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Duncan avatar image
Duncan asked

BMV-702 odd readings and correct settings

Hi

Over night I drew about 30Ahr from my battery bank and the BMV showed ~80% at the end of the night.

I would have expected the BMV to show 100% after the battery was fully charged - but no!

After a couple of hours of intermittent sun this morning, the BMV shows 100% while the MPPT charger is still in bulk mode (13.3V currently) delivering 200 watts when the sun is fully out. Based on history, the charger will not go into float until this afternoon.

The BMV reading is surely wrong and the battery status bogus. I thought that the BMV would show fully charged when the energy withdrawn overnight was replaced during the day (approximately).

I am confused. Can anyone help.

Ps, I have 240ahr bank and set the capacity to 120ahr in the BMV as I want zero capacity to show when the battery gets to half empty. Is this the correct thing to do?

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2 Answers
Justin Cook avatar image
Justin Cook answered ·

@Dunk O, without knowing the chemistry of your battery, the manner in which the shunt is connected, and the BMV settings, it's certainly premature to jump to conclusions about the accuracy of the BMV. That being said, a big clue is that, as you've mentioned, you have the battery capacity set incorrectly. Battery capacity should always be set at actual capacity, as specified in the user manual.

Please read and understand the user manual fully. After you have correctly set the battery capacity, and double-checked your connection of the shunt, please see Section 2.3 and Section 4.2.1 for the appropriate settings for your battery, and particularly see setting 16 to set your discharge floor. Factory default is already 50%.

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Duncan avatar image Duncan commented ·

Thanks very much. I will get onto it.

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Duncan avatar image Duncan commented ·

Setting 2: charged voltage - refers to section 3.7 of the manual, and for 12V batteries the setting is 13.2V (float). However, in section 2.3 BMV synchronises too early - absorption voltages are described 14.4V (absorption). These seem to be contradictory.
I would expect it setting has to be the absorption voltage minus 0.2V?

EDIT - Having just watched a Victron video on the BMV (https://youtu.be/4ReljfO3k6M) it seems that they differentiate between systems with, and systems without solar charging. Either I missed this in the manual, or it has not been well documented.


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Duncan avatar image Duncan Duncan commented ·

My system is: 2 parallel 120Ahr deep cycle AGM. I am using MPPT 75/15 and the BMV settings are based in the charging setting. The shunt is connected between the negative bus (includes inverter) and negative battery terminal of last battery in the pair. BMV settings are now:

1: 240 ahr

2: 14.5V (14.7-0.2)

3: 4% (4% of 240ahr bank = 9.6A, this seems way too high, I would expect less than 1-2A to indicate float charging with no specific loads on the battery. ???)(I am going to set this at 1.3%)

4: 3 min

5: 1.25

6: 95%

7: 0.1A

8: 3 min

16: 50% (this a relay cuttoff setting, how does it effect % capacity?)

When the battery is fully charged I will manually resynchronise today if these setting are correct?

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JohnC avatar image JohnC ♦ Duncan commented ·

Your 14.5V Charged V setting suggests that you want sync to happen while still in Absorb state. Premature? Once Float is achieved, that V will never be reached, so it won't sync at any tail setting.

The 1.3% tail would likely be appropriate (for me anyway), but not when CV is set so high. I'd set it below Float V so it would work at the appropriate time.

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Duncan avatar image
Duncan answered ·

Anyone care to comment further?

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Daniël Boekel (Victron Energy Staff) avatar image Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

Hi @Dunk O

@JohnC has given excellent advice, if you want more comments, please explain on what exactly. (explain what in the video contradicts, etc.)

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Duncan avatar image Duncan Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

The Victron video says for solar charging installations set BMV based on absorption voltage, but JohnC advises float voltage as I mention ( although I cannot see the full post here for some reason, full post can be found at https://community.victronenergy.com/questions/40410/bmv-702-readings.html?childToView=40550#comment-40550)

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