Understanding what "BATTERY 100%" really means - 100% yet still shut down low voltage

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panda-north asked Oct 1, '21

Understanding what "BATTERY 100%" really means - 100% yet still shut down low voltage

Hi All.

i have an all Victron off-grid system

48-5000-120-100-120V Inverter

MPPT-150 charger

Cerbo GX + Display

Victron DC shunt

(3) SimpliPhi 3.8 batteries @ 48V

I posess a basic knowledge set regarding electricity and such. i "designed" and installed this system myself - and it has performed flawlessly and to the specs which i targeted ...

QUESTION: What does "100%" mean ?

Can anyone explain (or point me to an article or treatise) that explains what the Cerbo Display unit is telling me as it relates to the "ACTUAL" status of my battery supply -

this morning i awoke early - just in time to hear a click and my entire system switched off/went dark...

(i subsequently figured out that my well pump had malfunctioned and run all night - fixed that problem promptly)

hmmmm. unplugged from my Victron system and dusted off my gen set so i could muddle through breakfast

as sun crested the horizon my Victron system started to "wake up"

one it had awoken - the display Showed 43.7 V (low) yet it also said "100%" in big happy numbers...

As i sit here and type this it is happily recharging, Voltage back up to 52.1V and its packin in 20A from the MPPT.

I can be awfully stupid - but i always remind people that i'm not as dumb as i look -

In the past i recall the %percentage%" battery level goin up and down with loads and charging -

BUT

how am i to understand batteries that are basically depleted low voltage (down to 43%) yet my fancy Cerbo Display is showing me an unwavering "100%".

What am i missing ? is there a way to understand this or is the display or shunt borked?

i would really like to be able to "trust the display" when it proclaims 78% that i actually have 78% of my usable Ah avail for all my fancy off grid amenities...

any thoughts or redirects to a knowledge base would be greatly appreciated

thx

/Steve (Panda_North Photon Collective)

battery capacity

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Answer 1 · 2021-10-01T16:51:08Z

Alexandra answered · Oct 01, 2021 at 04:51 PM

@Panda_North

Sometimes it syncronises to 100% when it is definitely not charged. The setting i have marked also causes this issue.

https://www.victronenergy.com/media/pg/SmartShunt/en/troubleshooting.html#UUID-46bfbd40-f8e2-0d29-43b6-5203ee31c6a4

screenshot-20211001-185011.jpg (240.6 KiB)

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kevgermany ♦♦ commented · Oct 02, 2021 at 09:47 PM

And system will automatically reset once batteries are really at 100%

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panda-north commented · Oct 06, 2021 at 05:04 PM

thx for your input - makes sense now -

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Answer 2 · 2021-10-03T01:02:06Z

Paul B answered · Oct 03, 2021 at 01:02 AM

Here is a Good video to look at

https://www.youtube.com/watch?v=mEN15Z_S4kE

I ALWAYS take voltage to be my primary check against SOC ie on a 12 volt system with LiFePo4 batteries then 14.2 volts, means you are at 100% full yes there are some finer points to look at, but basically its full

there are sheets out there that give you the SOC by voltage, but NOTE if you use these the readings must be taken at NO load and NO charge.

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panda-north commented · Oct 06, 2021 at 05:03 PM

Thx very much for your post and link to Youtube video. Watched it several times and am feeling much more confident about how things work and why my system displayed such crazy data

Thx again for contributing and helpin' me out.

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Answer 3 · 2021-10-03T05:28:42Z

Kevin Windrem answered · Oct 03, 2021 at 05:28 AM

Some explanation of how the Victron battery monitors (including SmartShunt) work.

The basic mechanism is to count energy in and out of the battery. Basically, the current in and out of the battery is accumulated and compared against the programmed capacity in amp-hours for the battery bank. Assuming the battery is fully charged and a capacity of 100 AH, SOC will calculate to 50% after 50 amps for 1 hour or 10 amps for 5 hours. Then if the battery begins receiving 10 amps, the battery will read 60% SOC after an hour. There is other math applied to account for nonlinear capacity with current and for charge vs discharge inefficiency.

When the battery is being charged, the voltage will rise and eventually the current will decrease. When a voltage threshold AND current is below the "tail" value, the monitor resets it's state of charge to 100%. This generally works, however if charging from solar, a cloud can fool the battery monitor into thinking the tail current has been reached.

Alexandra said, the battery monitor can also be set to reset SOC to 100% when it powers up. It's probalby a bad idea to set do this unless you know the battery will always be at 100% when the monitor powers up. This can also result in a bogus 100% SOC indication if for example, the battery protect disconnects everything including the battery monitor when voltage reaches the cutoff threshold.

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