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aare avatar image
aare asked

Parallel connected BMV-700 vs BMV-702 different readings

On 2016 we built a system using two battery monitors BMV-700 because we desired two relay outputs for more precise load control. This year we replaced one BMV-700 with BMV-702 for battery temperature monitoring. Now, readings of BMV-700 and BMV-702 are different about 10% for example BMV-700 showing 100% and BMV-702 90%. BMV-700 software has been updated recently and if we used two BMV-700 both readings were identical. What is the reason for that? System is built using one common shunt and both battery monitors are connected parallel. The other question is the type of temperature sensor? Does it have voltage or current outputs? If replacing other BMV-700 also with BMV-702 then for correct temperature reading it is important to know. If temperature is voltage signal then can we share this signal between two BMVs?

BMV Battery Monitor
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wkirby avatar image
wkirby answered ·

Are all of your parameters set up identical? For example, if the Peukert exponent setting is different, then you will experience different SOC readings between the two.

I do not know what type of sensor is used on the BMV series, sorry. I know that it is different from the MultiPlus one.

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aare avatar image aare commented ·

Peukert exponent and other relevant settings are identical. BMV-700 cannot measure battery temperature though

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aare avatar image
aare answered ·

So is there anyone who could answer this: is it thermocouple, resistance or some other type of sensor?

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markus avatar image markus ♦♦ commented ·

I think, the temperature sensor probe is a temperature dependent resistor.

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markus avatar image
markus answered ·

So you have connected the BMV 700 and the 702 PCBs in parallel to one shunt? That is not a common practice. I would connect them in serial, using 2 shunts..

Regards,

Markus

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boekel avatar image boekel ♦ commented ·

this shouldn't be a problem, as long as the connection is good (maybe he added mounting holes on the other side of the shunt)

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dtaschler avatar image
dtaschler answered ·

I am having the same issue with a BMV-702. My DMM measures 64.02Vdc, but the BMV is reporting 59.51Vdc. It is very concerning, especially with a critical battery system. I haven't quite found a solution. Changing Peukert's Exponent, temperature sensor, or any other setting should not affect the reported voltage. I understand a slightly different SOC calculation, but not voltage!

As for the temperature sensor for the BMV, they utilize a NTC thermistor inside the battery lug. One side of the thermistor is tied to battery plus (soldered to the lug) the other side goes to the auxiliary input of the BMV (B2 on the shunt PCB). It produces about 150kΩ at 25°C and has a B-parameter of roughly 4370K. It should not be used in parallel on two BMV units, as the resistance may change when the second device is added.

Hope some of this helps... I hope we can find answers on the voltage disparity.

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dtaschler avatar image dtaschler commented ·

Our issue was an open ground connection. It is reading good now.

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boekel avatar image
boekel answered ·

I would start by looking if the voltage / current readings are the same on both displays, if that is the case, than look into the settings again, might just be a setting overlooked.

if both settings and V/C readings are indeed also the same, I'd go to my dealer for a repair / replacement / etc.

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vuaillat answered ·

Regarding the topic, and as I also intend to mount 2 BMVs in parallel to monitor 1 battery park, I have 2 questions :

1. For @AARE : how did you connect the 2 BMV to the same shunt : using an UTP RJ12 male-female-female adaptor ?

2. For @MARKUS. You said : "So you have connected the BMV 700 and the 702 PCBs in parallel to one shunt? That is not a common practice. I would connect them in serial, using 2 shunts.. ". Could you be more precise in the way one can mount the 2 shunts in serial ?

Thanks


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markus avatar image markus ♦♦ commented ·

Hi @VUAILLAT

In a serial string, the current is always the same. So when connecting 2 BMV shunts in serial, the current readings on each BMV should be the same.

The discussion was about, mounting 2 BMV measurement PCBs to 1 shunt and so connecting 2 BMV monitors this way, should also work.

Don't mount your shunts in parallel.

Regards,

Markus

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vuaillat avatar image vuaillat markus ♦♦ commented ·

Thanks Markus,

Seems convergent to me that there are 2 ways, as you say :

1. Mount 2 complete shunts in SERIAL

2. Mount 2 boards (measurement PCBs) in PARALLEL on 1 shunt.
This way is explained somewhere else in this forum (

https://community.victronenergy.com/questions/1888/is-it-possible-to-have-2-bmv-units-operating-from.html )

Parallel is recommended, as it is said that the board reads the voltage bet. the 2 ends of the shunt and not the current.

Regards

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