BMV 700 SOC calculation

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steffenbrand2607 asked Oct 25, '23

BMV 700 SOC calculation

Dear everybody,

I am not quite sure whether my BMV 700 is calculating the SOC in the right way.

I have a 24 V system with two 150 Ah AGM Batteries in series. Fully charged when reaching 27.60 V (each 13.8 V).

During the night the batteries are discharged and and often reach a voltage level around 24.3 to 24.0 V. Then, the BMV 700 says the the SOC is 90 % respectiveley 86 %, see pictures below:

For me, 12.05 V are surely not 86% of the SOC as this surely needs to be quite lower as the chart (from the internet) shows:

What is wrong here? Can anybody help? Please also see my settings for the BMV 700:

Thanks a lot

Steffen

BMV Battery Monitor SOC

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Answer 1 · 2023-10-25T07:01:49Z

pwfarnell answered · Oct 25, 2023 at 07:01 AM

Your BMV may be synchronizing to 100% before they are actually full. What are you charging them with (solar or grid power) and what absorption voltage. You need to be charging then at something like 28.8V not 27.6V. In the BMV set the charged voltage to 28.4V and the tail current to 2% or less, I use 0.8% for my AGM batteries. You may have been undercharging your batteries.

The other check is to make sure your shunt is wired correctly so it is counting all loads. There MUST be only connection on the battery negative and that is the shunt and that goes to the battery side of the shunt. There MUST be only the battery connection on the battery side of the shunt. All other connections including, all loads, inverters, all chargers, system ground and on boats or vehicles connection to the chassis or hull MUST go to the system/load side of the shunt.

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Answer 2 · 2023-10-25T13:50:27Z

steffenbrand2607 answered · Oct 25, 2023 at 01:50 PM

@pwfarnell, Thank you for your answer. I only charge them with solar panels (2x405 Wp). Absorption voltage in the MPPT100|50 is 28.8 V, float is 27.6 V please see here:

I changed the settings in the BMV to the following values:

The installation was made by another electrician together with a Victron Energy retailer, so I assume that the shunt was connected properly. I will try whether the changed settings will have an effect on the SOC calculation and will let you know.

Thanks again and have a great evening.

Steffen

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kevgermany ♦♦ commented · Oct 25, 2023 at 06:10 PM

As it's solar, you might want to increase the absorption time. 6 hours is ok when sunny, but not in extended cloudy conditions when you have very low charge rates.
Definitely increase the charge detection time, 3 mins is too low for solar. 10-15 mins would be better. 15 mins works well for me in Bavaria.

Es gibt auch 'n Deutschsprachige Bereich.

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Answer 3 · 2023-10-25T18:40:03Z

steffenbrand2607 answered · Oct 25, 2023 at 06:40 PM

@kevgermany , thank you. To which time shall I increase the absorption time? I live here in Bavaria too and wheather is quite cloudy/rainy these days.

I will increase the charge detection time to 15 mins.

Thanks a lot.

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kevgermany ♦♦ commented · Oct 26, 2023 at 05:50 AM

For summer, 6hrs ok, for the rest of the year disable.

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