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Trickle charge 12v batter with test load

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

Reply to
W. eWatson
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...d a new battery. It has a shorted cell. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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.
Reply to
W. eWatson

Gel/AGM batteries last longer when kept charged - even with hard use. Partiall discharged batteries do not survive well. Example: We have a battery powered car for the grandkids (no, I didn't buy new, it was a freebie on Freecycle). I replaced the battery with a 12 volt 10 AH unit and used a Black & Decker 1A/2A smart charger (not the battery burner that came with the car). We're looking at year 4 of the battery's life.

Trying to charge your battery from another battery will NOT get it battery to full charge - it needs more than the idle voltage to get a complete charge. Depending on the type and usage of the battery, it should be charged to something between 13.8 and 14.2 volts.

Reply to
news

You can't charge a 12 volt lead acid battery by just paralleling it with another 12v led acid battery. 11.2 is only partially charged.

Reply to
Michael A. Terrell

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

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Reply to
Jasen Betts

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.

Reply to
W. eWatson

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.

Ed

Reply to
ehsjr

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