# question

```I don't understand victronenergy's adaptive algorithm well.
Victronenergy says that the absorption time should be longer with a very discharged battery and shorter with a slightly discharged battery.
But why?
whatever the previous discharge, when the battery reaches the absorption voltage, eg 14.4v, it means that it is at 80% charged.

Case 1: 100 Ah 0% battery, charged with 10 A charger. After 8 hours of bulk, it will be around 80%. And the absorption begins.

Case 2: 100 Ah 60% battery, charged with 10 A charger. After 2 hours of bulk it will be about 80%, so as in case 1. And absorption begins.

Why, according to the victronenergy algorithm, must the absorption of case 1 last 8 hours, while the absorption of case 2 must last 2 hours?
Why,if both batteries are about 80% charged when absorption starts?

According to electrical physics, providing a constant current of 10 amperes, any battery, at any level of discharge, will have the same level of charge when the absorption voltage is reached. That is about 80%.```

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@Francesco

Battery charging is a pretty complicated subject. I hear what you are saying about electrical physics but it is not as linear or as simple as that, in battery charging there are many factors that affect a battery's ability to accept charge. And as a rule 10AH discharge takes way more than 10AH charge to replace. As a battery is charging its internal resistance changes, and also its ability to accept the charge. There is so much information on particularly lead acid as adaptive charging is mainly applied in that chemistry. The article linked explains a bit more about charging batteries.

In short, the absorption phase is more about reversing the chemistry in the battery and obviously the more reaction the battery has had, the longer it takes to reverse it. The deeper the discharge the more sulfation of the plates has occurred and this is ultimately what renders them useless as it prevents chemical reactions from happening.

Alexandra

thankyou for the reply,and for he article. I agree with you for sulfatation, internal resistance etc....

But a question: its true that in the moment the battery reach the absorbtion voltage is ever at the same state of charge, either if its being discharge a little,or a lot??

i see varius charging diagram of lead acid battery,in witch the absorbtion fase last the same, with a 50% DOD than with a 100%DOD, to reach the 100% SOC.

Meanwhile what is changing is the time of bulk phase.

i have charged 70% dod battery and 20% dod battery, both with 2hr of absorbtion time, and in both case were on 13.1 v after 24 h open circuit. These means they were compleatly fully charged...Corrrect??

Can you give me your opinion also in these my other post, please??

https://community.victronenergy.com/questions/84568/new-victron-agm-battery-accurate-solar-charging-pa.html?childToView=84676#comment-84676

francesco

@Francesco

A battery is fully charged when it has reached the point when all it is taking is the energy it needs to float.

Since battery charging is not linear the 80% is more of a gauge as to whether your battery is at the target voltage. The next 20% is aimed at getting a fuller reversal of chemistry.

With each cycle your battery is technically loosing capacity. So is it at the same capacity or charge - no it wont be. It should be at the correct temperature compensated voltage though. Unless there are issues.

Absorption phase on solar is also affected by your solar day and voltage your battery and pv started up at.

Again I repeat battery charging is quite complicated. Especially if you are using a variable input like solar.

If your battery did not get the full absorption time it needed the day before then in theory you were not fully charged starting your next cycle during the eveing downwards. Also if you "mini cycled" the batteey during the day (drawing from it if the sun went behind a cloud) before it reached its full absorption you have also messed with the charge.

Alexandra

Thank for reply, i now waht you say,

Dont warry,

imagine that when i do my tests and report the data here i am very scrupulous: 1) the charge controller mppt always supplies more power than required. 2) I have been collecting data almost every day in various situations for 3 years. I also have a logger that records all solar data constantly every 10 minutes and can generate graphs.

I found this very interesting answer on a forum to roughly set a correct absorption time:

"""If you cannot get the information from the battery manufacturer,2 hours is a typical absorption time. Start with this and see what current is going into the battery ( not the total current becase some will be suppling loads) at the end of this time, just before it drops back to float. It should be about 2% of the battery capacity in amp hours. If the current is more increase the absorption time and vice versa. The absorption time is not as critical as the bulk and float voltages. So generally 2 hours will be near enough""

I did some tests with my new victron deep cycle, always in extreme insolation conditions (20A available for charge and 0 load):

-Always with a discharge of 20% dod (about 20 ah).

-battery at 12.65v / 12.69v open circuit at the start of the recharge

-data collected with victron smartshunt and mppt controller data logger.

It result that :

- after 1 hour of absorption at 14.4v the current absorbed was around 2.3A -After 1.5 hours of absorption, the current absorbed was around 1.9 amperes.

- after 2 or 3 hours of absorption the absorbed current remained around 1.5 / 1.8 amperes. - at the beginning of the floating at 13.7v, for a few minutes, the current was 0A, since the battery had a higher voltage than the charge controller !!! (= fully charged)

- after that it stabilized at 0.8A at 13.7v - after 4 hours of floating it was reduced to 0.3A / 0.4A.

I notice that - the victronenergy regulators interrupt the absorption with tail current <2A or 2% of capacity. - victronenergy regulators set maximum absorption at 1 hour with battery at> 12.6 v.

I concluded that to treat this new victron deepcycle well, under my conditions of use (daily discharge 10% max 20%, and always good weather and sunshine for about 20 A available from the charge controller during the entire recharge), I will use an absorption of 1 hour if dod <= 15%, 1.5 hours if dod = 20%. at 14.4 v. once every 28 days I will absorb 2 hours at 14.6v.

In all cases I interrupt the absorption when the current drops below 2A at 14.4V.

I have the strong fear of overcharging .... while a slight and occasional undercharge would be restored at the latest in 2 days, as I live in conditions in which the PV supplies every day 99% of the time from 3 to 5 times the energy consumed by the battery (1200/2000 wh potential vs 200 / 400wh discharged from the battery in the evening).

Does that seem correct to you?

francesco

@Francesco

It seems we have the opposite fears, I am always worried about undercharging as it is the worst offender for battery damage on lead based.

If you are happy that your battery is getting into the float stage and are satisfied that you are fully charging the battery then it really is up to you what you do after that.

I know plenty of guys who just leave their batteries connected to the Victron chargers and have not had issues or shortened life of their batteries. If the settings are within manufacturer recommendations then you are fine.

Alexandra
```thanks for your opinions.
Undercharging is certainly the worst cause of damage.
Anyway, victron, and the book “energy unlimited” say that AGM and GEL Vrla battery bad tolerate overchrging because of dryng of electrolite and the positive plate corrosion.
However I live in the Canary Islands in an RV with a 300 watt panel.
this produces a minimum of 900/1000 wh per day in December January, a maximum of 2000 wh per day the rest of the year.
My battery is discharged by 120/250 wh at most.
In addition, 99% of the day, it remains floating without any withdrawal until 06.00 pm or 8.00 pm.
I think that any possible undercharging (rainy day, or exceptional DOD discharge)will be restored in the following 2/3 days.
The last agm (probably of poorer quality than the new Victron deep cycle) lasted just over 3 years, that is about 1000 cycles with an average Dod of 15%.
With this I would like to reach the 1500/1800 cycles with the same dod.

With the algorithm I have decided (1 hour or 1.5 hours of absorption 14.4v, with tc -24mV / C, floating 7/10 hours at 13.7 v), the battery is at 13.18 v after 24 hours in open circuit .
Can I think what that means it is fully charged?

I add that the smartshunt, now that I have been able to configure it correctly with the parameters of the victronenergy battery data sheet (1.14 peukert coefficient, 96% charging efficiency), WORKS IN PHENOMENAL AND INCREDIBLY PRECISE MODE !!!
The SOC in discharge is very precise! And the Ah and SOC during charging, coincide and are synchronized exactly with the data provided by the MPPT controller Epever Tracer (despite being from another brand, this means that both instruments give very precise readings)! I am really satisfied.```

There are lots of variables but Alexandra is also correct,

how old are the batteries

whats the SG of all the cells

whats the Temperature.

different chemistry's

how fast they are charged

the list is long and varied between brands as well,

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The article say:

"" "full charge is reached when the current decreases to 3–5 percent of the Ah rating. A battery with high leakage may never attain this low saturation current, and a plateau timer takes over to end the charge" ""

My new victron 110ah, with 15/ 20% DOD(12.6 open circuit voltage), absorb 2A at the ond of 1hr of absorbtion at 14.4v. After, the current drop to 0.8/ 1A at the beginning of floating at 13.8v. And after 4hr of floating decrease at 0.4 A.

```If I try to extend the absorption time to 3hr (maximum allowed by my mppt controller), the current does not decrease less than 1.5 / 2 A.
In my opinion it means that it is fully charged with 1 hr of absorption, and with 3 hr I am overloading it by generating only gas.
Other than that, I measured the open circuit voltage after 24 hours of recharging, and it is 13.14v.
Both with 1 hour of absorption, both With 2 hours of absorption, and with 3 hours of absorption.
What should I do?```

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