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Old 11-04-2008, 07:41 AM   #1
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Hi all
yesterday evening I measured my 3 house batteries with the power hooked up and the batteries connected to the motor home. I checked the specific gravity with the following results. Battery 3 had 3 cells that read 1.26, & 1.25 (fair) respectively. The other 2 batteries checked at 1.3o (good). Yesterday at 3:30 I disconnected the power to the motor home and disconnected the batteries via the solenoid. Today at 11:00 AM, I measured them again and they all read the same as yesterday.
The batteries are Interstate Megatron RV/Marine batteries.
Will the cells that checked fair eventually recover with more charging? Does these tests indicate good battery condition, or are there more tests/conditions that I need to perform before I'll know the state of these batteries?
I haven't dry camped in the past, but I would like to be able to. Thanks in advance
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Old 11-04-2008, 07:41 AM   #2
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Hi all
yesterday evening I measured my 3 house batteries with the power hooked up and the batteries connected to the motor home. I checked the specific gravity with the following results. Battery 3 had 3 cells that read 1.26, & 1.25 (fair) respectively. The other 2 batteries checked at 1.3o (good). Yesterday at 3:30 I disconnected the power to the motor home and disconnected the batteries via the solenoid. Today at 11:00 AM, I measured them again and they all read the same as yesterday.
The batteries are Interstate Megatron RV/Marine batteries.
Will the cells that checked fair eventually recover with more charging? Does these tests indicate good battery condition, or are there more tests/conditions that I need to perform before I'll know the state of these batteries?
I haven't dry camped in the past, but I would like to be able to. Thanks in advance
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Old 11-04-2008, 09:29 AM   #3
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The batteries that are reading a lower specific gravity might need to have a conditioning or equalization cycle done to recombine the acid.

Your SG readings are accurate to the state of charge at I think 77 degrees F (25C.)

Don't give up on the lower reading ones until you try to equalize them.
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Old 11-04-2008, 09:35 AM   #4
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A forum search reveals this post from ChasA:
"To check the "state of charge", the battery should be "at rest", meaning it should not have had a charge or have any load for at least 6 hours. You have to disconnect the batteries some way, either by disconnecting the ground cables or turning the batteries off with a disconnect switch. In addition, you need to adjust you hydrometer readings for temperature. Please refer to www.batteryfaq.org it is an excellent source for information on maintaining batteries."
and from Norm Payne:
"Taking a voltage reading is not the best way to determine the condition of batteries. But if you check them with voltage do this. Charge the batteries until they are fully charged and then disconnect from shorepower. Press the Store switch or whatever it is called in your motorhome to disconnect 12 volts from everything. Wait at least two hours and take a voltage reading directly on the batteries with a good digital voltmeter. It should read 12.7 volts. If it is less your batteries are getting weak. (Taking a voltage reading with a load on the batteries will always give an incorrect reading.)

Equalizing is a way to make the cells "equal" to get more life out of batteries. To determine if your batteries will benefit from equalizing fully charge them and then let them sit a couple of hours with no load on them. Use a hydrometer and take reading from all of the cells. If some of the cells are lower than others equalizing will help bring the low cells up and make them "equal" to the other cells.

If you often drain your batteries below 50% they may need to be equalized every six months. If you are connected to shore power most of the time and seldom drain the batteries more than 20% they will probably never need to be equalized."

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Old 11-05-2008, 09:35 AM   #5
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"It should read 12.7 volts."

What is the range expected? IOW what is the +/-?
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Old 11-06-2008, 04:17 AM   #6
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Battery voltage needs to be measured with the battery at rest (as in disconnected) for about 24 hours and then some say you need to knock off the surface charge before measuring by discharging the battery at a rate of 10-20 amps (forgot what the exact number is) for a couple of minutes.

As to what voltage corresponds to a particular state of charge, the charts disagree as to what the voltage readings represent. I use 12.9 volts for 100% charge. Even so, voltage readings aren't particular useful for determining battery health/charge state per say.

Battery voltage can give you a rough idea of what the batteries are doing if they are under load. It takes some experience to get a feel for relating voltage to charge condition, even so this isn't more than a gross evaluation.
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Old 11-06-2008, 06:38 AM   #7
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John, can you explain the physics that allows for a battery to read 12.9V under the conditions you describe-

("the battery at rest (as in disconnected) for about 24 hours and then some say you need to knock off the surface charge before measuring by discharging the battery at a rate of 10-20 amps (forgot what the exact number is) for a couple of minutes.").
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Old 11-06-2008, 12:05 PM   #8
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An add on to John's comments.

If memory serves, a fairly new lead acid cell at full charge measures 2.15 Volts. Ergo 12.9 Volts for a 6 cell battery. Generally speaking, an at rest value of 12.8 Volts or better is pretty fully charged. As to chemistry, and AGM can be treated as a regular flooded if there is no specific setting in the charger for AGMs. Just avoid the use of the Gel setting because of the different temperature compensation and the lower charge currents that the Gels can accept (or you could clam that you have a 1000 Amp./Hour bank so the charger doesn't limit the current). The primary reason for having an AGM setting in the charger has to do with the fact that AGMs have a lower internal resistance and will accept a higher charge current for a given charge voltage than flooded (appears to the charger like a larger bank). There is also a slight difference in the temperature compensation but since it favors the safe side for the AGMs there should be no problem so the flooded setting will do just fine.
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Old 11-06-2008, 12:44 PM   #9
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Well said Mark!
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Old 11-06-2008, 01:03 PM   #10
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I suggest a visit to-

Lead Acid Battery

This seems to indicate that 12.8VDC is attainable only by reading the quiescent surface charge, not the conditions you described.

I realize this is nitpicky but as a research physicist, I just hate to see the laws of physics ripped apart.

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Old 11-06-2008, 04:15 PM   #11
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That's probably why the voltage to charge state charts vary a tenth or two of a volt - depends on knocking the surface charge off or not. It seems to be an academic curiosity.
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Old 11-07-2008, 01:21 PM   #12
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Hello hamguy,

I concur +/-. As the wikipedia article seems to show, this is not a very exact science, although it looks like it certainly should be. I know of no lead acid battery that any of us could buy that will behave as the simple chemical reaction formula would indicate. All battery manufacturers add other chemicals to their cells to enhance one characteristic or another. Antimony, tin, calcium or selenium, which will have an effect on measured voltage under varying conditions. The strength of the sulfuric acid will have an effect as well as temperature. To compound the problem, how much you pay for your meter has a profound effect as well. Analog meter movements do pretty well at 5% to 2% accuracy (think of the typical auto fuel gauge). Cheep digital meters are considered okay at 2% to 1%. Good meters are in the range of .5% to .25% and expensive meters like the Fluke 87V have a basic DC accuracy of .05%. But at over $300.00 even I don't have one. I currently use a Fluke 8060A with an accuracy of .05%. The difference between 5% and .05% when measuring 12.7 Volts is the difference between +/- 0.635 Volts and +/- 6.35 mV. The latter is the kind of accuracy you would need for precision monitoring. The prior has the potential to sulfate or boil a battery into extinction or cause you to replace a perfectly good charger! And if there are any' loads applied, then it also maters where you take the measurement as well. The preferred place is right at the batteries terminals, loaded or not.

It has been stated many times that voltage is not an accurate indication of state of charge. Neither is absolute specific gravity for similar reasons. Although specific gravity is generally more accurate, to know for sure, you need to know what the starting point is for each cell. Is it precisely 33.5% when fully charged or maybe a little higher or lower. The other major variable is how much lead dioxide has flaked off of the plates and fallen to the bottom of the cell. This will affect the total Amp./Hour capacity attainable from any particular cell as well as its absolute voltage readings as compared to its original new condition. All of these things are reasons why lead acid batteries are not' used as voltage references in electronic work. For that, the Clark cell or Weston cell are still in use today but even they require periodic recalibration against a known reference and no current can be used by the external circuitry during a measurement. (Tricky, what?)

This kind of precise knowledge would serve a Sub Commander well but for our requirements, the testing required would not be worth it. Plus the fact that Submarine batteries do receive 100% discharges, which we all know is very hard on cycle life, but since the taxpayer foots the bill, who cares. To precisely determine Amp./Hour capacity, for a given rate of discharge, the battery has to be discharged completely and the Amperes continually measured and summed. Rather impractical and damaging to the battery as well.

Most if not all manufacturers use an Amp./Hour value of 80% of design as the end of life for a battery. For us poor RVers, this is often impossible to determine. For most of us, it boils down to "Are my batteries performing less or more than 80%, and can I live with 70%". If not, it's time to replace.

There is no plain and simple answer. AGMs and Gels are sealed so you can't check specific gravity. All you can do is occasionally check to see that they are not being over-charged in the bulk, absorption, or float modes with voltages above that recommended by the manufacturer. Flooded batteries can be checked for specific gravity but if you follow the manufacturers recommendation, short of adding water and equalizing as reccomended, there is very little need to do anything other than ensure that recommended voltages are adhered to. I encourage everyone to check the manufacturers web site for the batteries they' are using and follow those recommendations not mine or anyone else's "Rule of Thumb".
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