Voltage difference MultiPlus-II and BMV-712 causing early low voltage cut off

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ngrms asked Jan 12, '23

Voltage difference MultiPlus-II and BMV-712 causing early low voltage cut off

It seems like the voltage measured by the BMV-712 and Multiplus-II are different:

When discharging, the BMV-712 reports a higher voltage than the Multiplus.
When charging, the BMV-712 reports a lower voltage than the Multiplus

This also results in the Multiplus-II (VE.Bus System) cutting power too early, as it thinks the SoC is dropping below 50%, while the BMV-712 is reporting a higher voltage and a SoC which seems to map on the Ah consumed vs the capacity of the battery bank.

The battery bank is a Landport AGM (16 x 260 Ah, configured as 4s4p. So +- 1000Ah 48V battery bank).

See screenshot for an example of such voltage discrepancy.

The system is configured to use the BMV-712 as battery monitor (see screenshot below). Which doesn't seem to have any impact on the MultiPlus-II detecting low SoC and cutting power.

Any advice?

Multiplus-II BMV Battery Monitor SOC low battery warning

screenshot-2023-01-12-165204.jpg (47.1 KiB)

screenshot-2023-01-12-170348.jpg (54.7 KiB)

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Answer 1 · 2023-01-12T23:33:34Z

klim8skeptic answered · Jan 12, 2023 at 11:33 PM

Normally such a big voltage difference between a Multi and battery will be due to some wiring problem. Voltage drop along excessively long or thin cables. Maybe additional voltage drop on crimps or connections. Or even a bad circuit breaker or fuse.

Normally multi's above 3000w will have a voltage sense cable connected to the battery to compensate for small differences in voltage readings between the multi and battery.

Is your v-sense wire connected to the battery? Is there a fuse inline between the battery and multi v-sense wire? Is the fuse ok?

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Answer 2 · 2023-01-13T03:28:31Z

Alistair Warburton answered · Jan 13, 2023 at 03:28 AM

Have you got Battery Monitor and Low SOC shutdown enabled in the Multi?

I am Fairly sure that when the 'system' is using something other than the Multi to determine battery state the Multi should have those features disabled. I am set up that way and it seems to work just fine. To be fair I am using a SmartShunt, not a BMV, and my system is anything but standard so I suggest you check what I am suggesting, if it may apply.

I also agree with the post above, that is a BIG difference. The currents don't match and neither W figure is correct but my own experience with battery measurements makes me think that measuring low currents in the Multi or the Shunt just isn't that accurate.

Is the discrepancy worse at higher currents? With that voltage drop those currents suggest a resistance between 0.7 and 0.9 Ohms, depending on which current you pick.

If the BMV is right, so circa 0.7, you would be dropping around 14V at 1kW... That you would notice in a hurry! I am assuming that isn't the case.

I don't have a good answer for you right now save to say something isn't right... I would be checking voltage at the Multi and the Bat terminals with a known good meter, in a range of conditions, as well as measuring the voltage drop of both the Neg and Pos sides of the circuit, as a whole, and across every unindividual item. That will tell you both If, and if so, where, you have a volt drop.

N.B. USE PPE and If you are not 100% sure you have a reliable test meter, in good condition, preferably with fused probes, and know exactly what you are doing... DO NOT mess with a high energy battery circuit at all! Those batteries could vaporise a spanner, let alone faulty meter or lead, and face full of molten metal is a life changing event. 50V is relatively safe but several thousand amps of fault current could set you on fire and blind you in an instant.

(Sorry if you know all this but some folks may not!)

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ngrms commented · Jan 13, 2023 at 07:32 PM

Thank you for the extensive information! The battery monitor is indeed enabled on the Multiplus 2 as well. I am also under the impression that this is causing the cutoff, as the alert originates from the Multiplus instead of BMV-712. Are there any things I should double check before disabling the battery monitor on the Multiplus, in order to not risk damaging the battery bank?

My system is running in ESS Optimized without batterylife mode btw.

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Alistair Warburton ngrms commented · Jan 13, 2023 at 08:03 PM

Provided your shutdown voltges re correctly set you should Be OK, In addition I think ther is a low SOC shutdown option within the ESS settings. I am unsure what or where and suggest you look this up.

I am just guessing here but I suspect that the low SOC shutdown based on the internal battery monitor of the Multi was introduced to get arround the issue of voltge sag under load...

If you only monitor terminal voltage, especially if you want conservative settings, you can run into issues...

Considder a small load over a long discharge period. Voltage sag is low, because the load is low, and the discharge will stop at a reasonable SOC aproximation baed on terminal voltage.

now take the same battery and apply a large load for a shorter period... Voltage sag will be high because the load is high and the voltage aproximation of SOC is now not correct. Your system will shut off early and quite possibly recover he terminal voltage, starting up again.

I have my 24V system configured for fairly low battery voltages but alarming on low SOC, based on the Smart shunt SOC value. In addition the batteries themselvs hav an internal BMS as a last line of defence.

My Pack AH rating is set a little les than the actual value to add headroom and will be adjusted about once every 6-12 months to compensate for ageing.

My 48V pack is a BYD and everything is handled by the BME/DVCC combo but I try to alarm high and keep deep discharge to a minimum.

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