SOC way to optimistic

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Anders asked Dec 12, '20

SOC way to optimistic

I have a challenge with my BMV-700 SOC. It displays to high SOC.

I have a 24V battery made of 12 x 2V 1100Ah AGM cells.They are 10 years old, so they are most likely not any longer 1100Ah of capasity, so I have downgreaded them to 1000Ah in the BMV settings. However that is not the problem (I think)

For the last six days the consumption has been net 8.7Kwh (Very little sun in Norway right now, but we do get a white Xmas)

The SOC of charge however only drops from 99% to 86.5% indicating a consumption of 12,% of (24Vx1000Ah) = 2.88Kwh - that's clearly incorrect.

What do I do wrong?

Snapshot right now: 13% <> 250Ah - Does the BMV think I have a 12V system?

BMV Battery Monitor SOC

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Alexandra ♦ commented · Dec 12, 2020 at 09:23 AM

@Anders

The peukert exponent may be to blame, what is it set to?

If the bmv is recent installed then it will be a synchronisation issue. May correct itself when you next fully charge up.

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Answer 1 · 2020-12-12T09:40:58Z

Anders answered · Dec 12, 2020 at 09:40 AM

Peukert is set to 1.25

Charge efficiency 90%

SOC was syncronised 100% last Saturday has been operational for many years

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Answer 2 · 2020-12-12T10:02:08Z

Paul B answered · Dec 12, 2020 at 10:02 AM

If they are 10 years old then they are probably under 50% of the orginal size, you would need to work out there real available capacity. you should be able to work this out by using voltage.

here is a ruff graph of voltage verse capacity Note when you measure make sure the batteries have no draw and no charge for at least 1 hr

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Anders commented · Dec 12, 2020 at 04:59 PM

I am not so concerned about the actual capacity. I can draw 9Kwh and Voltage is still over 24V - That indicates the actual battery capacity is the neighborhood of 15Kwh, that's OK

What is rather strange is that a 9Kwh draw is calculated to 12,5%, that is clearly wrong

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Fra commented · Dec 12, 2020 at 06:06 PM

This graph is curious.

The voltages for each percentage seem very low.

all the graphs that I have seen of the manufacturers of lead batteries (Yuasaa, Fiamm etc) indicate 12.9 / 12.96 for 100%.

12.6 for 80%.

12.2 / 12.3 for 50%.

And so on.

I am not very convinced by that graph you put.

Although perhaps your table is more realistic.

My two agm batteries in parallel 100ah + 90ah on the camper, coincide more with your table than with the tables of fiamm and yuasaa.

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