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Trickle charge 12v batter with test load

Discussion in 'Electronic Basics' started by W. eWatson, Aug 10, 2012.

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  1. W. eWatson

    W. eWatson Guest

    I have a 12v "alarm" battery from The Shack that I bought about 3 years
    ago. It weighs about 5 pounds. I use it to operate a small telescope,
    and haven't used it for about 7 weeks. It was weak, showing 9v on my
    digital VOM, so I charged it with a 12v 27 AH larger battery last night
    for maybe 6 hours and got it up to about 11v. After about 2 hours
    hooked to the telescope doing nothing but showing the small LCD screen,
    the scope shut down and key lights, and I noticed the voltage at about 8.5.

    I took it to an auto store and they used a test meter, and it showed 7v.
    My neighbor tells me that I should believe the test meter, since it
    presents a load. I borrowed his trickle battery charger, and will see
    the VOM reading later. How do I simulate a load to know that I might be
    able to get 3+ hours out of this battery? The battery should be capable
    of 7.2 AH. Maybe I just nee
  2. W. eWatson

    W. eWatson Guest

    Well, after about 2.5 hours of trickle charging, I see the voltage at
    11.2 v. I'll use it in an hour on the scope, and see if it holds for
    several hours.
  3. Jasen Betts

    Jasen Betts Guest

    When fully charged the voltage should go up past 13V. if it doesn't do
    that the battery is defective.
  4. W. eWatson

    W. eWatson Guest

    It got up to around 14v with trickle charing, but when I put it on the
    scope, it took about one hour to get it down to 8v. D-e-a-d.
  5. ehsjr

    ehsjr Guest

    Your battery which measured 9V on your VOM was definitely damaged,
    perhaps (probably?) by being discharged too far.

    When you allow an LA (lead acid) 12V battery to discharge below
    10.5 volts it will be damaged. The damage could range from a
    weakened to a totally dead battery. No amount of charging will
    reverse the damage, even if the battery voltage rises due to
    charging. You might be able to charge a battery damaged by
    being discharged too far, but its capacity will be lowered.

    Some rules of thumb for 12V LA batteries:
    * Never discharge below 10.5 volts
    * The deeper the discharge (even though *above* 10.5V), the
    greater the risk of damage
    * Never store a battery in a discharged state
    * The longer you wait to charge a battery after it has been
    discharged, the greater the risk of damage

    Your application should have a low voltage cutoff to protect
    the batteries from being discharged too far by the normal load,
    and, if possible, an automatic charging circuit.

    Finally, depending on usage and care, 3 years from your gel cells
    may be reasonable for what you are doing.

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