. Anyway it is at least 7 years old. I recently tried jump starting someone's car with it but it wouldn't, I think I was able to in the past though.

    Since it couldn't jump start the vehicle I though maybe its time to replace it or the battery inside. Is there a way to check how 'healthy' the battery is?
    I have a multimeter and checked the voltage with no load. I read close to 12V. then I read the voltage with load (air compressor turned on) and it read about 11.3V.
    I'm not sure what this means or if there are other readings I can take (like current).

    Thanks in advance
  2. jcsd
  3. Feb 18, 2013 #2

    Simon Bridge

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    Re: Is there a way to determine if a booster pack (battery) is still g

    The packs usually have a test circuit you can also take it to a battery testing station.
    The volt drop under load just indicates that it has an internal resistance and is normal.
  4. Feb 18, 2013 #3


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    Re: Is there a way to determine if a booster pack (battery) is still g

    Any competent mechanic will have testing equipment that can perform load tests on your battery. If your booster's battery seems normal under load tests, it is probably OK.
  5. Jul 22, 2013 #4
    I tested with a multimeter after fully charging over night. THe voltage reads 11.84 volts with no load.
    I was told that a good battery would read over 12V. Is this true. WHat does my low value indicate?
  6. Jul 22, 2013 #5


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    yes it should have been reading over 12V so the most likely reasons are either....
    1) ... your charger isn't working properly
    2) ... your battery is failing. is its reading only 11.8V with no load, then its likely to be much lower under load.

  7. Jul 22, 2013 #6
    12 volt lead-acid batteries are really 6 lead-acid cells connected in series. Fully charged each cell has a voltage of 2.1 volts, so a whole battery should be 12.6. If it has less then then this voltage then there are 2 possibilities. Either there is a problem with the charger or one of the cells is shorted. A shorted cell will reduce the voltage by 2.1 volts to a total of 10.5 volts.

    It is possible to overcharge cells so that they will produce more then 2.1 volts. This excess charge will dissipate within the battery even with no load present. Let your battery set for a couple of days. If the voltage settles down to 10.5 volts then you probably have a shorted cell. If it goes lower then the battery is self discharging. Either of these would be cause for battery replacement. If it stays at 11.8 suspect a charging problem. You could verify this by checking the booster packs voltage while it is charging. It may start out near battery voltage but should rise to 13-14 volts as the battery charges.

    Do not attempt any internal diagnostics of the battery pack while it is plugged in unless you know exactly what you are doing and what precautions are necessary. From your original question I suspect that you do not.
  8. Jul 23, 2013 #7


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    The best test is to see if it does its job = that is, you charge it for long enough and see if it will start the car. If it fails that test then I don't think that you (from what you say) could actually mend it; certainly, never even consider poking inside the battery!!! If the battery has been discharged to below half its capacity and, especially, been left in a discharged state, then it is probably dead. If you can get inside to see what the volts are across the battery itself then that could at least give you an idea if the internal charger is working.
    The way things are these days, it is very likely that a whole replacement booster unit will cost less (if you shop around) than a replacement battery to go inside your old one. Such is life.
  9. Jul 23, 2013 #8
    A seven year old lead acid [wet cell, typical auto type] battery is almost surely very weak. The only real way to test them is to measure their output under load and compare with the stated capacity.....As noted, any mechanic likely has a test set.

    Such a 'booster' battery gizmo is often too small to start an automobile, at least 8 cylinder varieties. [four cylinder automobile engines have smaller batteries than 8.] If a car battery costs,say $70, and a booster, say, $35, you can be sure its a considerably smaller battery.
    A freshly charged wet cell lead acid battery should read about 12.8 volts are room temperature...12 volts, if I recall, under no load, means it's only half charged. At about 11.5 volts or so it can barely run anything.

    If you have something like a spot light, try powering it from the booster...then compare with a regular functioning car battery hooked up separately...likely the booster battery will show a dimmer light....try maybe a one or two minute test.

    Try an online search for some voltages....This is the first I found and although I disagree with the values, you get the idea...They are talking the ability to provide power here....
    edit: voltages among lead acid batteries can also vary....other chemicals may be added to reduce gassing and so some manufacturers have higher charge voltages than others.
    Last edited: Jul 23, 2013
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