Possible to use BMV-712 with a LiFePO4 Battery with integrated BMS?

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nigelfxs asked May 10, '19

Possible to use BMV-712 with a LiFePO4 Battery with integrated BMS?

I have a system consisting of Multiplus-II, MPPT 150-70 and CCGX connected to a LiFePO4 battery that has an integrated BMS. The system is working well, except that the BMS does not accurately measure the battery SOC. I think the problem is that it does not properly measure low-value discharge currents - so the displayed SOC is generally higher than the actual SOC.

The BMS is built into the battery and all other functions (cell-balancing, voltage display, low/high voltage disconnect etc.) are working well.

Is it possible to add a BMV-712 to the system to replace the BMS SOC value, without loosing the other BMS info (e.g. battery temperature, warnings etc.)? I would like to use the BMV-712 SOC value to control the "Minimum SOC (unless grid fails)" function in ESS.

ESS BMV Battery Monitor Lithium Battery BMS CCGX Color Control

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Paul B answered · May 11, 2019 at 04:43 AM

Yes adding the BMV would not affect the BMS system in any way

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nigelfxs commented · May 11, 2019 at 12:31 PM

Thanks for the feedback. I guess that I would then need to select the BMV in Settings -> System Setup -> Battery Monitor.

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Answer 1 · 2019-05-22T21:57:05Z

fmerz answered · May 22, 2019 at 09:57 PM

Hi Nigel,

How do you know that your Battery BMS doesn't measure the SOC correctly? From the inverter...?

I have a similar problem, but I use a BMV-702 already.

My PV system:

16 x TrinaSolar 275W (4.4KW)

2 x Voltronic Axpert MKS 5KVA Inverter (parallel mode)

2 x YIY LiFePo4 batteries 51.2V / 150AH (300AH)

Basically my system works great, but also I had a difference in "Measured Amps" and "SOC state" between the BMV-702 and the battery internal BMS.

With standard recommended BMV settings for LiFePo4 (Peukert: 1.05, Charge Efficiency 99%) my BMV was too optimistic compared to the battery BMS, i.e. during discharging, used AmpsHours going down faster in the BMS than in the BMV, and related SOC state the same.

For example:
When BMV showed "150AH used" and "SOC 50%", the BMS shows something like "170AH used" and "SOC 44%".

I tried to play with the BMV settings, but I was not able to get the two monitors synced. At the moment my BMV settings are Peukert: 1.15, Charge efficiency 95%. Like this they show almost the same regarding AH used and SOC state.

I am definitly not sure if this is the right way, but at least it works for me.

If somebody have a better idea, how to get a BMV and a LiFePo4 BMS synced more accurately, I am more than open.

Cheers,
Felix

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nigelfxs commented · May 27, 2019 at 03:50 PM

Hi Felix

I think that the BMS integrated into my battery (52V, 230Ah, LiFEPO4) does not accurately measure low-current levels.

For example, if I have a small average load of 200W x 10 hours, I would expect the SOC to drop by about 2kWH/12kWh = 16.7%. In my case the BMS does not show any change in the SOC during this time (or maybe 1% max). If I put a medium or high load on the battery, then the BMS appears to show the correct change in SOC.

I would therefore like to use the BMV to accurately measure the SOC and therefore be usable by ESS to trigger the charger to top up the battery when needed. I would still like to have the BMS be able to give temperature warnings, error messages etc.

Getting back to your question: are you sure that the BMS really is "accurate"? It could be that the BMV is really showing the correct SOC and the BMS is inaccurate...

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Answer 2 · 2019-05-28T12:02:51Z

nigelfxs answered · May 28, 2019 at 12:02 PM

This problem where the BMS does not accurately measure low-level loads appears to be an issue with a number of Battery types. See the following link https://www.victronenergy.com/live/battery_compatibility:byd_b-box#known_issues

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