Inaccurate readings?

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mogal asked Apr 22, '21

Inaccurate readings?

Trying to understand and get a descent battery reading since I installed this BMV-712.

I added a RPi 3b+ with VenosOS for monitoring.

Attached is the latest readings from this week.

The numbers don't make sense to me.

I have two golf cart batteries (230Ah each) in series, so 12v with a useable 230 amps (Right?)

So if I have used 82 amps of the 230 as stated by the monitor shouldn't that be 64%, how does the monitor calculate 78%?

Is there a setting to mitigate this error? A setting I've missed?

Also, the (mostly) at rest voltage is 12.17 volts? Is this not considered a battery at roughly 50% charge?

(There are parasitic loads in the trailer, fridge, CO2 Dect. ect that I cannot remove)

I was hoping that this 712BMV help me determine the state of my battery, but I'm as confused as ever!

I just want to get the most out of the batteries without damaging them early.

Attached is the last week of use. battery started from full. Zero Cal was done with everything disconnected with the exception of the BMV itself.

BMV Battery Monitor SOC

battery.jpg (456.5 KiB)

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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

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Answer 1 · 2021-04-22T17:40:10Z

tozz answered · Apr 22, 2021 at 05:40 PM

> I have two golf cart batteries (230Ah each) in series, so 12v with a useable 230 amps (Right?)

Yes, if those 2 batteries are 6V batteries. Batteries in series increases voltage. Batteries in parallel increase Ah. So if you have 2 batteries in series at 12V each battery would be 6V

> So if I have used 82 amps of the 230 as stated by the monitor shouldn't that be 64%, how does the monitor calculate 78%?

I assume you mean 82 Ah (Amphours). So pulling 82 amps for an hour from your batteries. So yeah, roughly 64%.

You can't measure the capacity in a battery. Thus, a BMS does educated guesses based on parameters it can measure such as voltage, configurable parameters and (depending on your setup) how much current has been drawn through eg. a shunt.

Did you configure your BMV with the correct parameters? Stuff like battery capacity? And did you fully charge your batteries? A full charge is required!

Or.. in short.. have you read the manual?

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Matthias Lange - DE ♦ commented · Apr 22, 2021 at 05:52 PM

https://youtu.be/4ReljfO3k6M

https://youtu.be/mEN15Z_S4kE

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Answer 2 · 2021-04-22T20:36:55Z

mogal answered · Apr 22, 2021 at 08:36 PM

Hello, Yes I read the manuals, and watched the videos that Matthias posted. I'm not in range at the moment, but if I recall correctly, I have the settings as such:

Battery capacity: 230AH

Charged voltage: 13.1 (or 13.2?)

Tail current: 1%

Charged detection time: 3mins

Peukert at 1.25 as recommended by manufacturer for lead acid

Battery was fully charged at the start of the week.

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

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