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Old 10-18-2015, 08:32 AM   #21
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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?
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Old 10-18-2015, 08:51 AM   #22
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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.
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Old 10-18-2015, 08:58 AM   #23
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Peter

What would have been the outcome with Trojans vs the Surrettes?
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Old 10-18-2015, 09:49 AM   #24
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Quote:
Originally Posted by sunchaser View Post
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.
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Old 10-18-2015, 10:19 AM   #25
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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.
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Old 10-18-2015, 01:13 PM   #26
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Specific Gravity Change

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

Jim
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Old 10-21-2015, 11:33 AM   #27
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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.
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