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Sorry to be late answering, been off line for a while.

 

Our 100 amp alt starts charging at 100+ amps for several mins before commencing a gradual drop. Used to be like this with batts new but took longer to reach absorption stage after which everything behaved as it does now. (which would be expected as batt capacity is now smaller)

 

Ok....

 

I'm going to assume here that these are type 2 AGMs (see further down) because a type 1 AGM would make no sense whatsoever from these readings.

 

The charge current during acceptance will fall roughly exponentially. Starting at 100 amps and falling to 8 amps over about 2 hours. But initially it stays at 100 amps for several (how many is that?) minutes. Taking a few guesses here to fill in the gaps, and integrating over 2 hours gives us about 80 amphours returned during the charge.

 

Assuming you have no big loads on during the discharge then we can take the first voltage measurement of 24.6 volts as being (normally) about 60%. The second reading of 23.7 about 20%. That means you'd be using 40% of your battery capacity. 40% of your bank would be 192amphours. But you're only returning about 80 amphours and staying roughly still. From that we can conclude that your bank is down to about 40% SoH.

 

Note that we could try to adjust these figures and say that 24.6 is the fully charged voltage and readjust everything downwards but the end result would be the same.

 

Now then, when the SoH falls below (about) 50% the batteries are effectively broken. That's why the terminal voltage is so low. At 60% SoH this doesn't happern at all with wet cells. The voltage remains exactly the same as it was with brand new batteries. That isn't the case with AGMs. AGMs do show a drop in terminal voltage as they age.

 

The exact reason for this never became clear to me. I never found a proper explanation. The only thing that is different is that with a wet cell, plate shedding plays just as much a part as sulfation does in reducing capacity. In AGMs plate shedding still happens but the material stays where it is, squashed against the plates by the separators and matting. Thus it can still play a part in the reactions. This is the only real difference and, I suspect, strongly related to why AGMs show a reduced voltage as they age which wet cells do not.

 

Related to this (a subject which came up on here a while back) is that I have still never found a correct explanation as to why the open circuit terminal voltage drops during discharge. At a cursory glance it's obvious why it happens, there's less energy left in the battery, except that this "obvious" explanation is completely wrong and irrelevant.

 

Someone mentioned in an older post that their AGMs seemed to operate at a lower voltage than flooded lead acids which seems to be the case with ours but I think I recall you saying AGMs should be same which seems logical. Just seems a bit odd.

 

Cheers

 

Chris

 

There are two types of AGMs, One type shows a terminal voltage higher than wet cells. Round about 0.2 volts higher (on a 12 volt battery) at any particular SoC. The other type has a terminal voltage the same as wet cells. But it does indeed fall slightly as the battery ages.

 

Strongly related to all of this is the difference in the way wet cells and AGMs behave when they age and lose capacity. Wet cells behave as if they are just a smaller battery of the same type. Thus the terminal voltage remains the same for any given SoC throughout the life of the battery. AGMs don't do this. AGMs behave as if you simply cannot get them charged past a certain level. Thus the terminal voltage falls for any given state of charge as the battery gets older. Have another look at your figures and you'll see this ties in with your experience of the terminal voltage.

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Ok....

 

I'm going to assume here that these are type 2 AGMs (see further down) because a type 1 AGM would make no sense whatsoever from these readings.

 

The charge current during acceptance will fall roughly exponentially. Starting at 100 amps and falling to 8 amps over about 2 hours. But initially it stays at 100 amps for several (how many is that?) minutes. Taking a few guesses here to fill in the gaps, and integrating over 2 hours gives us about 80 amphours returned during the charge.

 

Assuming you have no big loads on during the discharge then we can take the first voltage measurement of 24.6 volts as being (normally) about 60%. The second reading of 23.7 about 20%. That means you'd be using 40% of your battery capacity. 40% of your bank would be 192amphours. But you're only returning about 80 amphours and staying roughly still. From that we can conclude that your bank is down to about 40% SoH.

 

Note that we could try to adjust these figures and say that 24.6 is the fully charged voltage and readjust everything downwards but the end result would be the same.

 

Now then, when the SoH falls below (about) 50% the batteries are effectively broken. That's why the terminal voltage is so low. At 60% SoH this doesn't happern at all with wet cells. The voltage remains exactly the same as it was with brand new batteries. That isn't the case with AGMs. AGMs do show a drop in terminal voltage as they age.

 

The exact reason for this never became clear to me. I never found a proper explanation. The only thing that is different is that with a wet cell, plate shedding plays just as much a part as sulfation does in reducing capacity. In AGMs plate shedding still happens but the material stays where it is, squashed against the plates by the separators and matting. Thus it can still play a part in the reactions. This is the only real difference and, I suspect, strongly related to why AGMs show a reduced voltage as they age which wet cells do not.

 

Related to this (a subject which came up on here a while back) is that I have still never found a correct explanation as to why the open circuit terminal voltage drops during discharge. At a cursory glance it's obvious why it happens, there's less energy left in the battery, except that this "obvious" explanation is completely wrong and irrelevant.

 

 

 

There are two types of AGMs, One type shows a terminal voltage higher than wet cells. Round about 0.2 volts higher (on a 12 volt battery) at any particular SoC. The other type has a terminal voltage the same as wet cells. But it does indeed fall slightly as the battery ages.

 

Strongly related to all of this is the difference in the way wet cells and AGMs behave when they age and lose capacity. Wet cells behave as if they are just a smaller battery of the same type. Thus the terminal voltage remains the same for any given SoC throughout the life of the battery. AGMs don't do this. AGMs behave as if you simply cannot get them charged past a certain level. Thus the terminal voltage falls for any given state of charge as the battery gets older. Have another look at your figures and you'll see this ties in with your experience of the terminal voltage.

 

Thanks for that, very interesting.

 

When we first took delivery of our AGMs I was told they were American (by the delivery driver) so I assumed they would require a lower charging voltage but 3 years later when voltages had deteriorated I found out from the suppliers that they they had never sold American AGMs and to increase our charging voltages. Bit confused on this but I am assuming type 2 AGM are not American but European spec?

 

Cheers

 

Chris

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Thanks for that, very interesting.

 

When we first took delivery of our AGMs I was told they were American (by the delivery driver) so I assumed they would require a lower charging voltage but 3 years later when voltages had deteriorated I found out from the suppliers that they they had never sold American AGMs and to increase our charging voltages. Bit confused on this but I am assuming type 2 AGM are not American but European spec?

 

Cheers

 

Chris

 

Type 1 are the original ones. They have lower recommended charge voltages but the terminal voltage is higher. They usually have a datasheet with lots of warnings about being damaged by high charge voltages. Almost all of the early AGMs were of this type. Hardly anyone is left making them now because they tend to sulfate up quite badly. Mainly because of the lower charge voltage.

 

The manufacturers nearly all changed to type 2 AGMs. They have a higher recommended charge voltage but the terminal voltage is lower. The datasheets often recommend periodic equalisation at a higher charge voltage, often as high as 15.5 volts and sometimes even 16 volts or higher. They are nearly all of this type now.

 

Both types haved been made all over the world. Lifeline started off as type 1, but later changed to type 2. Without actually telling anyone! (their datasheet did change but they didn't appear to alert anyone to this fact and many dealers were still peddling the old datasheet thus adding to the confusion). Optima Spirals have always been type 2. Trojan are type 1 (or at least they were last time I checked one). It all gets very confusing.

 

These "types" are purely arbitrary and merely a way to differentiate between the two when discussing charge voltages. They are not the same as the battery type numbers is SmartGauge. Many modern chargers have two settings for AGM because of this. And despite this there are still people around who don't believe there are two types of AGM. Even though lots of chargers have two different bloody settings for them!

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Type 1 are the original ones. They have lower recommended charge voltages but the terminal voltage is higher. They usually have a datasheet with lots of warnings about being damaged by high charge voltages. Almost all of the early AGMs were of this type. Hardly anyone is left making them now because they tend to sulfate up quite badly. Mainly because of the lower charge voltage.

 

The manufacturers nearly all changed to type 2 AGMs. They have a higher recommended charge voltage but the terminal voltage is lower. The datasheets often recommend periodic equalisation at a higher charge voltage, often as high as 15.5 volts and sometimes even 16 volts or higher. They are nearly all of this type now.

 

Both types haved been made all over the world. Lifeline started off as type 1, but later changed to type 2. Without actually telling anyone! (their datasheet did change but they didn't appear to alert anyone to this fact and many dealers were still peddling the old datasheet thus adding to the confusion). Optima Spirals have always been type 2. Trojan are type 1 (or at least they were last time I checked one). It all gets very confusing.

 

These "types" are purely arbitrary and merely a way to differentiate between the two when discussing charge voltages. They are not the same as the battery type numbers is SmartGauge. Many modern chargers have two settings for AGM because of this. And despite this there are still people around who don't believe there are two types of AGM. Even though lots of chargers have two different bloody settings for them!

 

 

Thanks for all that, very informative.

 

Now I can be pretty certain we have type 2 I think we will carry on with our weekly 8hr absorption but reset our Victron to 30v (adaptive charging is already disabled)

 

Cheers

 

Chris

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Ok....

 

I'm going to assume here that these are type 2 AGMs (see further down) because a type 1 AGM would make no sense whatsoever from these readings.

 

The charge current during acceptance will fall roughly exponentially. Starting at 100 amps and falling to 8 amps over about 2 hours. But initially it stays at 100 amps for several (how many is that?) minutes. Taking a few guesses here to fill in the gaps, and integrating over 2 hours gives us about 80 amphours returned during the charge.

 

 

 

Just finished a 2hour charge

 

103amps for 1.5 mins

95 after 3 mins

93 - 5

88 - 10

70 - 20

52 - 30

36 - 40

25 - 50

19 - 60

15 - 70

11 - 80

8 - 90

7 - 100

6 - 110

5 - 120

 

Chris

Edited by nb Innisfree
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  • 2 weeks later...
Just finished a 2hour charge

 

103amps for 1.5 mins

95 after 3 mins

93 - 5

88 - 10

70 - 20

52 - 30

36 - 40

25 - 50

19 - 60

15 - 70

11 - 80

8 - 90

7 - 100

6 - 110

5 - 120

 

Chris

 

Ok I just graphed (and yes they are roughly exonential) and then integrated those figures. Total comes to 71 amphours so my figures are pretty close. Your batteries aren't quite as bad as previous guesses showed. Maybe about 45% !

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Ok I just graphed (and yes they are roughly exonential) and then integrated those figures. Total comes to 71 amphours so my figures are pretty close. Your batteries aren't quite as bad as previous guesses showed. Maybe about 45% !

 

Thanks Gibbo, much appreciated

 

They do seem to have consistent performance so maybe we will be able to get a reasonable life from them.

 

Chris :lol:

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