Best Place for BMV 712 when batteries are split front and rear ?

[RAW]

Having the batteries split in the Merc and more AH in rear to that in Front I am wondering where best to place the BMV ?
The solar comes in at rear, most of the battery load will be from the rear? By missing front loads from being attached to the BMV then how skewed will the readings be I wonder ? Is it worth fitting in this situation or will I just get spurious figures ?

 

[trevskoda]

I have the same setup but two 90ah batts,one is from new in front footwell behind a kick board,second is a mid ships under sink along with water pump and fridge inverter.

 

[RAW]

I have the same setup

Are you using a Victron BMV with the set-up ? Ta

 

[SquirrellCook]

For your BMV to read from all the batteries you would need a single ground point.

 

[RAW]

For your BMV to read from all the batteries you would need a single ground point.

Do you mean, one place where all the negative's go - at the moment the rear is grounded to bodywork at rear
The front is grounded to Bodywork at front
So the rear and front cables to ground would need to go to one ground point i.e rear cables extended to reach front ground point or vice versa ? Is that what you mean ?

 

[SquirrellCook]

Think of it like filling a bath with a single combined fill and drain point. To know what you are using and replacing you need to monitor the flow at that single point. If some bugger is using a jug to help themselves or someone else is weeing in it you'll never know what's going on. (don't drink the bath water)

 

[RAW]

Think of it like filling a bath with a single combined fill and drain point. To know what you are using and replacing you need to monitor the flow at that single point. If some bugger is using a jug to help themselves or someone else is weeing in it you'll never know what's going on. (don't drink the bath water)

Yes, well in the current set-up that would take a lot more re-wiring and re-cabling and not sure that I want to do that yet so may leave it as a Winter Job, it would be quite a faff I think. Getting the Negative of the Batteries to the BMV would be easy enough but the Loads would be a completely different story.

 

[SquirrellCook]

If all the grounds for your loads go to the chassis and if all your batteries shared a common ground connection. The BMV would fit between the batteries ground connection and the chassis. That way all the loads and charging should pass through the BMV.

 

[wildebus]

This would be how I think your pair of batteries SHOULD be connected, followed by how it really should NOT be connected and finally how I *think* your setup is, and how to make the best of the situation until you can redo it - basically - properly.

If for anyone this whole speil is a bit 'TL: DR', just look at the pictures

1. How it should be

You have your two batteries connected to each other, +ve to +ve, -ve to -ve.
One Battery -ve is connected to Chassis ground
The other Battery +ve is connected to the +ve for any and all loads and charge systems (I show just two in the diagram above)

In this setup, both batteries are equally involved in the path for load and charge activity. It is always best to match the batteries technology and capacities as much as possible, but mismatched batteries can (to a degree) still be connected like this.


The way NOT to do it

Two batteries, not connected to each other in any way. This is not a battery bank, it is just two batteries in the same place.
The BMV will only know about stuff happening to that single battery. Having an expensive device like a BMV-712 connected to a setup like this is frankly, IMO, a waste of money.


A common but not ideal scenario

The Batteries -ve's have each been connected to their own load chassis-ground points. They have been brought together on the +ve's and then the various loads and charges distributed from this.

It is possible to use the BMV like this to get an idea of the Battery "bank" (as such) but how accurate that is will depend on how well the batteries are matched up in their power delivery and charge acceptance.
To use the BMV in this way, you would set its battery capacity to just the capacity of the battery it is connected to - so on a "bank" of two batteries as above, where LB-1 is 100Ah and LB-2 is 160Ah, and the BMV is on LB-2 as shown, set the capacity to 160Ah.
This will give you an approximation of the State of Charge.
The current displayed will be incorrect however and you will have to compensate for that outside the BMV. to get the true current, take the current shown, divide by the Ah of the battery it is connected to (so 160Ah in this example) and multiply by the total AH of the bank (100Ah + 160Ah in this example). So if the current shown was 10A, the actual current will be 16.25A (10A / 160Ah * 260Ah)
This setups reliability in terms of BMV data accuracy is very dependant on how closely the batteries are matched and how the cabling is done.

You can put in a control load or charge and check with a current clamp meter how the current is actually divided between the batteries to get a more accurate real-life number and tweak the Ah setting in the BMV accordingly, but this setup would really only be a half-way house until you recable the batteries correctly in order to not only get better information from the BMV but better performance from the Batteries as a whole.