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Fra avatar image
Fra asked

Victronenergy Agm deep cycle voltage drop after 7 months

Hello, I repeat a question that I have already asked in a simpler way, because I did not get an answer.

I have a Victronenergy AGm deep cycle 110ah for 7/8 months.
Since November I have noticed a drop in ocv (OPEN CIRCUIT VOLTAGE) of 0.07 / 0.08v for each discharge.

For instance:
Previously with a discharge of - 6ah it had ocv 13.00 / 12.97v,
Now 12.93 / 12.94v.

 before with discharge of - 12ah it had ocv 12.88 / 12.91v
Now it has 12.80 / 12.83v.

Previously with a discharge of - 20ah it had 12.73 / 12.75v ocv.
Now 12.66 / 12.68v.

This decline happened in November. For the first 6 months in the summer, the ocv remained constant.
The battery is recharged every day to 100% with the parameters of the victronenergy datasheet (abs voltage 14.4 / 14.5v, float 13.8v, abs time 1-3 hours, temperature coefficient - 24mV / C °).

The only thing that has changed is the temperature. Before it was on average 27/30 C °.
From November on average it is 20 / 22C °.

For more detailed information on the conditions of use of the battery read here:
https://community.victronenergy.com/questions/111143/victronenergy-agm-deep-cycle-performance-and-boost.html

Is this voltage drop normal? Is it the temperature? Or is the battery sulfating?

Thanks
MPPT ControllersAGM Battery
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2 Answers
Paul B avatar image
Paul B answered ·

A normal full gel ocv is around 12.7 to 12.8 vfor 100% SOC and a temp drop would also affect it a lot as well. the below is a average SOC Voltage chart at 25 deg C


1639060940465.png



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Fra avatar image Fra commented ·
Thank you. My agm victronenergy at 100% was 13.20v ocv after 24 hours unplugged, wehn was new.

Now is 13.10v.

Maybe is temperature. Maybe it stabilize at a slightly lower ocv with use.

I now that the first 2/3 months of use, capacity increase slightly and then stabilize.


Are my battery performance normal for you?

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snoobler avatar image
snoobler answered ·

Temperature.


Capacity is defined at 25°C. Warmer batteries have more capacity. Colder batteries have less capacity.


-X Ah @ 27/30°C is a different % DoD than at 20/22°C


The -24mV/°C is for charge voltage temperature compensation, i.e., 14.4V is defined at 25°C. At lower temps, absorption voltage needs to be HIGHER than 14.4V to get fully charged. Above 25°C voltage needs to be lower.


Have you programmed your charger with this temperature compensation value?


Example only, not necessarily absolutely accurate:


A 6Ah discharge BELOW 25°C might be 22% DoD while it's 18% DoD at 30°C.


Thus the greater DoD in cold would result in a lower resting voltage.


The smartshunt allows setting a parameter to adjust capacity based on temperature in Misc:


1639064784677.png


My particular FLA batteries are -1.1%/°C, but that varies with manufacturer and type.


1639064784677.png (48.0 KiB)
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Fra avatar image Fra commented ·

Halló, thanks you. Yes I programmed my mppt charger exactly with datasheet victronenergy parameters.

Abs 14.4v/14.5v. Temp coefficient - 24mv/C. Float 13.8v. Abs time from 1 to 2 hours depending of dod. My average dod is 15/20% daily.

One time per month I make 3 hours abs at 14.6v (a slightly equalization becouse is a vrla battery). Is unnecessary or dangerous?

You say:"X Ah @ 27/30°C is a different % DoD than at 20/22°C"

I agree with you, but I think these not justified a 0.08v drop in ocv.

A loss of 4% of capacity from 30 to 20 C Celsius, is not a 0.08v drop in ocv with same Ah discharge. It would be less.

I'm wrong?

I now all waht you say about cold and internal resistance and capacity drop. Thank you for your reply.

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