AGM batteries constantly charged. Can it damage them?

Batteries: 6 x AGM Mastervolt 12V 225Ah (3 sets of 2 12V batteries connected in series, 24V per set; sets connected in parallel) - https://www.mastervolt.com/products/agm-12v/agm-12-225-group-8d/

Charger: Mastervolt MASS 24/75C
https://www.mastervolt.com/products/mass-24v/mass-24-75-dnv-gl-lloyds/


There is some load connected to the batteries and charger, but I am not sure if the batteries are supplying any power (maybe charger is supplying all the power to the load; I will measure this in the next few days).

The charger is supposedly a smart one, charging in 3 phases and having a temperature sensor to lower or raise charging voltage depending on the battery temperature.

These are backup batteries and the system is designed so that these batteries are always being charged.

Question:

Can AGM batteries be damaged if they are constantly being charged by a supposedly smart charger?
 
No. Happens ALL THE TIME with RVs (Recreational Vehicles). A three-step (“phase”) charger with temperature sensor is best.
Thank you.

Then I have to keep looking for the reason why my batteries' capacity is dropping so fast.

The batteries CCA have dropped 50% in 2.5 years...
 
gubavac111,

The drop in CCA would have been nice to know in the original post.

Unless the load exceeds the output of the charger, the charger will supply all the current required by the load.

An equalizing charge applied periodically is always a good thing for batteries. In my RV I try to regularly discharge the batteries after temporarily offing the charger (but never below 50%!) just so the three-step charger will go into “high gear” once in a while. My AGMs are kept on smart trickle charger during off-season, and they are seven years old.

As a matter of curiosity: How are you measuring CCA? And when?
 
gubavac111,

The drop in CCA would have been nice to know in the original post.

Unless the load exceeds the output of the charger, the charger will supply all the current required by the load.

An equalizing charge applied periodically is always a good thing for batteries. In my RV I try to regularly discharge the batteries after temporarily offing the charger (but never below 50%!) just so the three-step charger will go into “high gear” once in a while. My AGMs are kept on smart trickle charger during off-season, and they are seven years old.

As a matter of curiosity: How are you measuring CCA? And when?

I am pretty sure all load is supplied by the charger, but I will double check it.

There is also a backup charger of the same type connected to these batteries.



How exactly do you perform charge equalization?



I am measuring CCA with the Midtronics MDX-700HD.

I am measuring CCA in a way that I just disconnect positive lead and then do the CCA test.
I know it is not correct way and I am not getting the realistic picture of the battery's capacity.

Correct me if I am wrong: When doing the CCA test this way, I will get the best possible results, right?
If I left the battery off the charger for about a day, the I would get more realistic results, right?

And even the results I obtain by measuring CCA this way show fast capacity drop.
 
Years ago, when a UPS system was a complete disaster, had a PLC and subsystem running on 24vDC battery set being permanent charged on what was effectively a DC PSU. Thinking of the long term effects of constant charging {and the PSU o/p being adjustable}, the charging voltage was set to 26.5vDC so not to fully charge.

The calculated 'up' time on power failure was something like 18hrs as the battery set was built in for UPS; but never got feedback from this site on how effective it was in the long term - this was before the 'millenium bug' and didn't get feedback on that either, not that it would have any detrimental effect.
 
I’ve only ever witnessed CCA testing of automobile batteries, and that was done completely disconnected from the vehicles. I’m not familiar with the device you are using but if you are following the manufacturer’s instructions and don’t feel good about the results try contacting the manufacturer for assistance.

The benefit—in my personal opinion—of AGM batteries is the lack of a need to maintain water levels in the cells. The downside of this is the inability to check specific gravity. I’m fairly certain AGM’s experience some sort of gassing, which must mean some kind of decrease in liquid level(maybe not—I’m SWAGging it here (Scientific Wild-Arsed Guessing). But, that can’t be checked, either.

So, I’m at a loss on this issue—that has morphed from the original post. I’m not sure AGM’s are the best option for emergency power supply—for the reasons above. But, I’ve never researched the issue; ALL of my experience is with lead-acid batteries (mostly 125 VDC systems).

Please write back to let us know how you fare in your investigation.
 
I’ve only ever witnessed CCA testing of automobile batteries, and that was done completely disconnected from the vehicles. I’m not familiar with the device you are using but if you are following the manufacturer’s instructions and don’t feel good about the results try contacting the manufacturer for assistance.

The benefit—in my personal opinion—of AGM batteries is the lack of a need to maintain water levels in the cells. The downside of this is the inability to check specific gravity. I’m fairly certain AGM’s experience some sort of gassing, which must mean some kind of decrease in liquid level(maybe not—I’m SWAGging it here (Scientific Wild-Arsed Guessing). But, that can’t be checked, either.

So, I’m at a loss on this issue—that has morphed from the original post. I’m not sure AGM’s are the best option for emergency power supply—for the reasons above. But, I’ve never researched the issue; ALL of my experience is with lead-acid batteries (mostly 125 VDC systems).

Please write back to let us know how you fare in your investigation.

So, I ve contacted manufacturer and got a reply:

They're saying that there is no problem whatsoever if batteries are constantly connected to the charger. You just need to make sure that the charger setting is set to the adequate battery type (in this case, AGM) and that temperature sensor is attached to the battery (which ensures voltage adjustment in case the temp is not between 20 and 30 ºC.

Since I have more important things to do, I will conclude my investigation here and simply replace these batteries and then observe how the new ones behave.

I cannot pinpoint anything that would cause this kind of CCA drop in such a short amount of time.
 
gubavac111,

Thank you for the update. I personally suspect an issue with the testing methodology—but I can’t prove that. I have not seen or heard of CCA testing in the context of what you are describing—a means of testing emergency power supply batteries. The only such testing I have ever witnessed is called “battery run-down” testing which is a method for testing the actual capacity of an emergency power supply battery to provide a quantity of current over a period of time—which in my opinion is not the same thing as CCA which is a term for automotive batteries used to start vehicles. Using a CCA “tester” for trying to determine battery capacity for automotive batteries used for emergency power supply capability (which you haven’t fully described as part of this post) is possibly a misapplication of the test or the tester.

A battery run-down test requires time, which my understanding of a CCA test doesn’t. CCA is a means of quantifying the ability to provide instantaneous amperes under adverse ambient weather conditions (cold-weather starting). Is that a requirement of your application of these batteries—providing a specific instantaneous amperage under cold-weather conditions? Or does the application require a certain amperage to be delivered over some period of time?

The system requirements should dictate the testing to ensure the ability to meet the requirements. If automotive batteries are being used for some application other than vehicle starting the maybe a different battery testing method is more appropriate. Not as simple or easy, but more appropriate.

But, again, not understanding the application we can’t really assess the testing methodology or results, can we?
 
gubavac111,

Thank you for the update. I personally suspect an issue with the testing methodology—but I can’t prove that. I have not seen or heard of CCA testing in the context of what you are describing—a means of testing emergency power supply batteries. The only such testing I have ever witnessed is called “battery run-down” testing which is a method for testing the actual capacity of an emergency power supply battery to provide a quantity of current over a period of time—which in my opinion is not the same thing as CCA which is a term for automotive batteries used to start vehicles. Using a CCA “tester” for trying to determine battery capacity for automotive batteries used for emergency power supply capability (which you haven’t fully described as part of this post) is possibly a misapplication of the test or the tester.

A battery run-down test requires time, which my understanding of a CCA test doesn’t. CCA is a means of quantifying the ability to provide instantaneous amperes under adverse ambient weather conditions (cold-weather starting). Is that a requirement of your application of these batteries—providing a specific instantaneous amperage under cold-weather conditions? Or does the application require a certain amperage to be delivered over some period of time?

The system requirements should dictate the testing to ensure the ability to meet the requirements. If automotive batteries are being used for some application other than vehicle starting the maybe a different battery testing method is more appropriate. Not as simple or easy, but more appropriate.

But, again, not understanding the application we can’t really assess the testing methodology or results, can we?

Thank you, I do believe you are right.

The best suited test for the batteries which serve as a backup batteries (meaning, they have to provide 50-100A current for 30-120 min; I will have to check the exact figure) is indeed a run-down test. That way I can see if the batteries will provide certain amount of current in a certain period.


But this begs the question - how will we know the condition of the individual battery? What if 5 batteries are good and one i bad? How do we find this out?

Remember, there are 6 batteries in total.
3 sets of 2 12V batteries. 2 batteries in each set are connected in series to give 24V.
All 3 sets are connected in parallel.
 
I didn’t forget/haven’t forgotten. Since you can’t check specific gravity, if the group were to fail the run-down test the you would have to separate the batteries and measure the individual voltages and maybe run an individual load test of some sort to see how long it takes to reach the lowest voltage recommended by the manufacturer. (For lead-acid wet cell batteries, for example, that’s 11.5 volts.)

That would be what I would suggest. At this point, again not knowing/understanding the application.

Best of luck!
 
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