pabgarde avatar image
pabgarde asked

How to completely disconnect BMV 702

I have a BMV 702 connected to a battery system. I use the relay (normally energized, closed) to control another relay where all positive loads are connected. When the BMV detects the battery is below 11V the BMV relay is opened and the load relay is open, disconnecting all the loads.

The problem is that in this situation the monitor is consuming 15mA which is a lot. Is there any "clean" way to disconnect the BMV to avoid discharging the battery when the system is not in use? A switch in the positive supply cable? Any other way to achieve the same but but having the relay normally de-energized? Other solutions?

Thank you!

BMV Battery Monitor
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2 Answers
kai avatar image
kai answered ·

when you say normally energised, do you mean normally open (N/O)? So when the coil is energised, it becomes closed?

The BMV itself will require power, and you can of course cut power to the BMV via a switch which would drop out a N/O load relay.

Theres a few ways of configuring relay arrangements, I'm not sure what you're trying to achieve though, is there a reason why the load relay has to be closed if you've switched the BMV off?

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pabgarde avatar image
pabgarde answered ·

My system is installed in a motorhome. I have a 200Ah AGM battery with 3 different chargers (solar regulator, alternator booster, 220V charger) and all the consumers (fridge, inverter, lights...). Between all the consumers and the battery I have a "load" relay (normally open) that is controlled by the BMV relay. The BMV relay by default closes the "load" relay but if the battery voltage goes below 11V the BMV relay opens the "load" relay and all the consumers are disconnected to avoid battery undercharge. The cable going from the BMV relay to the "load" relay also has an switch that I can also use to switch off the "load" relay manually.

I am using the relay with the following parameters:

  • 11. Relay mode = DEFAULT
  • 12. Invert relay = ON (normally energized = normally closed).
  • 18. Low voltage relay = 11V

In this mode the BMV consumes 15mA. That means 11A per month.

Sometimes the system is not used for several months and I manually switch everything off (chargers and consumers) but the BMV remains on. Now I want to know how to avoid the BMV to drain my battery while the system is not in use.

My concerns about using a switch in the BMV supply cable:

  • The supply cable needs to go directly to the battery and a switch with produce a small voltage drop.
  • If I switch the BMV off, I need to reconfigure it again when I connect it again.
  • Sometimes I leave the solar regulator on and if I switch the BMV off then I have no control over the battery status.

The only solution I found is to change manually the settings and change the relay to normally de-energized (normally open) but I don't want to be all the time changing the settings.

Thank you for your help.

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kai avatar image kai ♦ commented ·

Check/consider these:

1. Turn off backlighting if you have it configured to permanent on.

2. BMV 712 uses an internal bistable relay which will reduce consumption (relay only draws power when changing state; BMV electronics will still require power).

3a. Regardless of how you manage consumption, the BMV electronics will need current to operate (4mA), and if you don't want the reset then you have to pay the energy tax. Measure exactly how much your BMV only system (incl. all associated relays that would be energised) will draw, put in some margin and work out how long you can leave the system on before you have to attend. Note that there is no system protection in this option and it will flatten the battery if you don't attend.

3b. If your solar regulator is configured suitably (for long term float etc), can you leave regulator on and the BMV on, but the manual load switch off.

4. Converse of above - if you're only doing a full power down once or twice a year, maybe you can live with the inconvenience of restarting the BMV. i.e. switch off BMV as well as load manually.


You could configure your BMV relay to normally deenergised (as per BMV702 manual terminology; 12. Invert relay = off), but the load relay will need to be wired as a N/C (if it has a N/C contact; if its a heavy duty relay you may have to get a N/C version as they may not have both N/O and N/C contact as standard). This way when V>11, BMV relay will be open, and load relay will allow load. If V<11 then BMV relay will close and the load relay will disable load.

The issue with this is that the BMV electronics, the BMV relay and load relay will all be still drawing power when the system should be shutting down to stop battery from being drawn on further. If the system is potentially unattended when this condition is met, this is risky.

I suppose you could put in a switch to kill the BMV relay/load relay circuit (and the primary load path), in which case it is kind of similar to point 3a. You'll want to consider your risks (e.g. forgetting to flip the switch when you put into storage) before going down this path.

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