Voltage Drop

So I have an outback system. One day power died in house. Turned system off and on and power game back on. There was an error that said battery voltage error.

I have a 48v system consists on 16 6v 415amp rolls agm batteries. At the begining of the day voltage usually reads 48.6 .

I noticed three days ago that however after being fully charged the voltage dropped to 48.4 fast when under light loads.

So I went over to main power and then and solar charge the batteries. Today I disconnected them all and tested them after 8 hrs. They all read around 6.3v connected them up again . And after a light load with in thirty minutes voltage was down to 48.4. Can batteries fail all of a sudden. There three years old and maintained eell

Assuming S6-460AGM-RE batteries.

Worth noting that the cycle life of AGM is pretty poor. At 80% DoD, they're only going to last about 600 cycles. At 50% DoD, only 1200 cycles. With each cycle, your eating away at capacity, so whatever your daily "50%" is, becomes greater than 50% of the actual capacity. After 1200 cycles, your "50%" is actually closer to 62.5%.

Well maintained flooded (FLA) can dramatically outlast AGM.
AGM will outlast poorly maintained FLA.

6.3V resting is not fully charged for AGM. I would be very suspicious of anything below 6.5 after an 8 hour rest.

1. Maintained well? What have you done to maintain the AGM batteries? The only thing you can do is charge them according to specification.
2. Are you charging to 58.8V and holding 58.8V until the current going into the battery has dropped to 25A?
3. Are you floating at 55.2V
4. Have you confirmed the individual 6V are being charged to nearly exactly 7.35V and float at 6.9V?
5. Is temperature compensation active?
6. Working battery monitor with confidence that you haven't regularly discharged below 50%?
7. Have you read and followed this faithfully?

Consistent undercharging is the best way to kill Rolls batteries.

I never let voltage drop below 48 ever lots of times woul run generator for two hours into evening. It's just odd how this seems to of happened quickly

What's temperature compensation? Live in Bahamas. Thought agms would handle heat

wpmaura said:

I never let voltage drop below 48 ever lots of times woul run generator for two hours into evening. It's just odd how this seems to of happened quickly

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Not sure if you noticed, but I asked 7 questions. You answered #6.

I also indicated that a battery sitting at 6.3V after 6 hours is NOT fully charged.

A rapid change is almost always due to:

1. It's been going on awhile, and you just didn't notice.
2. Conditions have changed affecting your ability to fully charged... like a week of cloudy skies.

wpmaura said:

What's temperature compensation?

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Question #1 answered. NOT well maintained.
Question #5 answered. Temperature compensation is either not active, or your installer has it present, but you don't know about it.
Question #7 answered. A lack of knowledge of temperature compensation means you've never read the Rolls battery manual.

Please refer to the linked manual for temperature compensation. Assuming you are in the Bahamas due to your signature, temperature compensation is less important, but based on annual temperature range, you have likely been slightly over-volting the batteries regularly unless they are in a climate controlled environment of 25°C/77°F or if it's not active, and you just don't know about it.

Regardless of what you think, you have NOT maintained these batteries well at all.

Answers to #2, 3 and 4 are absolutely critical.

As for float and charge everything set to what rolls told me too. Which is basically what you said. But I pay attention. To the voltage in the am closely all the time. Never changed it for temp. Coldest it gets is 60 high is 98

Lifeline AGM recommends an equalization charge if sulfating with loss of capacity.
15.50 volts 6 to 8 hours. I recommend an hour a day and see if they are improving.
If they are shot there is nothing to lose.

https://lifelinebatteries.com/blog/can-i-equalize-agm-batteries/

Of course your manufacturer may have other recommendations.

Worst case start shopping for some lithium cells to build a replacement battery.

wpmaura said:

As for float and charge everything set to what rolls told me too. Which is basically what you said. But I pay attention. To the voltage in the am closely all the time. Never changed it for temp. Coldest it gets is 60 high is 98

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And what about the individual 6V batteries?

time2roll said:

Lifeline AGM recommends an equalization charge if sulfating with loss of capacity.
15.50 volts 6 to 8 hours. I recommend an hour a day and see if they are improving.
If they are shot there is nothing to lose.

https://lifelinebatteries.com/blog/can-i-equalize-agm-batteries/

Of course your manufacturer may have other recommendations.

Worst case start shopping for some lithium cells to build a replacement battery.

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Rolls doesn't do equalization on AGM.

sunshine_eggo said:

And what about the individual 6V batteries?

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not sure how to do that but i figure there screwed, going to just run solar during the day and switch to main power at night. Solar guy by me has quoted me for lithium batteries AES Rackmount by Discover anyone heard of them before

Discover makes top tier batteries.

We run their older lithium batteries 42-48-6650’s x 3. These work very well.

Their rack mount batteries are new. I would assume these to be excellent as well.

wpmaura said:

not sure how to do that but i figure there screwed, going to just run solar during the day and switch to main power at night. Solar guy by me has quoted me for lithium batteries AES Rackmount by Discover anyone heard of them before

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Wait until system reads 58.8V, then check all 6V batteries with a voltmeter.

Also worth repeating the exercise when batteries are at their lowest voltage in the morning.

I had AGMS's

in my camper the AGM batteries lasted almost 10 years, but they got plenty of absorb time between uses.. the ones in the cabin did not.

if you do not give them the proper absorb on a very regular basis they will lose capacity quickly. I had brand new AMG's at the cabin that only lasted 3 years. i just did not have nearly enough solar to bring them out of bulk, and then give them the 4 to 5 hours of absorb they would need due to battery bank size vs solar input.

I walked them down over three years...thats what made me swap to lithium. they charge fast enough that my solar panels are enough to get them charged to 100% on a daily basis in the summer and in the winter they hit it fairly often as well. the fact that lithium do not mind partial charge just makes it better.

wpmaura said:

But I pay attention. To the voltage in the am closely all the time. Never changed it for temp. Coldest it gets is 60 high is 98

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Voltage in spec sheet is quoted for 25 degrees C. Increase for lower temperature, decrease for higher. 98F isn't terribly high, but with thermocouple installed and temperature coefficient, would have reduced voltage somewhat. Lead-acid batteries heat up during charging, probably AGM not as much as FLA due to lower resistance, and voltage would be automatically reduced.

What are the voltages of the individual 6V batteries?

sunshine_eggo said:

Wait until system reads 58.8V, then check all 6V batteries with a voltmeter.

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If some are low, they could be brought up with 6V charger. I just did that with a batch of used batteries I acquired. If one or two are excessively low and don't recover, you may be able to reconfigure for a 42V nominal bank (if inverter/charger supports that.) Or a single string of 8 batteries for 48V. Possibly a low battery could be equalized (gently, because AGM) if one of its 3 cells is lower than the other.

Do you know how many Ah or kWh you use per night?

sunshine_eggo said:

Worth noting that the cycle life of AGM is pretty poor. At 80% DoD, they're only going to last about 600 cycles. At 50% DoD, only 1200 cycles. With each cycle, your eating away at capacity, so whatever your daily "50%" is, becomes greater than 50% of the actual capacity. After 1200 cycles, your "50%" is actually closer to 62.5%.

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So 50% DoD would be 3 or 4 years. But 15% DoD could last a decade.
You've got twice the capacity of my battery, and live in a mild climate, unless you're running A/C at night.
AGM should be fine for light loads at night, kept floating much of the daytime with PV keeping up with loads.

wpmaura said:

I never let voltage drop below 48 ever lots of times woul run generator for two hours into evening. It's just odd how this seems to of happened quickly

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48V under heavy load could be an OK SoC, but 48V under light or no load would be completely discharged.

Do your batteries have cross connect wires at every 6V battery (2p8s), or just two sets of 8 in series, then connected in parallel (8s2p)?
How are inverter and charger cables connected to the bank? To "corners" of array, or to both ends of one 8s string? You could provide a photo. There is a way to balance charge/discharge perfectly, and a wrong way which won't treat all the same.

yep i think between us all we have hit the high points for AGM... Long absorb, temperature compensation for overly cold or overly hot, and proper wiring. it is one of the three that is killing your batteries.

Daddy Tanuki said:

yep i think between us all we have hit the high points for AGM... Long absorb, temperature compensation for overly cold or overly hot, and proper wiring. it is one of the three that is killing your batteries.

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With "fully charged" batteries showing 6.3V after an 8 hour sit, they're just not being fully charged if they're not actually failing.

sunshine_eggo said:

With "fully charged" batteries showing 6.3V after an 8 hour sit, they're just not being fully charged if they're not actually failing.

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That would be my honest guess as well. AGMs once walked down are pretty much shot as you cannot reverse sulfation like you can with FLA

Daddy Tanuki said:

That would be my honest guess as well. AGMs once walked down are pretty much shot as you cannot reverse sulfation like you can with FLA

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This "all of a sudden" thing makes me thing charge conditions or usage has changed vs. the batteries crapping out.

On the other hand, not having a good handle on the behavior of the individual 6V can set you up for rapid degradation.

sunshine_eggo said:

This "all of a sudden" thing makes me thing charge conditions or usage has changed vs. the batteries crapping out.

On the other hand, not having a good handle on the behavior of the individual 6V can set you up for rapid degradation.

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how do i know what the charging on the individual batteries is doing. I say all of a sudden becuase the system is setup to shut off when ever the voltage go to 48v and and that never happeend once until it did a week or so ago.

wpmaura said:

how do i know what the charging on the individual batteries is doing. I say all of a sudden becuase the system is setup to shut off when ever the voltage go to 48v and and that never happeend once until it did a week or so ago.

Click to expand...

sunshine_eggo said:

6.3V resting is not fully charged for AGM. I would be very suspicious of anything below 6.5 after an 8 hour rest.

1. Maintained well? What have you done to maintain the AGM batteries? The only thing you can do is charge them according to specification.
2. Are you charging to 58.8V and holding 58.8V until the current going into the battery has dropped to 25A?
3. Are you floating at 55.2V
4. Have you confirmed the individual 6V are being charged to nearly exactly 7.35V and float at 6.9V?
5. Is temperature compensation active?
6. Working battery monitor with confidence that you haven't regularly discharged below 50%?
7. Have you read and followed this faithfully?

Click to expand...

sunshine_eggo said:

Wait until system reads 58.8V, then check all 6V batteries with a voltmeter.

Also worth repeating the exercise when batteries are at their lowest voltage in the morning.

Click to expand...