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Ron Webb avatar image

BMV-712 with two 6-Volt batteries setup

I am finally picking up my new travel trailer in two days. It will be coming with two 6-volt Interstate GC2-ECL-UTL batteries connected in series to give me a 12-volt system. In anticipation, I have purchased a Victron BMV-712 and have a couple of questions as to how I should set up my particular setup.

From reading through the manual, it appears I have a choice to add a temperature sensor or a midpoint sensor but not both, is this correct? As there are two batteries involved, which would be the better variable to monitor, temperature or a midpoint? Does the answer differ due to the batteries being flooded lead-acid batteries as opposed to AGM or Lithium?

Any other guidance for installation? The batteries will be in battery boxes (one for each battery) installed on the "A-frame" of this conventional travel trailer. There will also be a battery disconnect switch installed on this "A-frame." My understanding is that I should install the shunt inside one of the battery boxes, connecting the shunt to the negative terminal feed of "battery one" and the wire attached to "B1" of the shunt connected to the positive terminal of "battery two", is this correct? If I go with a temperature probe option, I believe I'd connect the red wire to "B1" and the black wire to "B2" on the shunt, correct? If I were to go with the midpoint option, it would be taking the second red wire and connecting it to either the negative terminal of "battery 1" or the positive terminal of "battery 2" (which should be wired together anyway, which is why I don't think it matters which way I went.), correct? I then plan on running the data wire to inside the coach to where the display will be installed.

When it comes to this "UTP" or data cable, is it a straight-through cable? (I could test to see if this is the case but figured I'd ask before getting started.) I was thinking of making my own, as I have a ton of CAT6 cable and a crimping tool for 4P, 6,P or 8P modular connectors. I know that UTP is untwisted pairs, as opposed to the isolated twisted pairs of CAT6, is that really an issue? If so, I guess I could either cut the wire that came with or just buy some UTP cable (UTP is easier to crimp than CAT6). I also have a tool to electrically verify which wire on one connector connects to which wire on the other connector to make sure there is no crosstalk or shorts... it also verifies the "electrical length" of the cable. My reasoning for "rolling my own" cable is that it will make pulling it into the coach from outside easier, as well as having the length of the cable being exactly how long I need it as opposed to having a large coil of excess cable on one end.


Lastly, the two 6-volt Interstate GC2-ECL-UTL batteries look like the "Ah@20 hours" is 225 Ah; that is what I set the capacity to in setup, correct?

BMV Battery MonitorinstallationTemperature Sensor
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If you intend to add solar panels and a MPPT in the future I would go with the temp sensor. It will improve the chargers behavior and allow it to work more efficiently while better taking car of your batteries.

2 Answers
Mark avatar image

If you will be installing a MPPT charge controller then I would probably recommend going for the temperature sensor, as you can Bluetooth link the BMV-712 wit a SmartSolar MPPT via a 'VE Smart Network'. This will improve the charge cycle voltages based on the data the BMV provides.

If you use the temperature sensor it doubles as the positive power feed so the cables that come with the BMV are not used.

The shunt must be connected to the negative terminal of the 2nd battery in series - not in between the 2 batteries. Also ensure its wired the correct way around (one sides labeled 'battery' & the other 'charger & loads'.

The UTP cable is NOT basic/flat telephone cable. It is 6 core UNSHEILDED TWISTED PAIR with RJ12 / 6P6C plugs (straight through / same at both ends).

There is no problem to cut one end off the 10 meter cable that comes with the BMV, thread it through your install and then crimp on a new plug. Just copy the wire colours/positions in the plug you cut off.

The battery capacity entered should be the 20 hour discharge rating as this is the nominal rating that the BMV bases its other calculations from. The BMV will then compensate for the actual discharge rate at any point in time using the 'Peukert Exponent' entered (normally 1.25 for lead acid batteries).

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JohnC avatar image

Hi Ron. Good you read the manual. Males aren't normally congratulated for that. Lot to learn there..

Your 'choices' are indeed one or the other. Take your pick, but the temp sensor is an optional accessory. Mid-point V will prewarn you about batt cell failure when that day ultimately arrives..

The 'data cable' is just an RJ12 telephone cable.

The '225Ah' is a yes. If you're going to daily-cycle them you could also consider lowering that to their C10 or even C5 rating.

I use GC2 pattern batts myself, they're pretty rugged. Have fun tuning the BMV for a realistic SOC output.. :)

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