48v Battery Bank with 12v down step & original 12v house battery -- can I use common negative cable?

[mattnyc]

Hi All,

I have a 48v LIfePO4 480AH bank in my RV. I have a 48v to 12v step down converter. I left the house 12V battery alone, connected to the alternator and such. At this point I've got two completely different systems. I was wanting to use my car radio off of the 48v->12v system as to not tax my starter battery. I installed a 2-way switch under the car console so I can switch between using the legacy house accessory battery vs the 48v->12v battery.

Here is my question>

Can both the 48v->12v and the 12v native battery share a common negative? I ask because today I just got done hooking up my side cameras and still finishing up the system. I had some of those negative cables pretty loose, but the thing caught fire as I was going down the road! I'm not sure if it was due to a short from some other wire that had tape on it and came uncovered and caused a ark or not. I am suspecting that. However I'm going through all the wires and redoing all of the connections without any tape and making sure its all solid, and now I'm nervous about them sharing the same negative as the cause. It was the negative wire that seemed to be the source of the fire, but I can't be sure. The negative wire was shoddily connected in a temp fashion with probably a high resistance.

 

[mattnyc]

hmmm welp just got done rewiring everything into these enclosed terminals to be safe...

And I went to connect the negative from the 48v->12v step down converter to the accessory power original 12v negative, and got a short circuit, sparks.... hmm weird. I never heard of negative to negative cause sparks.

So its definitely that... now hmmm what to do with my problem.

Is there such a thing as a 3-independant terminal switch? I was using one that that had 2-independant terminal's... one to control acc power and the other to control the b+ battery power going to the radio & amp.... but I guess i'll need a third to control which negative to use... although, I am not really comfortable having the negative being switched by the same switch as the postive.

Currently I'm using this cheap switch



And btw I'm using this 48v->12v step down converter, it shares a common ground with the 48v and the 12v, at least thats how i have it wired and its working...

https://www.amazon.com/gp/product/B07QVJNQF2

Cllena DC 36V 48V Step Down to 12V 60A 720W Voltage Reducer Converter, Waterproof DC/DC Buck Transformer Power Supply​

hope I don't die guys!

 

[JoeHam]

The buck converter draws power when you connect it, hence the sparks. If the positive lead was already connected the sparks will be when you connect the negatives.

Be certain any switches or terminals you use are rated for the voltage and current you are using.

Given your history keep a fire extinguisher handy!

 

[mattnyc]

Well these Sparks aren't on the converter itself but rather when I connect a load cable running from the load positive and the negative of the buck to a device I want to draw power from. But I'm connecting the negative and instead ofmto the device directly, I'm connecting it to the negative of the original 12v in the RV that the radio gets accessory power from.

These aren't just Sparks when I freshly connect the buck into the system. These are arking that if I leave connected will burn the whole thing down.

 

[JoeHam]

These aren't just Sparks when I freshly connect the buck into the system. These are arking that if I leave connected will burn the whole thing down.

Well that aint right!

Properly rated and properly connected components should NEVER behave like that.

I’m not sure what you’re doing from your description but get that sorted out and buy more fire extinguishers!

 

[mattnyc]

I ended up using a 3-pole switch and adding the negative into the switch so when I switch from native 12v to the step down'd 12v they are completely isolated. working well so far. Still have no answer as to why when I touch both negatives together it causes a short. Mind boggling but... everything is def wired properly I've checked everything..

So the 3-poles in the on-off-on are

1: battery +
2: accessory +
3: negative

So now the car radio and amp and such on that circuit can run off the starter battery or the solar battery by way of switch.

 

[willo]

Yes, you can have a common ground with these systems. IMO it's safer.
I did the same thing on my old class c - ran power from house/starter to a simple switch to feed the radio from one or the other.

Edit, not sure on the spark issue - perhaps this is due to your floating ground?

 

[HaldorEE]

The vehicle electrical system connects the battery/alternator negative to the chassis ground so that if a wire ever shorts out to the chassis ground it will blow the fuse and this will lead to you finding and fixing the problem.

You need to do the exact same thing for your house system for exactly the same reason. This means that your starter and house electrical system are both going to be connected to the vehicle chassis ground.

However, there is another issue, you don't want to connect the electrical system to chassis ground in more than one place (otherwise you will have current flow through the vehicle frame which can cause corrosion and mysterious operational problems). If your house system only uses solar or shore power charging, then you can ground the house battery to chassis ground without concern. However, when you add alternator charging you have two options on how to manage this:

• Connect the starter positive and negative to the house system and depend on the starter negative to chassis ground connection (remove or do not add a second negative to chassis ground wire for the house system).
• Use an isolated DC-DC charger controller (like a Victron Orion-TR Isolated Charger) and add a house battery negative to chassis ground wire. This option makes sense in the two conditions below.
  • If you are using alternator charging from a tow vehicle to a trailer then this pretty much the only rational solution since you will lose the battery negative to chassis ground connection when you unplug the trailer from the tow vehicle.
  • If you are adding alternator charging to an existing system then an isolated DC-DC charger is probably easier than removing an existing battery negative to chassis ground connection.

 

[HaldorEE]

If you are seeing sparking then this indicates you are passing current through the chassis ground. I highly recommend you find why this is happening and fix it. There is absolutely no reason to use chassis ground for current return, this is a lazy practice that can cause problems.

 

[willo]

If you are seeing sparking then this indicates you are passing current through the chassis ground. I highly recommend you find why this is happening and fix it. There is absolutely no reason to use chassis ground for current return, this is a lazy practice that can cause problems.

Be sure to note that 'chassis ground for current return' - AKA you should ground out the chassis but you should have a good rated current return pathway. There's no way I'd just connect the negative of my 3kw inverter to the friggin chassis, for example. I use a bus bar for this, which also has a chassis ground line.