Mid-point voltage deviations occasionally when discharging, but not always?

Question

question

vulture asked Feb 16, '21

Mid-point voltage deviations occasionally when discharging, but not always?

I have 4 12v AGMs wired in parallel/series for 24v bank with their midpoint monitored from my BMV-712. In the past month or so, once or twice a week I get a midpoint voltage alarm when I discharge below 74%.

The batteries are less than a year old and I think I've taken care of them pretty well, never getting them below 60%. It's very weird that the midpoint voltage doesn't deviate /every/ time my bank is discharged, only some times. For example, in that graph the midpoint voltage was totally fine on the deepest discharge on february 12. In addition, I am sampling from one of the two series midpoints, and it is not linked to the other one because I read here that it could be bad if one of the batteries totally fails. Should I try connecting the two midpoints? Or is there anything else that might be causing his behavior? The batteries never get below 45-degrees.

BMV Battery Monitor AGM Battery

captuewrfre.png (149.1 KiB)

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Modifications Space

Node Red Space

Answer 1 · 2021-02-16T10:37:57Z

JohnC answered · Feb 16, 2021 at 10:37 AM

Hi @vulture

I can see 2 occasions on your graphs when that happens. Both times the V's have gone through-the-floor. They're essentially flat.

So the BMV is misrepresenting that, but why? My first suspicion is that you may have fed the total of your batt Ah ratings into the BMV without halving it for 24V. So your SOC is showing double what it really is, and to continue on like this will harm your batts.

If that's not it you'll need to provide more detail on your system and how it's set up. For now, don't believe the SOC.

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Answer 2 · 2021-03-11T20:55:21Z

neoneddy answered · Mar 11, 2021 at 08:55 PM

What @JohnC Said is correct.

SOC with any battery chemistry is a best guess and needs to be sanity checked from time to time.

I have a 10 12v agm batteries in 24v configuration, so I've been around the block on these.

I see a few things that give me concern:

I'm not seeing flat spots on the voltage graph. - AGM batteries need to have some absorbion time, ideally 2-6 hours depending on state of charge from the previous night.
Related to 1, you're not hitting 100%, not even close. I'm guessing your BMV is syncing to 100% when a voltage is hit, you need to set that charged voltage to .2v higher than absorb voltage, otherwise anytime you hit absorb the BMV will think you're at 100%, when you're really at 80%.
Solar chargers also don't seem like they are in bulk / absorb long, look into the battery settings there.

Here is what I'd do, as soon as possible get these batteries on a charger and get them soaking at 14.4v or higher (per 12v battery) for a good 8 hours. Then, disconnect any chargers and run them down until low voltage hits, 11v or 10.8v. Look at what the BMV shows for amps drawn, that is your new capacity, set it on the BMV and go from there.

Attached are my Settings for the Solar chargers and BMV

1615495990521.png (170.8 KiB)

1615496041253.png (179.5 KiB)

2 |3000

Attachments: Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

question