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Old 01-11-2012, 03:24   #1
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24 hr resting, specific gravity reading

How do yo take a 24 hr resting, specific gravity reading, from a flooded lead acid battery bank, that you cant take off line?

It's a very simple procedure. It requires a turkey baster style battery syringe, and a real float style Hydrometer.

Modification of the hydrometer makes it easy, remove the bulb from the top, and the suction tube from the bottom. Now replace the suction tube on the bottom with a rubber stopper.

Now take the syringe, draw out enough electrolyte from your pilot cell, and fill the modified hydrometer, wait 24 hrs to take the reading, then remove the electrolyte back to the cell from which it came.

What's your trick?

Lloyd
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Old 01-11-2012, 05:51   #2
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Is a 24 hour resting period required for taking SG readings? I thought that applied as relates to trying to use voltage to determine state of charge.

Also, I don't see how taking readings as described above would accomplish your intent to provide a reading based in a 24 hour resting period. If a 24 hour resting period WAS required for SG readings, once you remove the electrolyte from the battery you take it away from the chemical process that would affect it. There would be not interaction with the lead plates. The end result would be that the SG readings would be the same at the 24 hour point as they were when you first drew the sample, wouldn't they?

Frank
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Old 01-11-2012, 06:00   #3
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Re: 24 hr resting, specific gravity reading

My first job out of school was a Quality Eng in a battery mfg plant. I performed tests such as these. The 24 hours refers to the delay since last charge. Ideally, the battery terminals are disconnected.
We had a special hydrometer which incorporated the float in the "turkey baster" part. A smaller diameter tube on the bottom to insert into the cell opening and draw up the acid, take your reading and squeeze the electrolyte back into the cell. You don't want to leave the cell low or dry of acid - it will kill it. Also, don't forget to correct for temperature. Your hydrometer should have a correction chart with it, if not, probably easy to find on line.
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Old 01-11-2012, 07:21   #4
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Re: 24 hr resting, specific gravity reading

Removing electrolyte for 24hrs will not effect its reading - it will be the same as when first sampled, corrected for any temperature difference.

If you are taking SG readings to determine when to end an equalization run, then taking them "live" is OK because you are only looking for that point where all the cell readings begin to remain consistent in time.

If you do want a true resting state reading and you have more than one battery, just isolate all but one and run the house off one battery overnight when the draw is lowest. Measure the isolated batteries in the morning, reconnect them, charge and isolate the remaining single battery the next night.

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Old 01-11-2012, 07:26   #5
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Re: 24 hr resting, specific gravity reading

Quote:
Originally Posted by FlyingCloud1937 View Post
How do yo take a 24 hr resting, specific gravity reading, from a flooded lead acid battery bank, that you cant take off line?

It's a very simple procedure. It requires a turkey baster style battery syringe, and a real float style Hydrometer.

Modification of the hydrometer makes it easy, remove the bulb from the top, and the suction tube from the bottom. Now replace the suction tube on the bottom with a rubber stopper.

Now take the syringe, draw out enough electrolyte from your pilot cell, and fill the modified hydrometer, wait 24 hrs to take the reading, then remove the electrolyte back to the cell from which it came.

What's your trick?

Lloyd

First I don't think most boaters have a clue that, like OCV readings, the SG readings also need a rest period. 98% of the boat owners I come across who proudly tell me how they SG test their batteries are doing it incorrectly.

The best they often get is some ruined clothes and a feeling of having accomplished something, even if the opposite is true. They also use a tool akin to using a pitch fork for doing brain surgery. Using inaccurate $4.00 tools, the wrong testing methods, not compensating for temp and misinterpreting the results due to the former mistakes usually gain little...

My trick is to fully charge the battery then simply take the battery off line. In most house banks this is quite easy and I always have a cable or two I can use to by-pass a battery or two..

I would also question the validity of removing the acid from the chemical process to attain a resting reading? I question if it can yield accurate results? By doing this it also seems that unless you have three or six hydrometers on hand that this will take 3 - 6 days to do??

I actually use my hydrometers rarely and actually prefer my sight refractometer. It removes/wastes less battery acid, has a more clearly readable scale, to me anyway, and I find it is just easier to use. I have also not broken one like I have my expensive Freas hydrometers.. Broken two of them... Doh'....

For most instances, unless you are seeing noted performance changes in the battery, taking regular SG readings can do more harm than good. This becomes more true especially if done incorrectly or contaminates are introduced to the cells...

Just my .02....
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Old 01-11-2012, 07:58   #6
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Re: 24 hr resting, specific gravity reading

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Originally Posted by Maine Sail View Post
First I don't think most boaters have a clue that, like OCV readings, the SG readings also need a rest period. 98% of the boat owners I come across who proudly tell me how they SG test their batteries are doing it incorrectly.
I for one didn't know that a resting period is required for SG readings. Then again, I only take SG readings when equalizing batteries. But it is always nice to learn something new!

Frank
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Old 01-11-2012, 10:19   #7
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Re: 24 hr resting, specific gravity reading

OK, the above posts have relevance to laboratory conditions. But really, how many people care other than if the specific gravity....taken with a mid-range priced hydrometer on charged batteries are not in uppermost readings (green areas) on all cells?

I do not fault the technique that provides accuracy, I do think however most folks just want to know their batteries are fine. I will state from experience, every time I have had a failed battery, it was immediately obvious both when the battery's charge didn't last as expected along with finding a cell or two with abnormally low specific gravity as compared to the other cells.

My tests are provide info more like the so called "idiot light" in my car. Things are fine or things are poop.

Foggy
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Old 01-11-2012, 10:28   #8
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Re: 24 hr resting, specific gravity reading

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OK, the above posts have relevance to laboratory conditions. But really, how many people care other than if the specific gravity....taken with a mid-range priced hydrometer on charged batteries are not in uppermost readings (green areas) on all cells?

I do not fault the technique that provides accuracy, I do think however most folks just want to know their batteries are fine. I will state from experience, every time I have had a failed battery, it was immediately obvious both when the battery's charge didn't last as expected along with finding a cell or two with abnormally low specific gravity as compared to the other cells.

My tests are provide info more like the so called "idiot light" in my car. Things are fine or things are poop.

Foggy
I agree with you, however to even get relative or ballpark SG readings, you do need to let them rest off charge for a while. A "green" reading while charging or immediately after can easily be a "yellow" or "red" reading after a short sit.

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Old 01-11-2012, 19:10   #9
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Re: 24 hr resting, specific gravity reading

Wow, I'm confused. I've never heard of letting a battery rest before taking an SG reading. Surrette (Rolls) has a tech paper or two on how to take SG readings and says nothing about this.

What does letting the battery rest accomplish? The only thing I can think of is electrolyte striation (denser electrolyte sinks, and weaker rises). But this happens in batteries that are not charged enough and/or fast enough to encourage mixing. If anything, it seems that letting a battery rest will allow striation of the electrolyte and result or less accurate readings, not more accurate.

I'd love to know more. Could there be confusion between SG readings and voltage readings? Voltage readings do require a battery to have rested to be very meaningful.
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Old 01-11-2012, 20:27   #10
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Re: 24 hr resting, specific gravity reading

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I agree with you, however to even get relative or ballpark SG readings, you do need to let them rest off charge for a while. A "green" reading while charging or immediately after can easily be a "yellow" or "red" reading after a short sit.

Mark
Exactly

A Quote from: Table 1: BCI standard for SoC estimation of a battery with antimony. The readings are taken atroom temperature of 26C (78F); the battery had rested for 24 hours after charge or discharge.

Again this is hard to do when a battery bank can't be taken off line for 24 hrs. So the pilot cell can help spot problem in batteries before they become sulfated.

How else do you really know when to equalize a bank.

Lloyd

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Old 01-11-2012, 22:47   #11
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Re: 24 hr resting, specific gravity reading

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Wow, I'm confused. I've never heard of letting a battery rest before taking an SG reading. Surrette (Rolls) has a tech paper or two on how to take SG readings and says nothing about this.

What does letting the battery rest accomplish? The only thing I can think of is electrolyte striation (denser electrolyte sinks, and weaker rises). But this happens in batteries that are not charged enough and/or fast enough to encourage mixing. If anything, it seems that letting a battery rest will allow striation of the electrolyte and result or less accurate readings, not more accurate.

I'd love to know more. Could there be confusion between SG readings and voltage readings? Voltage readings do require a battery to have rested to be very meaningful.
Absolutely no confusion..google, ieee 450

Lloyd

Voltage in a bat is directly related to spg in a bat. resting voltage changes, just as resting spg changes.
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Old 02-11-2012, 07:26   #12
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Re: 24 hr resting, specific gravity reading

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Voltage in a bat is directly related to spg in a bat. resting voltage changes, just as resting spg changes.
True, but it changes as a result of interaction with the lead plates. Taking the electrolyte out of the battery, and then testing it 24 hours later, doesn't give you a proper "resting" reading.

If you can't take the batteries offline for 24 hours, then do the best you can. A reading 15-20 minutes after all charging has been stopped, and all loads have been turned off, will be much better than a reading WHILE it is charging or WHILE it is under load. 24 hours is ideal, but even 24 minutes is much, much better than nothing.
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Old 02-11-2012, 07:46   #13
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Re: 24 hr resting, specific gravity reading

While charging, the chemical reaction and its rate toward returning sulfate to solution is driven by voltage and heat. For a short time after the charging source is removed, the plate voltage and temperature of the battery is still high enough to drive the reaction.

So for accurate readings, it is best to wait a short time to allow the chemical reaction to stop. However, this does not require the resting state that checking OCV does. It only requires 20 minutes or so - after which the resting SG will not change, even though the OCV remains in flux for many hours.

The BCI table referred to above was for measuring OCV, not specific gravity.

In other words, absolute SG readings are only directly related to OCV when the OCV is measured after a long rest time. The SG readings do not need this rest time - only the OCV readings do.

Removing electrolyte and waiting before reading the value will not change the chemical composition because half of the reaction components are left behind - no change can take place no matter how long one waits. So the original post doesn't make much sense.

SG readings are most useful when used as relative measurements to show cell-to-cell differences or to determine when to end an equalization cycle. Used this way, no resting is really required because an absolute value is not needed.

Absolute SG measurements are only useful if one has regularly taken these measurements since the battery was new and kept careful records of them. If this is done, then the health and sulphation of the battery can be monitored. However, a good load test or drain test gives the capacity information most are looking for.

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Old 02-11-2012, 08:04   #14
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Re: 24 hr resting, specific gravity reading

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While charging, the chemical reaction and its rate toward returning sulfate to solution is driven by voltage and heat. For a short time after the charging source is removed, the plate voltage and temperature of the battery is still high enough to drive the reaction.

So for accurate readings, it is best to wait a short time to allow the chemical reaction to stop. However, this does not require the resting state that checking OCV does. It only requires 20 minutes or so - after which the resting SG will not change, even though the OCV remains in flux for many hours.

The BCI table referred to above was for measuring OCV, not specific gravity.

In other words, absolute SG readings are only directly related to OCV when the OCV is measured after a long rest time. The SG readings do not need this rest time - only the OCV readings do.

Removing electrolyte and waiting before reading the value will not change the chemical composition because half of the reaction components are left behind - no change can take place no matter how long one waits. So the original post doesn't make much sense.

SG readings are most useful when used as relative measurements to show cell-to-cell differences or to determine when to end an equalization cycle. Used this way, no resting is really required because an absolute value is not needed.

Absolute SG measurements are only useful if one has regularly taken these measurements since the battery was new and kept careful records of them. If this is done, then the health and sulphation of the battery can be monitored. However, a good load test or drain test gives the capacity information most are looking for.

Mark
This is much closer to my understanding. Granted, stratified electrolyte will measure differently at different levels, but of all the ways to check SOC and to verify that your charge protocol is returning all cells to full charge, I've always thought it's the best. I just don't take readings after filling with water. So far I haven't see any variation as cautioned in the IEEE document. That paper also says that lead-calcium is more susceptible to stratification than lead-antimony.

By the way, I don't understand how the acid conversion can continue without current flow, even if there is still an elevated temp and/or elevated voltage.
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