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Author Topic: Testing acid lead battery  (Read 9294 times)
K7RNO
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Posts: 279




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« on: September 02, 2013, 03:18:18 PM »

I have reason to believe that my UPS batteries are on their way out but I am not sure at what level they are.
Both are of the 12V/17Ah UPS variety, sealed.

Both batteries show 13.6V when charged and they work just fine to operate my QRP rig at 5W for hours. However, when I raise the power to 10W, they soon drop below 11V under TX load. At that voltage and with more than 2A drawn, the XCVR switches down to 5W.

I only have the XCVR's ampere- and voltmeter available for testing. How can I tell where the batteries are at?
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73,
aRNO
NAQCC #6870, SKCC #11131
KD0REQ
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Posts: 1006




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« Reply #1 on: September 02, 2013, 06:30:50 PM »

batteries are well tested under load.  in an auto battery, they use a 50 or 60 amp "carbon pile" resistor.  in your UPS application, if you don't get 75% of rated runtime, change 'em out before they swell and won't slide out.

I've been using some BEST UPS at home for 15 years in one case, and they test on request, as well as on start-up.  the APCs don't, but a runtime test with load is easy enough to do, start the stopwatch and pull the AC plug.

sounds like if you're not getting at least 8 minutes, it's battery time.
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K7RNO
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Posts: 279




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« Reply #2 on: September 02, 2013, 09:05:15 PM »

Makes sense, REQ, but I don't have these in a UPS. They came out of a UPS (not mine) after scheduled maintenance/replacement.

I could run something like a 50W (~4A) light bulb on it and monitor voltage drop over time.
Am I thinking right that a 17Ah bat should keep that bulb burning for 4 hours?

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73,
aRNO
NAQCC #6870, SKCC #11131
KD0REQ
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Posts: 1006




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« Reply #3 on: September 03, 2013, 08:24:24 AM »

that's if you assume the voltage stays constant for the whole discharge period, and then drops to 0.

that's not now it works, once you reach 10.5 volts on a 12-volt nominal lead/acid battery, it is considered fully discharged.  you damage the battery by going further.  in the case of a deep-cycle battery, which a gelcell is NOT, once you hit around 8 volts, you damage the battery quickly, possibly permanently, by sulfation.  this is why UPS have a cutoff point at which it shuts down, to protect the battery.

go online and find any major gelcell manufacturer's discharge cycle.  intersect the curve with 10.5 volts, and that's the maximum load that should be put on the battery for the maximum expected time.  that's new at 68 degrees ambient.  derate the specified amount for higher temperatures.

that is what you can draw for a designated period.  if the battery can't do 80 percent of that, it's in the yellow, and that's why it was replaced.  you can sneak some life out of it yet, but it's not full-service any more.
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WD8KNI
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Posts: 152




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« Reply #4 on: September 06, 2013, 06:50:35 PM »

if you have access to an ESR meter put across the battery, internal resistance should be below 1 ohm on a good SLA battery.  Internal resistance goes up as batteries go bad.  There is a reason that UPS batteries get changed on a schedule.  --  Fred
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WX7G
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Posts: 6194




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« Reply #5 on: September 07, 2013, 07:04:42 AM »

The internal impedance of a 17 A-H battery should be approximately 50 milliohms (not 1 ohm).

You can load it with the QRP radio and see what the decrease in terminal voltage is from the resting voltage (off the charger for some time). For a 2 amp load I would expect the battery to not drop more than 100-200 mV.

I believe the 17 A-H rating is for a 20 amp discharge dropping the voltage to 10.5 volts. That is a discharge current of 850 mA.
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WD8KNI
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« Reply #6 on: September 07, 2013, 05:57:14 PM »

Last time I checked 50 milliohms is below 1 ohm.. like I said.  Not many ESR meters have a the best they do is tenths of ohms.. range.  However I bow to your brilliance..  Fred
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KA5IPF
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Posts: 1029


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« Reply #7 on: September 08, 2013, 08:10:09 AM »

Haul them down to your local Wally World and let them test them on their computerized tester. You will get a printout and all. No charge....
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W6EM
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Posts: 877




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« Reply #8 on: September 10, 2013, 07:34:26 PM »

How soon does the battery terminal voltage drop to 11V with only a 2A load?  Seconds, minutes, or hours?  If a 17AH battery's coming off a charger and assumed to be fully charged with a no-load terminal voltage of 13.6V, and the terminal voltage drops immediately below 12V when loaded with only 2A, it's one sick, sulfated puppy.  Ripe for the lead recycler....
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