Specific Gravity Change

[CaptTPT]

If a fully charged new 12V Flooded Lead Acid battery's electrolyte has a SG of 1.265-1.275 at 77 degrees F, what will be the expected SG (roughly) when the battery is used for several years, possibly sulfated, but still "fully charged" as indicated by your boats battery monitoring system?

 

[DavidM]

Sulfation takes the SO4 radical which is part of the H2SO4 sulfuric acid molecule out of solution when it precipitates. So there won't be as much acid in solution to get up to the original, fully charged specific gravity.

All of which is a long way of saying that the SG will be lower than normal if the battery is sulfated even if fully charged. That is why it has less capacity.

David

 

[sunchaser]

Visit the Trojan battery site. All sorts of good battery info there.

 

[CaptTPT]

So the question is how predictable the SG is, when full charged, for assessing the quality or remaining capacity of the batteries.

 

[sunchaser]

So the question is how predictable the SG is, when full charged, for assessing the quality or remaining capacity of the batteries.

SG is the "quick test" gold standard for assessing battery health. Not sure what you mean by remaining capacity. Did you go to the Trojan site?

 

[CaptTPT]

Yes. They do not address this question. Simply want to know if you can guess anything about the reduction in amp hour capacity by the drop in SG when full charged over time.

 

[BruceK]

A hydrometer test says more about individual cell condition than charge capacity.
According to Ed Sherman, 75%SOC = 1.225=12.3 Open Circuit Voltage, whereas 12.6v and above OCV=100%SOC=1.26-1.28SG. That might help a little, I`d only say " not as good as they were once".

 

[FF]

The better use for SG is to see if there is a difference in the cells ,

where one may be dragging the batt down ,if so" the end is near!"

 

[twistedtree]

I've got a different take on it. SG is the ULTIMATE test of a battery's charge state, and arguable the only accurate test. A battery is a chemical reaction, and the state of the chemistry is 100% reflective of the battery's state of charge. It's much more accurate than an amp-hr counter, or load tester, or impedance tester, or voltage measurement.

djmarchand was spot on describing the chemistry and what happens when there is sulfation. A sulfated battery will appear to be full charged to any 3-state charger, i.e. it will have reached terminal voltage and terminal charge acceptance rate. Yet the SG level will be below the target level, and the battery will have corresponding reduced capacity. That's when it's time to equalize to recombine some of the SO4.

The trouble with SG as a measurement is that it's a PITA to do. It's messy, tedious (especially if you have a large battery bank), and access is often difficult on a boat. And flooded batteries require maintenance to top up the water level. All this had helped make AGM batteries so popular on boats, but with them you can't take an SG reading even if you wanted to.

Now, after all that, I need to confess that I have AGMs on my boat. The advantage of being able to read SG just isn't worth the hassle. We also have an off-grid house and it has big flooded batteries. They are a PITA to maintain, and consume a huge amount of water (long story why that's so, but I don't recommend Surrette batteries...). When they die I will probably switch to AGMs or perhaps even the new LiFo batteries.

 

[cafesport]

General rule of thumb is get ready for new batts when fully charged sg is off by 5 or more percent. Good source for tons of info on all chemistries is http://batteryuniversity.com/learn/article/how_to_measure_state_of_charge


Spell check via iPhone.

 

[sunchaser]

Yes, a SG test can be a PITA. But SG is the subject the OP posed. On many boats measuring SG is indeed a hassle and messy as Peter mentions. So AGMs are the less hassle answer. This is one case where a - ahem- DeFever is superior to some Nordhavns, easy battery access.

I have never had the need or interest to routinely measure SGs. Spot check only. If my LAs or AGMs are acting up and older than 5 years I chuck them. Good voltage measurement and checking the chargers is all I fret about. As Cafe mentions, battery U is a welcome SG site.

The only boat batteries that have recently let me down are the AGMs feeding the thrusters, toast after 6 years. LAs on house bank are cheap, low maintenance and long lasting.

 

[DavidM]

I am always amused by those who buy AGM batteries to avoid having to check or add water to flooded cell batteries and even more amused at those who find that checking SG is too difficult.

I have an ancient, rotating pointer type SG tester I bought at an auto parts store for a few bucks. It takes only a few seconds to pop the caps off of a battery, poke the tube into the electrolyte, suck up electrolyte with the squeeze bulb. After reading the SG I blow out the electrolyte with the squeeze bulb and leave it on the top of the battery where any drips will be contained and close the battery box.

SG readings are the only absolute test of battery SOC. Voltage readings, to be even approximately accurate require that the battery rest for several hours. Battery monitors are affected by monitor settings, battery sulfation and the vagueries of the Peukert equation.

That is one of the reasons I use flooded cell batteries for my house bank. Cost and the knowledge that golf cart batteries are truly designed for deep cycle use are another.

David

 

[CaptTPT]

I've got a different take on it. SG is the ULTIMATE test of a battery's charge state, and arguable the only accurate test. A battery is a chemical reaction, and the state of the chemistry is 100% reflective of the battery's state of charge. It's much more accurate than an amp-hr counter, or load tester, or impedance tester, or voltage measurement.

djmarchand was spot on describing the chemistry and what happens when there is sulfation. A sulfated battery will appear to be full charged to any 3-state charger, i.e. it will have reached terminal voltage and terminal charge acceptance rate. Yet the SG level will be below the target level, and the battery will have corresponding reduced capacity. That's when it's time to equalize to recombine some of the SO4.

The trouble with SG as a measurement is that it's a PITA to do. It's messy, tedious (especially if you have a large battery bank), and access is often difficult on a boat. And flooded batteries require maintenance to top up the water level. All this had helped make AGM batteries so popular on boats, but with them you can't take an SG reading even if you wanted to.

Now, after all that, I need to confess that I have AGMs on my boat. The advantage of being able to read SG just isn't worth the hassle. We also have an off-grid house and it has big flooded batteries. They are a PITA to maintain, and consume a huge amount of water (long story why that's so, but I don't recommend Surrette batteries...). When they die I will probably switch to AGMs or perhaps even the new LiFo batteries.

This gets to precisely my question. I'm not talking about a bad cell. That's easy. I'm talking about a battery that is deteriorating as expected over time but still functioning properly. Should be predictable by measuring the SG. What is the SG where you say time to get new. I posed the question to tech support at Trogen. Waiting for response.

 

[sunchaser]

This gets to precisely my question. I'm not talking about a bad cell. That's easy. I'm talking about a battery that is deteriorating as expected over time but still functioning properly. Should be predictable by measuring the SG. What is the SG where you say time to get new. I posed the question to tech support at Trogen. Waiting for response.

The SG will drop as the battery is discharged and increase as the battery is recharged. A brand new fully charged battery can be a starting point for both SG and volts, but a measurement of battery health is also dependent upon charging system performance.

Batteries often fail due to charging system issues, which is why many use BMKs to monitor charger and battery. So knowing the SG of a battery as it ages is also a hint as to how your charger is doing.

 

[CaptTPT]

I've got a different take on it. SG is the ULTIMATE test of a battery's charge state, and arguable the only accurate test. A battery is a chemical reaction, and the state of the chemistry is 100% reflective of the battery's state of charge. It's much more accurate than an amp-hr counter, or load tester, or impedance tester, or voltage measurement.

djmarchand was spot on describing the chemistry and what happens when there is sulfation. A sulfated battery will appear to be full charged to any 3-state charger, i.e. it will have reached terminal voltage and terminal charge acceptance rate. Yet the SG level will be below the target level, and the battery will have corresponding reduced capacity. That's when it's time to equalize to recombine some of the SO4.

The trouble with SG as a measurement is that it's a PITA to do. It's messy, tedious (especially if you have a large battery bank), and access is often difficult on a boat. And flooded batteries require maintenance to top up the water level. All this had helped make AGM batteries so popular on boats, but with them you can't take an SG reading even if you wanted to.

Now, after all that, I need to confess that I have AGMs on my boat. The advantage of being able to read SG just isn't worth the hassle. We also have an off-grid house and it has big flooded batteries. They are a PITA to maintain, and consume a huge amount of water (long story why that's so, but I don't recommend Surrette batteries...). When they die I will probably switch to AGMs or perhaps even the new LiFo batteries.

This gets to precisely my question. I'm not talking about a bad cell. That's easy. I'm talking about a battery that is deteriorating as expected over time but still functioning properly. Should be predictable by measuring the SG. What is the SG where you say time to get new. I posed the question to tech support at Trogen. Waiting for response.

 

[caltexflanc]

A deteriorating battery will not hold a charge, the SG won't come up. That's why the cheap hydrometers you get at the auto places show "good" "ok" "replace" instead of the numerical results. Twisted Tree summed it up well.

I once had a conversation with Trojan about this when I was breaking in a new bank of their L16s. I was getting impatient, they assured me to to trust the SG and sure enough after a few charges it synched up with my Magnum battery monitor perfectly at all SOCs.

I've found Trojan (and Rolls for that matter) tech support easy to reach by phone and quite chatty.

 

[twistedtree]

This gets to precisely my question. I'm not talking about a bad cell. That's easy. I'm talking about a battery that is deteriorating as expected over time but still functioning properly. Should be predictable by measuring the SG. What is the SG where you say time to get new. I posed the question to tech support at Trogen. Waiting for response.

There are a lot of failure modes for batteries, so I don't think SG is a good way to gauge end of life. It really only tells you whether you are getting back to full charge. If not, it's likely due to sulfations, and that sulfation might be irreversible. So SG can tell you that you are getting to the end due to sulfation, but only for that particular failure mode. If you are measuring SG, then you will get an early warning of sulfation and you can equalize to reverse it. So you can prevent death by sulfation.

But there are lots of other things that can kill your batteries. I've had them come back to full SG level with no problem, but collapse during discharge well before they should. I can't tell you why.

 

[C lectric]

I no longer have wet cells, not because I was too lazy, at least not entirely, but because of access, which was extremely poor. Still is for several batteries although I moved the eng. starting ones but I am keeping the batts. all the same type, not having different types aboard.

What I found was Wet cell batteries [golfs], when new, in good shape and properly charged will have very even S.G. levels, cell to cell.

As the batteries age those cell to cell S.G. levels will start to differ and when they reached ~ 25 points different, cell to cell, that battery is in trouble. It won't be just one cell that starts to drop its S.G. level, as in a dead cell, but there will be one or more cells that change faster than the others. That's what should be watched for as not all the cells will drop at the same rate.

Keep some records. They don't need to be highly detailed but should should have enough info to show the changes.

If I remember correctly when new and after several cycles, 10-15, the S.G. would go to about 1300 uncorrected for temp., remaining like that for one-two years and then slowly start to refuse to approach that level. I would still get many years out of them but eventually the inability to hold a charge would be noticeable particularily as those few cells simply refused to come up.

I noticed that 1300 would hold on several cells but drop more quickly on some others over several days. Over the years that 1300 would no longer be achieved.
How hard you work them will have a huge impact on the overall life.

So if after a FULL charge and a rest period you start seeing one or more cells differing by/approaching up to ~ 25 points that battery is in trouble.
I got that cell/cell difference from the battery mfgr., Trojan, which was many years ago and what I saw appeared to align with thier advice . I might have been kidding myself but over the years but I went through a few sets before I went with the Gel cells.

It is unlikely that you will see all cells dropping at the same rate. It will be one or two cells per battery that will determine the overall life of that battery.

I used the glass float type hydrometer, still have it, with the temperature scale on the side and used to check S.G. religiously.
Even so for a GOOD S.G. reading some time needs to be allowed for the cell to settle down as the liquid may still be gassing and mixing itself due to gassing.

By the way, my hydrometer holder is a piece of black ABS plumbing pipe
[could be white pvc plumbing pipe also] with a cap glued on the one end and the length enough so the hydrometer bulb was what it hung by in the holder. The holder had a hole drilled in a piece of wood to hold it and the hydr. near vertical. I could easily lift the whole thing out of the wood piece to use the hydr. The acid will not affect those materials.

Enough of my tome. Just my experience.

 

[FF]

"It really only tells you whether you are getting back to full charge. If not, it's likely due to sulfations, and that sulfation might be irreversible."

True enough , but the good folks that make DE Sulfation units would argue as to weather its reversible.

I hope Practical Sailor can figure a way to test the units now on the market.

Even with a SOC meter and observing the 50% discharge rule , many boaters go far too long before recharging , and a 60% SOC batt will still be sulfating.

This is why many batt mfg ask for a slight overcharge on each recharge.

Charging and esp overcharging or Equalizing will cost water, so Hydro Caps or a rational boat builder is nice.

 

[twistedtree]

C Lectric, the difference in SG between cells that you observed is the result of one cell beginning to sulfate before the others. The cells in your battery are wired in series, so get charged in series. Slight differences in resistance of each cell results in slightly different charge voltages at each cell. So on a 12V battery charging at 14.8V, the individual cell voltages might be 2.5V on 5 cells and 2.3V on the 6th. This is an extreme example meant only to illustrate what's happening. Every charge cycle that 6th cell is being charged at a lower voltage than the other cells, so it always gets slightly undercharged. This will be reflected in reduced SG on the low cell.

An equalization charge will cause all the cells to be charged at a higher voltage. The 6th cells will again charge at a reduced voltage compared to the other cells, but high enough to bring it back to full charge. The overcharging is mostly harmless with the only side effect being excess production of H and O2, plus heat. The production of H and O2 is why you typically need to replenish with water after an EQ cycle. And if you EQ too much, the excess O2 can lead to premature corrosion inside the battery, but you have to being going pretty nuts with EQ for that to be a problem.

So that's a bit of what's behind the policy of checking for unequal SG between cells, and the call for an EQ cycle.

 

[sunchaser]

There is some excellent information and hands on experience exhibited on this thread. Takeaways:

-SG is a good measurement of LA battery health
-AGM health is less certain, faith based
-AGMs don't have to be physically (via SG) checked, can't do it anyway
-LAs cost much less than AGMs
-In either case charging systems must be adequate
-Battery and system monitoring is essential to insure long life
-Operating life under similar conditions is about the same

One question, are gels and AGMs of similar pros and cons?

 

[twistedtree]

The other place where SG readings are very useful is when you first install batteries and are fine tuning your charger(s). If you check the SG after a full charge cycle, it should be back at 1270 (or whatever your baseline is). If it isn't, or if it slowly drops aver the course of multiple charge cycles, you have an indication that your full charge cycle isn't really a full charge cycle and needs to be jacked up a bit. This is the issue I had with my Surrette batteries, and why I strongly recommend AGAINST them for renewable energy applications, and for most boating applications.

A quick bit of background - we have an off-grid house with solar, giant Surrette batteries (1300ah @ 48V), and a backup generator with autostart. This particular system has been in service for about 8 years now, with similar systems preceding it over the past 15 years. But it's really just the same as a boat, except it's run aground.

Anyway, when I first installed the Surrette's, I too thought I had bought the most bad-ass batteries on the planet and was ready for spectacular life and performance. But after a few months in service, I found that the SG was quite low across the whole bank. I was charging per their specs to the correct voltage, and running until the acceptance current was down to 2% of capacity. But still low SG.

So I called Surrette, which in and of itself was a problem. Talk about a company where nobody is home....But finally I got connected with a guy who helped me out a bit, then at some point I got shulffed off to a dealer in WA somewhere.

The dealer in WA kept telling me I needed to run the Absorb mode for 6hrs, and I kept telling him the sun doesn't shine that long. His background was clearly forklifts and large commercial marine applications where there is a 24x7 source of power for charging. We got nowhere.

I finally got a paper from Surrette about different charge parameters for solar applications. This was the reveal that these batteries are really not suitable for such an application, even though they are heavily advertised for exactly that. Remember that 6hr absorb charge the dealer kept telling me I needed? Well, he was right. I gather it's characteristic of very heavy plate batteries, and perhaps also related to the amount on antimony in the plates, but it takes a LONG time to bring these batteries back to full charge. Way longer than the sun shines each day.

The work around is to use a really hot charge voltage. By jacking up the bulk/absorb voltage, you can get the battery back to full charge. I finally settled on a 60V bulk/absorb, and a terminal return current of 1% not 2%. That 60V charge on a 48V system is equivalent to a 30V charge on a 24V system, or a 15V charge on a 12V system. It's pretty close to doing an equalization every time you charge.

All this was sorted out under the instruction of Surrette, and using SG measurements to confirm that after successive charge cycles the batteries were really coming back to full charge.

You might be thinking, who cares? The problem is that it takes way more power to get these batteries charged than more traditional cells like Trojans, and with a limited source of charge power, that really matters, hence my assertion that these batteries are unsuitable for solar applications, or any off-grid application. The same problem occurs if you are charging from a generator or main engine - you have to run it MUCH longer than with other batteries.

The other side effect is water consumption. The hot charge voltage cracks a lot of water into H and O2. I have to top up every 3 months or so, and it take about 10 gal of distilled water to do that. Yes, that's right 10 gallons. And yes, I have hydrocaps. They help, but still.....

Lessons learned are:

1) Never buy a Surrette battery for any sort of off-grid application

2) SG measurements are an important way to confirm that you have a good charge protocol set up.

3) Monitoring SG and filling with water is a giant PITA. In fact, I'm actively procrastinating on that task right now. OK, I'm being a whimp, and DMarchand can laugh at me. That's fair. But you can probably see why I'm tainted on the subject.

 

[sunchaser]

Peter

What would have been the outcome with Trojans vs the Surrettes?

 

[twistedtree]

Peter

What would have been the outcome with Trojans vs the Surrettes?

I had Trojan L16s before the Surrettes and they had no trouble coming back to full charge. If I did flooded LA again, I'd go back to Trojans or Deka. The problem with them is that each cell's AH capacity is much lower than the giant Surrettes, so more cells required for equivalent capacity. With the Surrettes I have 24 cells to monitor and water, all ni one series string. With L16s I would have 3x that, or 72 cells to monitor and water in 3 strings. That starts making AGMs look pretty good.

What I really want is for the Surrettes to last long enough for me to experiment with and hopefully build a reliable LiFePO4 battery bank. Their properties are REALLY attractive, but I'm not sure the overall system technology is mature enough for use beyond a science project.

LA batteries have two undesirable properties that we are all familiar with:

1) LA is typically sized for 50% max discharge. This means LA battery banks are twice the capacity needed by the application. That's a lot of extra expense, space, and weight that's not doing anything for us.

2) LA has a declining charge acceptance rate between 80% and 100% charge level. This means we have to run generators much longer, and with declining load in order to get back to full charge. It can also leave available solar charge power unused when you really want to extract every drop. In practice, this leads to chronic undercharging, and subsequent sulfation, since nobody wants to run their generator long enough to reach full charge.

LiFePO4 batteries don't have either of these restrictions. You can draw them down pretty much 100% (it might be 80% or 90%, but it's WAY more than 50%). So you get to use everything you buy, not just half of it. And of course you can size you battery bank to match what you need, not twice what you need. So it takes less space, and offsets the higher cost of LiFePO4.

And LiFePO4 will accept full charge rate right up until they are full, so you can make much better use of your generator, solar, or whatever. And they also have a very high allowable charge rate, so it you really want to run you generator balls to the wall and use 200A, or 300A, or 400A of charging, you can. I really like the idea of a shorter generator run time, and actually achieving full charge at the same time.

I've only given this a little thought, but would probably build a LiFePO4 system in parallel with the LA bank so I can keep the LA bank in place and switch back to it if I have problems.

 

[twistedtree]

Sorry for being such a blabber mouth here, but you've got me going. Actually, you've provided me a good way to avoid doing that SG check on my batteries.......

I've had two cell failures on my super expensive, super uber all that Surrette batteries. About 2 years into service I started getting low voltage alarms. SG was fine on all cells, but I ultimately tracked down one battery (each battery is two cells, so 4V) where the voltage would be fine, then plummet during discharge. One nice thing about the Surrettes is that you can pull off the cover (there is an outer case with the actual cells inside) and access the individual cells which are bolted together. I was able to further isolate the problem to a single cell. Again, its SG was fine, but as it would discharge the voltage would plummet and actually reverse voltage. After a huge run-around from Surrette, they finally agreed to replace it under warranty, but..... I would have to pay shipping at a cost of about $200, and it would take 2-3 months to get a replacement cell. WTF?

So I reconfigured the bank to run at 44V instead of 48V, adjusting charge parameters accordingly, and kept running until the replacement cell arrived. BTW, this is an other advantage to having individual cells accessible. The new cell ultimately arrived and with the help of a couple of friends, we were able to get it replaced. BTW, one of these batteries weights 325lb, and an individual cell half that, so handling them is not easy.

But the new cell always ran with very high SG. 1325 to be specific. I asked Surrette about it and they just hand waved it away and said it would be fine. Well, about a year ago I had another cell crap out, and guess which one it was? That's right, the same darn cell.

This time I got a little smarter and called around, ultimately finding a dealer who would sell be a replacement cell for $300 including shipping. This was way more attractive than dealing with Surrette on a pro-rated warranty claim, so I ordered the battery.

As an aside, at the time I considered dumping the whole bank and going to AGMs, but I couldn't find anyone who had them in stock in L16 size, so I abandoned that approach. And besides, if for $300 I could get another couple of years out of the Surrettes, it might give me time to switch to LiFePO4...

This time the replacement battery took nearly 4 months to get. But it's installed and I'm up and running at full voltage again. Just need to go do another check on SG, water level, etc. to be sure all continues to be well.

 

[JDCAVE]

Peter: Excellent post! Keep up the good "Blabbing"!

Jim
Sent from my iPad using Trawler Forum

 

[CaptTPT]

Response from Trojan Tech Support:Specific gravity is only a good indication of “state-of-charge”. An actual discharge capacity test will tell you how much capacity your batteries will have at the exact time. There are no prediction tables or charts.