BMV700 - SoC drifting off in a BYD setup

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mariowagner asked Nov 27, '18

BMV700 - SoC drifting off in a BYD setup

Hi,

sorry to bug you again. But there is an issue related to the BMV700 I can’t figure out. We use BYD B-Boxes with 7.5kWh, Multiplus 48/3000, MPPT 250/85 and a BMV700.

But for some reason at some sites the shown SoC suddenly drifts of, showing way too high %-values, whilst the Ah-Counter is about right all the time.

e.g:

site 1 showing: -76 Ah used of 150Ah -> 52,8 %

site 2 showing: -106 Ah used of 150Ah -> 52,9 %

My BMV settings (on v3.08):

01: 150Ah

02: 54.8V

03: 6%

04: 10 min

05: 1.05

On sites with Victron LiFePO4 batts i've never seen this drift so far.

In the meantime I solved the problem by calculating the SoC myself via modbus extracted data, but I still don’t know what causes the BMV behaviour. Anyone any idea?

BYD bmv 700

soc1.jpg (52.7 KiB)

soc2.jpg (55.0 KiB)

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Guy Stewart (Victron Community Manager) ♦♦ commented · Nov 27, 2018 at 11:57 PM

Are these sites regularly (ie daily) reaching the 100% recharge calibration settings you have set?

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mariowagner Guy Stewart (Victron Community Manager) ♦♦ commented · Nov 28, 2018 at 12:53 AM

hello guy. no. currently those batts are fully recharged every 10 - 25 days. dependent on solar conditions. we got winter time

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Modifications Space

Node Red Space

Answer 1 · 2018-11-28T22:52:15Z

Guy Stewart (Victron Community Manager) answered · Nov 28, 2018 at 10:52 PM

I have contacted you via email for this specific situation.

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mariowagner commented · Nov 29, 2018 at 10:34 AM

thx! i've set the system to english :-)

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mariowagner commented · Dec 11, 2018 at 10:27 AM

hi. Sorry to bother again. I see that u must be swamped with questions ... :-) Have u had time to look at my installations?

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Answer 2 · 2018-11-29T05:38:08Z

mvader (Victron Energy) answered · Nov 29, 2018 at 05:38 AM

Hi, from what you say I thinks it’s very unlikely, for both those values (Ah and SOC) to come from the BMV.

More likely you are looking at Ah from the BMV and then SOC of the BYD.

I recommend to verify that first: don’t look at the aggregate (and especially not in the soon-to-be-replaced-with-something-better VRM-app), but look on VRM and then download the excel sheet and check the columns pertaining to the BMV.

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Answer 3 · 2018-11-29T10:16:21Z

mariowagner answered · Nov 29, 2018 at 10:16 AM

hi. thx for your effort!!!

@mvader: no its not the BYD SoC. BMV 700 doesnt show the correct value. see pics attached. BMV is set as the SoC monitoring unit. XLS shows incorrect SoC valued for instance 260 = BMV.

soc-pr1.jpg (24.7 KiB)

soc-pr2.jpg (18.0 KiB)

soc-pr3.jpg (19.3 KiB)

soc-pr4.jpg (23.0 KiB)

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Answer 4 · 2018-11-29T11:20:42Z

mariowagner answered · Nov 29, 2018 at 11:20 AM

history of the BMV shows strange values. -201 Ah is impossible.

are those values taken into consideration when the BMV calculates ists SOC?

soc-pr5.jpg (19.7 KiB)

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boekel ♦ commented · Dec 11, 2018 at 06:06 PM

What setting do you have in the BMV?

Peukert factor should probably be about 1.00

next suspect:

06. Charge Efficiency Factor - standard is 95%, so when not synchronizing (charged voltage / tail current settings) the SOC will drift away because the Lithium batteries are more efficient. setting might have to be 98-100% (this is not total efficiency, only Ah in-out)

It will actually help to not have the BMV to equalise at first, so you can get these settings dialed in correctly. (And maybe share the findings here).

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Answer 5 · 2018-12-11T11:03:55Z

Guy Stewart (Victron Community Manager) answered · Dec 11, 2018 at 11:03 AM

I haven't looked closely here yet, but perhaps a consequence of an incorrect Peukert co-efficient/exponent setting for that battery type.

If the current drawn is very low, the battery will appear to have more usable capacity.

If the current drawn is very high, it will have less.

The Ah value will be the same, but 'capacity' will be different relative to the rate at which it was charged or discharged.

This is quite true for lead acid batteries (and the default configuration). However in batteries with very low internal resistance (like your lithium) and very different charge or discharge profiles, you might get a distortion like you are setting in the readings.

Normally, when batteries get a full recharge regularly, this is not a major issue. But if they go a long time without a 'recalibration' at 100%, then this miscount will add up.

This is only one possible explanation. Try setting the BMV battery setting for Peukert exponent to 1 and see if that behaves closer to your expectation.

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