Victron BVM-700 - single battery or battery bank

[Unix]

Hi all,

On my NarrowBoat is a BMV-700 battery monitor; this was installed by a previous owner.

I lifted the floor to do a sketch of the batteries and their connections.

There's a single 12v starter battery and four 6v batteries connected in series/parallel to give a 12v bank.

The BMV-700 is monitoring the battery bank but the Victron manual seems to only refer to the 700 model being designed for a single battery.  I tried to upload a photo of a page from the manual but failed to show any progress at all.

Is it acceptable to monitor a battery bank with this monitor?

[nicknorman]

Bear in mind a battery is a collection of cells. Inside a 6v battery there are 3 cells, so in two 6v batteries there are 6 cells, which is the same number as a 12v battery. So as far as the BMV 700 is concerned, it doesn’t care whether those 6 cells happen to be in one casing or two. It also doesn’t know or care if there are several groups of 6 smaller cells in parallel vs just one set of large cells.

 

So to directly answer your question, yes the BMV700 is quite happy monitoring a battery bank made up of several cells and/or batteries, provided they sum to 12v and the correct total capacity is set up in the BMV configuration.

What the BMV700 can’t to is monitor more than one battery bank. So it can monitor the various cells and batteries that make up your domestic battery bank, but it can’t also monitor your starter battery. You would need a BMV702 or 712 to monitor two different battery banks, and even then it just gives voltage for the engine battery, not state of charge.

[Unix]

Thank you nicknorman. I completely understand that a battery can be internal cells in one unit, or many units making a bank. The battery monitor will not know the difference. I just wondered why the manual seemed to focus on a single battery.. Your answer makes sense.

I also have a Merlin Smartguage installed; again by the previous owner. So far I've read that this Merlin can be a two or three wire installation. However I haven't finished sketching out the wiring under the floor. I have an OutBack solar device wired in as well.

I really need to see what wire is going to which monitor. It isn't that tidy under the floor and some wires and cables aren't easy to follow.

I have multiple green wires connecting to tbe battery negative and haven't seen where they go yet.

Thank you for your reply.

 

[nicknorman]

By way of general explanation the two monitors work on completely different principles:

 

1/ The BMV measures current passing through a shunt to calculate charge into and out of the battery. You have to manually set the actual  battery capacity in the BMV settings. It then knows the State of Charge (SoC). One issue is that as the actual capacity of the battery degrades over time, unless the owner updates the capacity in the settings, the SoC can become optimistic, sometimes extremely optimistic.

2/ The Smartgauge monitors only voltage, but it has a clever algorithm from which it can derive the SoC. It is pretty good during discharge, not very good during charge. It works out the SoC based on the actual capacity of the battery, not on the notional capacity on the label. With new batteries you would therefore expect these two monitors to read much the same during discharge. If the Smartgauge is showing a lower reading during discharge, this indicates that the battery capacity has degraded and the BMV set capacity needs to be updated.

The Smartgauge has a third wire which is connected to the starter battery postive (via a fuse, of course) so it can show starter battery voltage, but it does not calculate starter battery SoC.

 

These sorts of monitors often get installed badly such that they don't work properly. Here are some tips for sucessful installation:

 

1/ The BMV can only work properly if ALL the current flowing into and out of the battery, passes through the shunt. The shunt measures current in the negative side. Therefore in any battery bank monitored for SoC by a BMV (normally the domestic battery bank) it is vital that nothing is connected to the negative terminals of the domestic battery except each other, and the BMV shunt. EVERYTHING ELSE must be connected to the other side of the shunt. This includes the engine battery negative. So when you report several green wires connected to the domestic negative, I am suspicious!

 

2/ For every RULE IN BLOCK CAPITALS there is of course an exception! The Smartguage must have a very precise measure of battery terminal voltage and so must be connected directly to the battery terminals, so this is the exception to 1/ in that it must be connected directly to domestic battery negative, not to the other side of the shunt.

 

So in summary, on the domestic battery negative terminals you should have the other domestic battery negatives, the shunt, and the Smartgauge negative. NOTHING ELSE! Any other negatives should be connected to the other side of the shunt.

 

 

Edited February 10, 2021 by nicknorman

[Unix]

Thank you for that nicknorman.  I will look tomorrow to see where these wires on the negative go to.

Actualy I've just looked at a photo of my battery bank and it appears that one of the green wires goes to some terminal block where it's connected to a black wire - which, I'm sure, goes up to the merlin smartguage. The other green wire disappears off screen to where my mains battery charger is located.

Out of interest, I saw that you commented on someone's problem with their powerline T105 battery bank - four batts in series/parallel. That's the same set up that I have.

I will attempt an image upload.

 

[Awayonmyboat]

10 hours ago, nicknorman said:

 

1/ .......This includes the engine battery negative.

 

Nick, thankyou for the above bit of advice. I installed a meter last month and whilst I made sure all other negative connections were on the far side of the shunt, the engine battery connection was something I did not move simply leaving it as was (connected directly to the nearest domestic battery). Now thinking about it I understand why it needs to be moved. Time to get the tools out again.