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richie99 avatar image
richie99 asked

Incorrect over voltage readings with BMV-712 on starter battery

41737469-6e9f-4b9d-928a-0908e196fd56.jpegThe starter battery reading on my BMV-712 is suddenly not accurate, having checked my starter battery voltage with a meter.

I have two 12V 300Ah lithium leisure batteries and one 12V lead acid starter which now has just started giving false over voltage readings of around +3V. I’ve checked the fuse on the lynx power in 1000 and it buzzes out ok. I discussed it with a Victron engineer who says there is an issue with this setup (lithium) and to just disconnect the connection to the shunt for the lead acid battery and accept that I won’t get a reading on BMV-712. Consequently I’ve had to disable the over/under voltage alarms and he says I need to fit another battery monitor just for the starter battery. This seems odd as it was doing just fine for eight months? Although the fuse buzzes out ok he says it may be worth replacing it just in case. I’m yet to do that. Ultimately, we’re users not installers!

Has anyone else come across this? Any advice/help would be appreciated. Thanks


BMV Battery Monitor
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3 Answers
raymiller avatar image
raymiller answered ·

Most probably a couple of possibilities;

First, check the common battery and cable connections to make sure no high resistance joints causing voltage drops.

Second, the supplied fuse and holder for the BMV's can give problems, I've had a fuse that was not of the correct rating, causing a voltage error on the BMV, as well the holder is poor mechanically. I replaced both with a poly switch (self-healing fuse by Littelfuse) of suitable rating. With a multimeter measure the voltage drop across between the battery and BMV, should be <0.1V.

I would also caution about the sensor/supply connections on the BMV, you need to exercise care when connecting the ferals, make sure you release the tension with a small screwdriver when installing to prevent damage to the receptacle, if this is damaged the connection becomes unreliable.


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kevgermany avatar image
kevgermany answered ·

Have you got a good earth connection between the lithium and lead batteries?

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David avatar image
David answered ·

From the way you describe the install, the advice you were given by the 'Victron Engineer' (put in quotes as not sure if it was someone actually from Victron?) seems odd. There is no problem I am aware of monitoring a Starter Battery on the Aux input no matter what type of Battery you have on the Primary input. I would also not agree with having to add a dedicated shunt to monitor the Starter.
May well not know all the facts, but TBH I find the advice as quoted suspect.

The only time I can think where you cannot do just what you have been doing is if you don't have a common ground on the batteries - in which case, I would expect to see no voltage rather than an overvoltage.

Once you have checked for loose conections and you still have a problem, what I would try if I were you is swap the two inputs on the Shunt and see what happens in terms of voltage reading. The Current/Ah/Watts/SOC reading will not make any sense when you do this, but it will let you know if the wrong voltage readings swap from Aux to Main (which points to an issue with the connection INTO the shunt) or if the Wrong readings stay on Aux (which points to an issue with the Shunt/BMV). Undervoltages are not uncommon with resistance in cabling/fuses, but overvoltage (especially in the area of 3V) is much more uncommon.

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