question

krby avatar image

Leaving MultiPlus voltage sense connected when battery disconnect switch is OFF.

I'm building a portable-ish system with a MultiPlus 24/3000. The main switch on the MultiPlus will not be easily accessible. I'll have the positive line run from the battery thru a disconnect switch on its way to the MultiPlus. The negative directly from battery to MultiPlus. The disconnect switch will be in the OFF position for weeks at a time.

I'm going to hook up a separate smaller cable to the voltage sense terminals on the MultiPlus. I am planning on hooking the voltage sense cable directly to the battery terminals (not the disconnect switch) but I want to know if it is ok to leave the disconnect off while the voltage sense cable is connected.

Is there a problem with this? Is there a little current flowing on the voltage sense cable even with the disconnect switch OFF? If so, how much?




MultiPlus Quattro Inverter ChargerMultiplus-IIvoltage
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I hear and understand all the concerns about the drawbacks to a main battery switch. But, at least on a boat, I consider the battery switch a vital safety device. In the event of a fire (and most fires on boats are electrical in nature) you NEED to be able to shut down all power--everywhere--RIGHT AWAY. It seems to me this would apply to terrestrial systems too, but I admit haven't given those enough thought to be sure.


Arcing at the switch only occurs if there is high current flowing. If there is any routine need to turn off the switch, then just power down all the operating equipment before opening the main disconnect and any arcing will be minimum.


Standard practice has usually been to switch just the positive side, unless the boat was built in France, where they put switches on BOTH legs. I have never figured out why...
2 Answers
Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) answered ·

Hi @krby

I normally advice to not use any switches, as every time you connect with the switch, a big spark inside the switch damages it.

But if you really want to: make sure to indeed always leave the negative connected.

the inverter can be switched on/ off with an external switch, or with a 'remote control panel' (DMC)

https://www.victronenergy.com/panel-systems-remote-monitoring/digital-multi-control-panel-gx

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I'm aware of the pre-charge / spark problem, but was trying to simplify for this question. I'm actually going to use a multi position switch that always passes thru a position that pre-charges the inverter's capacitors thru an appropriately sized resistor.


But, I didn't know if I could leave the voltage sense wire connected while the disconnect was OFF, or what side effects this would have.

Ah glad to hear you've got that covered.

It shouldn't be a problem, side effects can be: when multi is powered from AC with battery disconnected: you can get a battery sense error when the voltage difference between the sense input and battery terminals is too big.

jimsryker avatar image jimsryker Daniël Boekel (Victron Energy Staff) ♦♦ ·
@Daniel Boekel. I was planning to install two battery switches. A positive and a negative. BattleBorn Batteries recommends a battery switch on the negative. I plan both to be installed before the Victron Distribution Buss. I too planned to install a momentary switch and resistor to “pre-charge“ the capacitors prior to turning on the positive battery switch. I do have the digital multi-control panel you mention above, so I can shut it down when I’m just using 12VDC. If you recommend no switches, how can I isolate the batteries when my RV is parked until the next adventure? Thanks for any information.
raymiller avatar image
raymiller answered ·

Avoid any high current switchers apart from say a fuse switch as they all have typically have a much higher resistance. I've been testing all cables and connections using a 10A constant current power supply and measuring the voltage drop (mV) across the various high current components. It is surprising what you find just using ohms law. That is where I found a 300A switch was unstable for isolating an inverter even though in theory it was should have worked, brand new, its resistance was different each time I tried with my test setup and was sensitive to high vibration.

Cheers

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