BMV-702 reads each battery voltage in two parallel connected batteries? Nope!

[Wetjala]

This applies to those punters who have parallel connected batteries installed into either their T-Van, like mine (Mark IV), or a Topaz – both of which can have at most two batteries connected in parallel (effectively doubling the nett battery Ah or capacity available).

Question: "Have you looked at the wiring between the batteries and from the batteries to the Victron BMV-702?" Like me you have probably assumed that the voltages shown on the Victron BMV-702 for the each of the batteries are a true indication of the battery voltages. Wrong! Why? For the simple fact that the voltage of each battery connected in parallel cannot be measured independently (at least not without some pretty fancy electronics between the batteries); the voltage measured will be an average across both parallel connected batteries with any slight differences displayed on the BMV-702 main and auxiliary voltage readings attributable to the slight voltage drops over the different length cables.

The Victron BMV-702 shunt had two input signals inputs which the BMV-702 can display. The first input always indicates the voltage of the main battery (or in a more general sense the main battery bank) and provides the power for the BMV-702 to actually operate. The second input can be configured to indicate the temperature of the main battery (or a battery in a battery bank) using the appropriate sensor, or to indicate the voltage from a separate auxiliary battery (or battery bank), or to indicate the midpoint voltage of batteries connected in series – the latter of which doesn't apply to the T-Van or Topaz battery arrangement. Note I italicized separate. Separate as in independent or not connected in parallel. (Also refer BMV Manual Section 3.10.1 Auxiliary battery monitoring. "This configuration provides basic monitoring of a second battery, displaying its voltage. This is useful for systems with a separate starter battery.")

I guess my point here is that if you have dual battery setups with the Victron BMV-702 wired to sense the voltage of both parallel connected batteries then it's a false belief that you are observing the actual independent voltage of each battery. What you are actually seeing in both readouts is the average voltage across both batteries which if the batteries are matched and healthy should be close to the actual voltage of each battery - just not the actual voltage of each individual battery. In reality the readings are in an engineering sense redundant (duplicates). So in my opinion for the T-Van/Topaz, it would be better to utilize the second auxiliary input to measure battery temperature so the charging profile can be adjusted (manually) to suit the actual temperature of the battery, or not connect it all. Perhaps previous T-Van/Topaz models did not connect the batteries in parallel in which case the voltage readings would be independent and meaningful.

I'm by no means an expert on any of this so feel free to correct my understanding and in particular how using the auxiliary input to the BMV-702 to read essentially the same voltage as on the main input will be of any practical use (other than show slight voltage differences caused cable length losses and not battery level differences).

[TimH]

Spot on.
I had problems with my batteries charging properly.
I assumed that the seperate battery readings on the 702 reflected what each battery was doing.
It was only after much mucking around that I realised that this wasn't so. I disconnected both batteries and then connected each one in turn to the charging circuit and identified the problem that one battery had failed and wasn't charging.
Definitely educational.

Tim


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[TimH]

Maybe you could use the 2nd voltage monitoring to show the incoming voltage from the car.
I got voltage readouts of the main and aux car batteries in the cabin so I could see what sort of voltage drop I'm getting over the cable run from the car to the Tvan.
Always good to have more information than you need.
Tim


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[Frankrhona]

Agree 100% with the above explanation. I set up a 4 position battery switch: Off, Bat 1, Bat 2 or BOTH, similar to how many yachts and motor boats are set up. For me this has the advantage of being able to completely isolate the batteries when at home.
https://www.whitworths.com.au/4-position-mini-battery-selector-switch-1-2-both-off-with-afd

If you wired each input for your Victron to each battery this would give you a wealth of information. You could test each battery with a load or no load and solar charging voltages.
Just an idea.
Cheers,
Andrew

[robstevens]

If you could connect a manual on-off switch between the two batteries, leaving it "On" normally, then by switching it "Off" you would get the actual readings of each battery... One of those 100amp battery isolator switches would do,

[Ray]

or another idea is a trippable circuit breaker between batteries that offers some protection and allows isolation between batteries, I use a 70 amp one.

Ray

[boobook]

But.......if the two batteries are wired in parallel, then by definition they are at the same voltage. That's the whole point.

You could wire the second ( primary) voltmeter wire to the second battery but it would be of no value.

A valid use for the primary volt gauge could be to measure the input to a DC to DC charger, or the Solar panel ( it will measure up to 70V)

[Phil G]

So in my opinion for the T-Van/Topaz, it would be better to utilize the second auxiliary input to measure battery temperature so the charging profile can be adjusted (manually) to suit the actual temperature of the battery, or not connect it all.

Just wondering what charger yours has, and how you can adjust the charging profile manually? 
Both the Projecta IDC25 and Redarc Manager 30 already have remote temperature sensing - a temp probe is installed on the  battery so charging voltage is decreased as the battery gets warmer.
Cheers
Phil

[Wetjala]

Both solar & mains chargers in my MkIV are Projecta brands and do not have temperature compensation - at least none that is mentioned in the manuals.

I have now disconnected the redundant voltage monitoring of the second battery from the shunt and replaced it with the Victron temperature sensor to measure the battery temperature in the primary battery box. The BMV was reconfigured accordingly.

Also, as suggested by "Frankrhona" above, I too have installed a four position battery switch allowing me to check the voltage of each battery independently (in case a cell collapses, etc.) and isolate a bad battery quickly and easily should that be required when traveling remotely. By switching the 0Vdc from each battery that connects to the shunt input (closest to the RJ11 plug) it is now much safer when removing the batteries by mitigating any adverse effect if the battery positive accidentally comes in contact with the chassis (the resulting voltage arc & high current would blow a fair sized hole in the panel). The only drain from the batteries now is self discharge. After the past few months in storage and with no discharge possible from attached devices nor any top-up charging, both batteries are keeping the open-circuit voltage above 13Vdc.

Re: Boobooks comment above (" . . . if the two batteries are wired in parallel, then by definition they are at the same voltage. That's the whole point.) Yes I know but that's how Track wired it and yes it's wrong. I guess one point I was trying to make in my original post was to alert punters to the fact that you can not read the separate voltages each of the batteries installed in T-Van's or Topazes (assuming both batteries are installed) without modifcation from the original build but perhaps that point got lost in the rest of 'noise' in my post.