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Old 06-02-2008, 19:18   #1
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Evaluating 12vdc Flooded-Cell Batteries

Evaluating a 12 Volt flooded-cell battery of unknown history and quality

To do this you would need to fully charge the battery (making sure that a full 14.4V or more is reached during acceptance charging @ or around 25 C). You would then measure each cell separately, obviously the total has meaning as well yet it is the VARIATION in cell-to-cell readings that reveals the need for equalization or evidence of permanent sulphatation (sulphation is not permanent, sulphatation is).

The three things to determine: 1. That the battery is fully charged, and if so that it does not need equalization or that equalization does not yield a reversal of sulphation cell-to-cell. 2. The approximate Amp-hour rating. 3. The battery internal cell resistance.

The internal resistance is a very important and little acknowledged attribute of any battery, lead-acid or otherwise. It is the internal resistance multiplied by a heavy load current which results in a “voltage burden” to your system and directly increases losses in the energy stored by the battery that you cannot use.

Notice that the fully charged terminal voltage is not on the list because it will be sufficiently high (about 12.7V without surface charge) when the other conditions are met. Surface charge may be removed by applying around a 5% of the Amp-hour rating in Amps and noting the terminal voltage. It will linearly decrease until it becomes constant. Remove the load at that time. Note that most nominal 12V flooded-cell batteries have a specific gravity that results in a 12.66 Volt terminal reading yet many of you notice readings of 12.8Volts or so....this is due to a lingering surface charge after the application of a charge voltage. Similarly many gel-cell and AGM batteries exhibit terminal voltages of 13 Volts or so even though the readings drop to 12.9 Volts (AGM and gell-cell batteries have higher specific gravities than do most flooded-cell batteries and, therefore, the terminal voltage is higher. Terminal voltage, Vt = 6(0.85 + SG) where SG is the specific gravity of the electrolyte for a 12 Volt lead-acid battery (yes, this also includes AGM and gell-cell batteries).

Note that state-of-charge means little to you. It is state of capacity that is important and must be inferred from a relatively simple test. Given two batteries in separate boxes that do not allow you to inspect other than terminal voltage: They both read 12.65 Volts. Which one has a greater capacity? One cannot know. What good does it do to have a degraded start battery that exhibits a so-called full state of charge. One day it will easily start the engine. The next day it will not because it may have degraded from a 1 Amp-hour state of capacity to a 3/4 Amp-hour in one day's use and may not start the engine. To be sure, that same battery began as a 75 Amp-hour battery and for 6 years always started the engine. One day the final denouement occurs to one's dismay.

Because it is hard on the life of any lead-acid battery to fully discharge it in order to make an attempt to determine capacity it is not advised to do so unless one has many batteries available to waste before choosing a similar one that has not been discharged. What WILL be revealing, though, is to discharge the battery at the theoretical 20 hour rate for an hour (at least 1/2 hour), let the battery recover for at least an hour and measure the standing voltage and use a "look-up" table to determine approximate state-of-charge and using simple algebra calculate the capacity from that information. At the end of the hour observe the terminal voltage under load and then the voltage immediately after the load is removed. Note the current just before removing the load. Calculate the internal resistance by dividing the difference in voltage by the difference in current (as an example for a 180 Amp-hour rated battery the current difference will be 180 divided by 20 minus zero...zero because after removing the load the current should be zero). For this test note as well the initial change in voltage from no load to the 20 hour rated load applied. Calculate that initial resistance as well making sure that initially there is no surface charge on the battery which will make the results lower (better) than the actual value.

So, the 20 hour rated current is 9 Amps in this example. Using this test you will notice that the terminal voltage drops linearly for awhile then settles down to a constant value. I would expect that voltage to be approximately 12.5X Volts or better for a flooded-cell 180 Amp-hour “real” battery.

The approximate capacity, C = (measured test Amp-hours)/(1-decimalSOC), where the decimalSOC is the decimal value (0 to 1) of the state of charge shown by a chart showing state of charge versus voltage. Now to be more accurate one should divide C by 20 to get the 20 hour capacity current discharge rating and repeat the test at that current to recalculate a refined value of C. **I've corrected my original mistake in this formula caught by cal40john as seen in his improved description in the following entry at 23:56, thanks to John!

Obviously the use of an accurate battery monitor is almost essential in making such tests.

AGM and gel-cell batteries require similar tests yet require slightly different observations and tests. More on that later.
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Old 07-02-2008, 00:56   #2
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Maybe I'm not reading your message right Rick, but here's how it makes sense to me

Quote:
Originally Posted by Rick View Post
Evaluating a 12 Volt flooded-cell battery of unknown history and quality

To do this you would need to fully charge the battery (making sure that a full 14.4V or more is reached during acceptance charging @ or around 25 C). You would then measure each cell separately, obviously the total has meaning as well yet it is the VARIATION in cell-to-cell readings that reveals the need for equalization or evidence of permanent sulphatation (sulphation is not permanent, sulphatation is).

The three things to determine: 1. That the battery is fully charged, and if so that it does not need equalization or that equalization does not yield a reversal of sulphation cell-to-cell. 2. The approximate Amp-hour rating. 3. The battery internal cell resistance.

The internal resistance is a very important and little acknowledged attribute of any battery, lead-acid or otherwise). It is the internal resistance multiplied by a heavy load current which results in a “voltage burden” to your system and directly increases losses in the energy stored by the battery that you cannot use.

Notice that the fully charged terminal voltage is not on the list because it will be sufficiently high (about 12.7V without surface charge) when the other conditions are met. Surface charge may be removed by applying around a 5% of the Amp-hour rating in Amps and noting the terminal voltage. It will linearly decrease until it becomes constant. Remove the load at that time. Note that most nominal 12V flooded-cell batteries have a specific gravity that results in a 12.66 Volt terminal reading yet many of you notice readings of 12.8Volts or so....this is due to a lingering surface charge after the application of a charge voltage. Similarly many gel-cell and AGM batteries exhibit terminal voltages of 13 Volts or so even though the readings drop to 12.9 Volts (AGM and gell-cell batteries have higher specific gravities than do most flooded-cell batteries and, therefore, the terminal voltage is higher. Terminal voltage, Vt = 6(0.85 = SG) where SG is the specific gravity of the electrolyte for a 12 Volt lead-acid battery (yes, this also includes AGM and gell-cell batteries).

Note that state-of-charge means little to you. It is state of capacity that is important and must be inferred from a relatively simple test. Given two batteries in separate boxes that do not allow you to inspect other than terminal voltage: They both read 12.65 Volts. Which one has a greater capacity? One cannot know. What good does it do to have a degraded start battery that exhibits a so-called full state of charge. One day it will easily start the engine. The next day it will not because it may have degraded from a 1 Amp-hour state of capacity to a 3/4 Amp-hour in one day's use and may not start the engine. To be sure, that same battery began as a 75 Amp-hour battery and for 6 years always started the engine. One day the final denouement occurs to one's dismay.

Because it is hard on the life of any lead-acid battery to fully discharge it in order to make an attempt to determine capacity it is not advised to do so unless one has many batteries available to waste before choosing a similar one that has not been discharged. What WILL be revealing, though, is to discharge the battery at the theoretical 20 hour rate for an hour (at least 1/2 hour), let the battery recover for at least an hour and measure the standing voltage and use a "look-up" table to determine approximate state-of-charge and using simple algebra calculate the capacity from that information. At the end of the hour observe the terminal voltage under load and then the voltage immediately after the load is removed. Note the current just before removing the load. Calculate the internal resistance by dividing the difference in voltage by the difference in current (as an example for a 180 Amp-hour rated battery the current difference will be 180 divided by 20 minus zero...zero because after removing the load the current should be zero).
At this point as I read it, we're supposed to be calculating internal resistance.

Let's say no load voltage is 12.7 v and load voltage is 12.5 v with a 20 amp load, then (12.7-12.5)/(20-0)= 0.01 ohms. An older battery will have higher internal resistance, and will have a greater voltage drop with a load. This will make the battery less useful, particularly with higher current loads.

To rephrase what Rick said above, plot volts versus current, the slope of the line is the internal resistance of the battery. Since you can't put your voltmeter across the internal resistance to measure the voltage drop across it, you measure the voltage at the terminal at two different loads and calculate the slope.
i=internal
T=terminal voltage
V(B)=internal battery voltage (constant as is R(i))
V(i)=voltage across internal resistance
V(T)= voltage at terminal (or across external resistor)

slope or R(i) = (V(i1)- V(i2))/(I1-I2)
V(B)=V(T)+V(i)
solve for V(i)
V(i)=V(B)-V(T)
substitute
R(i)= (V(B)-V(T1))-(V(B)-V(T2))/(I1-I2)
R(i)=(V(T2)-V(T1))/(I1-I2)

Notice how V(T2) is over I1

Quote:
Originally Posted by Rick View Post
For this test note as well the initial change in voltage from no load to the 20 hour rated load applied. Calculate that initial resistance as well making sure that initially there is no surface charge on the battery which will make the results lower (better) than the actual value.

So, the 20 hour rated current is 9 Amps in this example. Using this test you will notice that the terminal voltage drops linearly for awhile then settles down to a constant value. I would expect that voltage to be approximately 12.5X Volts or better for a flooded-cell 180 Amp-hour “real” battery.

The approximate capacity, C = (measured test Amp-hours)/(decimalSOC), where
Once again, maybe I'm reading it wrong, but I would put
C = (measured test Amp-hours)/(1-decimalSOC)

I put a 9 amp load for one hour, so 9 amp-hrs consumed. If I read a voltage at the end of that period of say 12.63, which my state of charge chart (not presented in this thread) says is a state of charge of 95%, then C=9/(1-0.95) = 180 amp-hour capacity at full charge. In this case my battery is still as new.

Now use this battery for 3 years (or 3 months) and after 9 amp-hours, you read 12.42 volts, which state of charge chart says 80%.
C=9/(1-0.80) = 45 amp-hour capacity at full charge.

As Rick states below, the 9 amps out of this old battery is much greater than the c20 rate, so redo the test 45/20=2.25 amps and you will get a better capacity.

John


Quote:
Originally Posted by Rick View Post
the decimalSOC is the decimal value (0 to 1) of the state of charge shown by a chart showing state of charge versus voltage. Now to be more accurate one should divide C by 20 to get the 20 hour capacity current discharge rating and repeat the test at that current to recalculate a refined value of C.

Obviously the use of an accurate battery monitor is almost essential in making such tests.

AGM and gel-cell batteries require similar tests yet require slightly different observations and tests. More on that later.
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Old 07-02-2008, 01:28   #3
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BDM

Yes, well put! Obviously I had a BDM, brain-dead-moment regarding the denominator which you corrected with the 1-decimalSOC.

Actually the medical term is "cruiseheimers".

Thanks for running through the logic.
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Old 07-02-2008, 04:26   #4
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I added a sticky so we don't lose this one. Great work guys!
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Old 07-02-2008, 12:11   #5
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Quote:
Originally Posted by Rick View Post
Evaluating a 12 Volt flooded-cell battery of unknown history and quality

Terminal voltage, Vt = 6(0.85 = SG) where SG is the specific gravity of the electrolyte for a 12 Volt lead-acid battery (yes, this also includes AGM and gell-cell batteries).
I'm guessing that the above equation is actually Vt= 6(0.85+SG)
since that gives reasonable voltage numbers.

Is this a hard equation, or a guesstimate? I have found three tables, all disagree and one of the table has voltages and SG at 5 degree intervals of which none of the columns match the equation.

Teardrops & Tiny Travel Trailers :: View topic - Battery State of Charge Chart


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Old 07-02-2008, 12:30   #6
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From a practical standpoint, I would never buy somebody's used battery...given the typical lead-acid is only good for roughly 5 years anyway. Why chance the unknown in order to save a few bucks?
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Old 07-02-2008, 13:02   #7
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Rick,

Thanks for your detailed post re: evaluating old batteries.

I'd be very interested if you could tell me from the graph below:
Gallery :: Miscellaneous 2007 :: VoltageDecayComparison

what your evaluation method would come up with in terms of capacity.

FYI, these were batteries used in a 12-month plus test of pulsers. The gelled golf-cart batteries were over 10 years old (had been removed from a sailboat), and the T-105's were about 3-4 years old but not performing very well.

My interest is peaked in your calculations, because I have estimates of capacity of these battery banks (of 2 each) derived from direct measurements using a $600 internal resistance computer.

Would be fun to compare :-)

Thanks,

Bill

BTW, the voltage measurements were taken with a calibrated Fluke 189 meter; the "load" consisted of several 12V light bulbs in parallel (and measurements were taken of amperage, AH, and KWH).

Note also that the T-105s actually show an INCREASE in voltage after 15 minutes or so. This is typical of the behavior shown, i.e., their voltage drops way below what you'd expect, then picks up a bit. Here's a graph of the first 20 minutes of each battery bank:
Gallery :: Miscellaneous 2007 :: First20MinsLoad

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Old 07-02-2008, 14:43   #8
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Quote:
Originally Posted by David M View Post
From a practical standpoint, I would never buy somebody's used battery...given the typical lead-acid is only good for roughly 5 years anyway. Why chance the unknown in order to save a few bucks?
I believe there are more reasons than buying used batteries that this could be used for.
How about a more quantitative way to evaluate batteries you own and have been using as a reason?

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Old 07-02-2008, 16:23   #9
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Quote:
Originally Posted by cal40john View Post
I believe there are more reasons than buying used batteries that this could be used for.
How about a more quantitative way to evaluate batteries you own and have been using as a reason?

John
True John...I did not see it from that angle.
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Old 07-02-2008, 18:20   #10
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SG and voltage

John,
First I apologise for not paying sufficient attention to the details, like failing to hit "shift" for a "+" and getting a "=" as you found, kinda like not checking my work.

One source for the formula giving voltage versus SG comes from an old IBE (a large manufacturer of batteries in this country) book and I've seen it quoted elsewhere as well. IN addition, I've verified the general accuracy.

I note a definite departure in the voltages versus SG for the first chart provided in your link in that it shows 12.478 Volts for a 1.271 SG @ 65 deg F when, for absolutely sure a 1.26 SG at 65 deg will show a much higher voltage (12.66 V). Of this I am sure. I question, therefore, all those voltages quoted.

The next chart down (in green and red) show a voltage for a 0% charged battery as being 10.5V. 10.5V is the end of test voltage for a 20 hour current discharge rate for making capacity tests. Obviously the voltage will rise after removal of the test current to a higher voltage (11.76 Volts) so this table is also suspect as not meeting standards.

According to Storage Batteries, by Vinal, aside from voltage variations caused by temperature of the electrolyte there are slight variations depending upon the grid makeup such as pure lead (like AGM and Gell-cell batteries and those grids of lead-antimony. There are also differences between the results made by various well-known electrochemists yet these variations occupy the second place past the decimal, i.e.; 2.1x on a volts-per-cell reading. Therefore, the third place past the decimal for nominally 12V batteries is of a highly dubious accuracy.

A comment was made about voltage during discharge rising after time. This is due to the discharge current being sufficiently high to cause electrolyte heating and plate heating which yields an increase in voltage. The electrolyte temperature increase also can result in a lower apparent cell resistance.

Please keep in mind that using open-circuit voltge to infer state of charge is not very accurate by itself. Using my method of determining capacity would be fortunate to be within 10% yet that is sufficient for energy planning purposes. I have long used (and advocate using) accurate battery monitors to make internal resistance checks and by noting voltage under noted load current along with actual Amp-hour discharge measurements one CAN infer just what is the general quality of one's battery. This is not supposed to be used to buy used batteries.
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Old 07-02-2008, 18:50   #11
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Bill,
Here are a few of my thoughts;
"I have estimates of capacity of these battery banks (of 2 each) derived from direct measurements using a $600 internal resistance computer."
I don't understand that statement. If direct measurements of capacity was made what is "estimated"? What is meant by "internal resistance computer"?

The first link shows that the CG01 battery, for instance, yielded 152 Amp-hours at a 15.2 hour rate. Without open circuit rested voltages my method cannot be used. There is another way to calculate what Amp-hour ratings that battery would have at different rates. One would make another test and use Peukert's equations for the calculations. These are well known.

The second graph does not indicate an absence of a surface charge so that the internal resistance can be easily calculated. After the voltages stabilize to a "flat curve" the load would have to be removed allowing the open-circuit voltage to stabilize and be measured. The rise in voltage to a "flat curve" by the Trojan is not unusual.

The curves by themselves offer no means to compare or evaluate the efficacy of having applied a pulse charger. There certainly is no means of comparison to how the batteries would have performed by merely equalizing them for 24 hours which is a lot shorter than weeks of pulse charging. BTW; equalizing requires a constant-current source with a compliance voltage of 17 Volts or so.
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Old 07-02-2008, 19:16   #12
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Quote:
Originally Posted by Rick View Post
John,
First I apologise for not paying sufficient attention to the details, like failing to hit "shift" for a "+" and getting a "=" as you found, kinda like not checking my work.

One source for the formula giving voltage versus SG comes from an old IBE (a large manufacturer of batteries in this country) book and I've seen it quoted elsewhere as well. IN addition, I've verified the general accuracy.

I note a definite departure in the voltages versus SG for the first chart provided in your link in that it shows 12.478 Volts for a 1.271 SG @ 65 deg F when, for absolutely sure a 1.26 SG at 65 deg will show a much higher voltage (12.66 V). Of this I am sure. I question, therefore, all those voltages quoted.

The next chart down (in green and red) show a voltage for a 0% charged battery as being 10.5V. 10.5V is the end of test voltage for a 20 hour current discharge rate for making capacity tests. Obviously the voltage will rise after removal of the test current to a higher voltage (11.76 Volts) so this table is also suspect as not meeting standards.

According to Storage Batteries, by Vinal, aside from voltage variations caused by temperature of the electrolyte there are slight variations depending upon the grid makeup such as pure lead (like AGM and Gell-cell batteries and those grids of lead-antimony. There are also differences between the results made by various well-known electrochemists yet these variations occupy the second place past the decimal, i.e.; 2.1x on a volts-per-cell reading. Therefore, the third place past the decimal for nominally 12V batteries is of a highly dubious accuracy.

A comment was made about voltage during discharge rising after time. This is due to the discharge current being sufficiently high to cause electrolyte heating and plate heating which yields an increase in voltage. The electrolyte temperature increase also can result in a lower apparent cell resistance.
Looks like the table on the Trojan site uses the equation.

Trojan Battery Company - Testing

The differences from the table to the equation were a constant. Shifting the 0.85 constant to 0.845 made the equation match the Trojan table which is at 80 degrees.

John
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Old 07-02-2008, 20:06   #13
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Rick,

Thanks for your response. Yes, of course, I forgot you need the open circuit or "resting" voltage for your equation. It's late and I'm tired...sorry.

We did do resting voltages before and after the load tests. One typical pattern is shown here: Gallery :: Miscellaneous 2007 :: TG_01_LoadTest3PhaseII
where it can be seen that after being pulled down to 10.5 volts, the no-load voltage climbed after awhile to around 11.5.

The internal resistance tests were made with a Midtronics digital battery tester, supposedly the tester of choice when we started the tests about 18 months ago. After using it on about 20 different batteries, I think it's OK but not necessarily definitive. So many variables....:-)

Yes, I know about the pulsing vs. equalization thing. Our testing was to try to determine whether the little pulsing devices sold by a number of manufacturers really did anything. We worked up a testing regime which involved a series of full charges and discharges to 10.5VDC under load (all done WITHOUT the pulsers), followed by a series of cycles of pulsing for a few days, then load testing, then recharging, then pulsing for a few more days, etc. Tons of data, a few surprises, learned a lot.

You'll be interested to know that after almost six months of testing using the above methodology, we decided to equalize the T-105s ONE TIME to see what would happen. Presto...the results were much more positive than had been the pulsing/charging/discharging over the previous six months. See, e.g., this graph: Gallery :: Miscellaneous 2007 :: First20minEqual

The red and blue curves show the difference that one equalization made to these batteries after they had been "treated" for six months.

Tired..gotta go.

Best,

Bill
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Old 07-02-2008, 21:45   #14
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a question related to this thread, we run trojan 105's and charge using Trace SW2612 inverter Charger, we normally charge when voltage drops to 12.1 to 12.2 V but as we have temperature probe on batterries should I read temp compensated voltage or actual voltage to start charging cycle?
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Old 10-02-2008, 12:32   #15
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temperature compensated rest voltage

Use temperature compensated voltages. For 10 degrees F there will not be a significant difference for your purposes as the error will most likely be much larger due to the likelyhood that you will not always be reading a true rest voltage unaffected by discharge current values unless you always have a hour rest time with no load.

This is another reason to use a real battery monitor.
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