question

FRA avatar image

Which absorption Tail Current Indicates Real 100% Soc?

Good morning.
I'm trying to maximize the charge of my agm victronenergy 110 Ah.

I made some considerations:
Solar charging can never complete 100%.
Because time is not enough.
Correct solar charging reaches 99%.

My battery is exclusively maintained by solar mppt.

I stop ABS when the tail current on the smartshuunt drops below 2%, i.e. below 2.2A, for at least 15 minutes.

This usually occurs after 1.5 hours / 2.5 hours of ABS.
In fact, my previous dod is always 15/20%, no more.
My mppt allows a maximum abs time of 3 hours. Or an all day ABS, raising the floating voltage to the ABS value.
After 3 hours of ABS, the tail current manages to drop to 1.5A.

I have read that many battery manufacturers believe that 100% is reached only after 16/20 hours of charging.
When the tail current drops to very small values. Much, much smaller than 2%.

Is it better to stop Abs with tail current of 2-2.5%, or wait for tail current of 1-1.4% ??

Do you think it is preferable to keep Abs time for 3 hours every day? ?
And once a week even do an abs time all day long?

I thought that if my recharge reaches only 99.x% and not 100%, my battery will experience an inevitable and slow sulfation.
Because the 0.x% that is not recharged every day will slowly transform the sulphate with irreversible crystals.
It's right?
BMV Battery MonitorVRMSOC
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3 Answers
kevgermany avatar image
kevgermany answered ·

I think you're going to find that there are no absolute answers and that over time your battery will degrade. Excess charging leads to plate erosion, which also shortens battery life. Extending absorption times will overcharge if you have good power input. And some kit allows you to set max absorption time for this reason.

The reason for setting a low tail current/long time detection with solar is because of variable output from the panels. With other charging there's a default tail of 4%, shorter time periods. The regular absorption if a battery is in float/storage is designed to combat sulphation. But as you're in absorption daily, it's not needed.

I thought about this long and hard when setting my system up, decided on lead carbon. Now I just leave the kit to do its thing while I get on with other things.

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Thank you for your opinion.

I read a lot of opinions around.
FOR example Optima and Rolls argue that it is strictly necessary that Abs is prolonged for at least 1 hour after it drops to 2%, to reach full charge. Not a few minutes like victronenergy controllers do.
Otherwise, silfation occurs.
But agm ROLLS and OPTIMA have much thicker plates than my victronenergy agm, or spiral plates.

In your opinion it is preferable that I stop Abs with tail current <2%, or <1.5% in my case with mppt controller?

Instead I'm pretty sure 4% is way too high and doesn't even remotely indicate a full charge. 

YYou say:
"" that over time your battery will degrade."" 
I don't understand the sense of these, I'm not English : do you mean the normal battery aging or
Do you mean I'm overcharging my battery now?

Thank you. 
Normal aging.

As snoobler said follow the maker's recommendations for your battery. Hooefully they know better than us ;-)


snoobler avatar image
snoobler answered ·

Follow the battery manufacturer's instructions. Rolls, Trojan, Optima, Crown, etc. all have different criteria.


If your battery monitor isn't hitting 100%, but your MPPT is meeting the fully charged criteria, your battery monitor isn't configured correctly.


In the absence of manufacturer specific data, Current: 0.1C, 14.4V absorption, 1-3% tail current and 13.6V float are all pretty safe numbers.


AGM are sulfation resistant. Limiting your DoD to 20% (80% charge remaining) means the risk of sulfation is extremely low provided you charge per manufacturer's criteria.



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Thanks you snoobler.

My battery monitor with my personal settings (tail current 1.3%\charging voltage 13.5v/charging time 10 minutes, charging efficiency 95%) or (tail current 2%/charging voltage 14.2v/charging time 10 minutes) indicates 96/98% wehn the mppt controller drop below 2% current during abs phase.

In these moment jump on 100% due to synchronization.


If it don't synchronize for 2/3 days due to cloud etc, it tends to accumulate 1-2% of error per day.

Error like delay respect to "real" state of charge.


With victronenergy default settings, the battery monitor anticipate the 100% A LOT, respect to mppt criteria and real soc.

Default settings in my case are totally wrong. (tail current 4%,charging voltage 13.2v).

A lead acid battery(especially wet battery with high internal resistance) that accepts 4% of current at 13.2v is NOT EVEN REMOTELY near to full charge.

Not with solar charge, not with 230v charger, not with a alternator.

I don't understand why is a default setting.



I apply strictly the victronenergy parameters for charging : abs 14.4/14.5v, floating 13.8v, charging current 0.05-0.1C, temp coefficient - 24mV/C°.

But victronenergy don't say anything about tail current for fully charge or abs time for its battery in datasheet.


I read that in float charging mode, a standard fully charged AGM flat plate have a tail current or ends current at floating voltage (13.8v)of 0.3mA/Ah.

These means 0.033 A or 33mA or 0.03% in my 110ah victronenergy aGm. At 13.8v and 25C°.

Is IMPOSSIBLE to reach in 8 hours of solar charge in cyclic use.

My battery reach max 0.8A of tail current at 13.8v at the end of a sun day in the better charging conditions. These is 24 times bigger than waht a fully charged agm in float charging should accept.

"float end current in a fully charged FIAMM AGM is 0.3mA/ah at 13.8v and 25C°"

surce : FIAMM AGM VRLA TECHNICAL MANUAL)

Means these that my battery is NEVER FULLY CHARGED, but ever 99% charged?

If every day, a 0.x% remain not recharged sulphate, and every day these sulphate add up to the one of the previous day, the capacity will decrease, very slowly, but will decrease.

Or I'm wrong?

snoobler avatar image
snoobler answered ·

You're wrong. Sulfation difference between 99% and 100% is nothing.


Batteries charged to 13.8V are not cycled. Using this to compare to your batteries is wrong.


You are applying non Victron resources to Victron. Stop.


You are obsessing about details that don't matter. Use this.


https://www.victronenergy.com/upload/documents/Datasheet-GEL-and-AGM-Batteries-EN.pdf


Stop confusing yourself. You are sorting grains of sand to find the perfect grain. Takes a lot of time, but means nothing in the end.

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Thanks, I know by heart the datasheet of the victronenergy vrla and apply it precisely.
I use different sources just for the missing data.
The tail current at the end of charging is not indicated in the victronenergy datasheet.
The Fiamm battery that I have shown as an example with an end-of-charge current of 0.3mA / ah is practically identical in construction to the victronenergy agm. Stop

Maybe I am obsessive in some aspects and waste a lot of time, but I like it and I assure you that being obsessive has allowed me to learn much more about batteries than some specialized technicians I find in motorhome and solar energy stores, who sometimes give advice without knowing in the least what they are talking about.
Scientific study is made up of precise quantities. Not "more or less, roughly, broadly" etc ...

Reading many sources on lead-acid batteries is not confusing ... Because the theory behind it is always the same.
Reading many different sources is the only way to learn a subject and the laws that govern that subject, for someone like me who has not done a specific school for that subject.
I'm like you, I have to know the details. Drives my wife nuts when I don't understand her because she's vague/not precise or says one thing, meaning another.

But it comes to a point, like with tail current, that you realise there are no exact answers, and even if there are good ones, it changes with the situation.

Al avatar image Al kevgermany ♦♦ ·

@kevgermany "I'm like you, I have to know the details. Drives my wife nuts when I don't understand her because she's vague/not precise or says one thing, meaning another."

I think we must be sharing the same wife, lol