What happens when BMV712 sees charging beyond 100% SOC?

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Tomasz Czernecki asked Apr 26, '21

What happens when BMV712 sees charging beyond 100% SOC?

I have a BMV712 + MPPT 75/15 in my new RV, and I've been collecting data through various conditions. After a lot of analysis, I'm convinced Johannes's video from earlier this year is providing the 'best' settings for my solar scenario, where I'm setting Charged Voltage = 14.2V (14.4V absorption minus 0.2V) and tail current around 5%. The downside is that we will synchronize when we're at approximately 95% actual SOC, and if we charge exclusively via shore power (which has a lower 13.8V absorption voltage), we will never synchronize. Setting lower tail current is not an option because we don't always stay in absorption mode long enough to see this lower current, and we don't want to set a lower charged voltage + lower tail current in float mode because clouds could result in a false synchronization.

So on to my question: when we synchronize, whether it's manually or automatically, we often will do so at a level slightly below the actual 100% SOC. What happens if we continue charging (regardless of whether it's in absorption or float) beyond this point? Specifically, will the BMV start counting down from 100% SOC after that additional charge is discharged? Or will it ignore that additional charge and just start counting down as soon as it sees any net discharge on the battery?

Here's an example:

- 200Ah battery, charged to 95% (190Ah), and synchronization event happens, setting the BMV to 100% SOC.

- Since we're not 'truly' at 100% (but rather 95%), an additional 10Ah of charge continues to be applied before true float/maintenance state reached.

- After that point, we apply a load that consumes a net 5Ah power off the battery.

What's the BMV going to indicate?

- Will it internally calculate that we're at 100% + 5Ah capacity (effectively, 102.5%) and keep displaying 100% SOC until we've discharged more than the 10Ah "extra" we added post-sync?

- Or will it ignore that extra 10Ah of additional charge and just show that we're now at 100% - 5Ah = 97.5% SOC?

In simplified terms, when does the BMV start counting down from 100%, and how does the BMV handle additional charge beyond what it thinks is 100% SOC?

My guess is that anything above assumed 100% SOC is ignored/clipped, but the engineer in me wishes there was a way to better reflect 'true' SOC rather than jumping to 100% SOC when we're in reality only around 95%. The quest for accuracy :)

Thanks!

Tom

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Answer 1 · 2021-04-26T15:04:39Z

rslifkin answered · Apr 26, 2021 at 03:04 PM

From my understanding, anything beyond 100% gets "clipped", as you described it.

To avoid early sync, I've got my settings (with AGM batteries) as follows. 13.2 volts, 0.5%, 15 minute sync time. So it'll only sync after dropping to float and accepting less than 0.5% (just over 2 amps in my case) for 15 minutes. Which means it usually syncs 15 - 30 minutes after dropping to float, at which point it's typically calculating somewhere between 99 and 100% anyway based on amps returned. I figure I'd rather have it sync late than early for best accuracy.

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Tomasz Czernecki commented · Apr 26, 2021 at 06:50 PM

Thanks for the answer - my issue with those particular settings is I've personally recorded (under approx 80% SOC) that while it's sunny I'm pulling 7-8A into the battery (162 watt panel array), but once it goes cloudy I'm only pulling 0.9A (less than 0.5% tail), but still at 13.6V (higher than your 13.2V charge voltage). This 'cloudy' state would result in a premature sync event (with 13.2V charge / 0.5% tail settings), even with an extended measurement time (eg. 15 mins vs. default 3 min).

If your case is non-solar and shore-power/converter-only config, your numbers would definitely work, but if using solar, I would get false syncs @ 13.2V.. Johannes explains this in the following video (12:53 mark): https://www.youtube.com/watch?v=mEN15Z_S4kE and the scenario he goes through is exactly what I saw on my setup..

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Matthias Lange - DE ♦ Tomasz Czernecki commented · Apr 26, 2021 at 07:06 PM

Yes with small PV arrays it can be tricky to find the right settings.

Higher charged voltage, lower tail current, longer detection time.

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rslifkin Tomasz Czernecki commented · Apr 26, 2021 at 07:35 PM

I'm using both shore power and solar depending on the day (my setup is on a boat). But I've found that it's unlikely for me to end up in the window of being at or above 13.2 volts, charging, but at less than 2.1 amps, and stay in that window for 15 minutes continuously. If it does happen, the batteries would have to already be pretty close to full.

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