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vicky avatar image
vicky asked

BMV-712 weird readings and calculations ?

Bought a BMV-712 as an enhancement for my Smart 100/50. My intention is to get a better view on the SoC and remaining capacity of my solar batteries of my cabin. As the 100/50 is very far away in a remote location I prefer to test things prior to go there. So here is what I did.
I took an older 12V / 42A wet battery and connected the BMV-712 and shunt. Configured the 42Ah in the app. Then I charged the battery for 24 hours to be 100% sure it is @100% SoC, charger displayed "float". Then I set the Soc=100% in the app.
Next I connected two 12V/21W bulbs to discharge the battery.

I expected to see correct values for decreasing SoC and remaining battery time after a while.

However the values I see after a few minutes are:
9.87V @ 3A (battery seems to be pretty done), but instead of seeing a low SoC, the SoC still is at 94.6% ! I would expect almost zero when the battery is at 9.87V.
Am I wrong with my setup and expectations? Do I need to repeat some steps to allow the BMV to learn?



BMV Battery Monitor
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3 Answers
Alexandra avatar image
Alexandra answered ·
@Vicky

The two possibilities are:-

1. The battery was not fully charged when you began the test. And the bmv began sync at 100%. This can be corrected by putting it through a charge cycle and if your tail current is correct it will know when it is charged again.

2. The capacity of the battery you programmed in is its when new capacity. The battery you tested on is not new, probably sulphated.

Batteries lose capacity over time. So the 42Ah is no longer the rating for your older battery. It is now whatever was discharged out (check it up in history) if you were fully charged when you began.

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kevgermany avatar image
kevgermany answered ·

To add to Alexandra's explanation.

BMV calculates state of charge by measuring current flow in and out of the battery. Not the voltage. So current based readings based on the initial 100% reading would give a small amount of discharge calculated, which would be correct.

When you charge the battery, the BMV automatically resynchronizes the 100% value once battery is full. This is based on a combination of volts and what's called tail current - the small current passed by a proper charger once the battery is full. Time, charge efficiency and other factors also taken into account.

You can set/adjust these parameters, it's documented in the manual. You might want to, especially if your main charge is solar and you get a lot of cloud interfering with the charge rate.

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vicky avatar image
vicky answered ·

Thank you both for the helpful explanations., makes things much clearer.

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