question

JoeOffgrid avatar image

Use same vehicle ground for negative of battery, PV, and load. Negative of PV connected to metal roof of van

Any reason I shouldn't do like that?

Can somebody explain why the manual says: "Never ground both the minus of solar array and the minus of battery."?

Ground would be identical to my minus lead to the controller.

MPPT - Solar Charge ControllerGrounding
10 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

3 Answers
scoraigwind avatar image
scoraigwind answered ·

I am also very curious to know exactly why it is forbidden to connect the PV negative directly to the battery negative (common, grounded, negative busbar). In the USA this would be very normal for many charge controllers (Outback Flexmax, Midnite Classic). The advantage is that you can use a single-pole circuit breaker or fuse on PV positive. If PV negative is not grounded via this busbar then a two-pole breaker seems to be required.

But sometimes there is a functional reason why the PV negative must go via the controller. In the above discussion it has been mentioned that there could be a current measurement shunt in the controller on the PV negative. But this is speculation.

I would like to ask if any Victron staff can tell me whether the battery negative and PV negative are connected directly to each other in the SmartSolar and BlueSolar controllers, or whether there is something in there like a shunt, or what exactly is the deal? If they are connected directly together and the battery negative is grounded, then can we say that we effectively have a negative ground PV array? (Of course the actual ground connection of the negative busbar must only be in one place in the system, and not at the PV as well as at the battery.)

Share
10 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

DanC avatar image
DanC answered ·

The positive and negative wiring of the panels needs to be connected directly to the solar charge controller, and nothing else, in order for the controller to accurately monitor the panel output. Otherwise the controller cannot accurately adjust the charging current running from the controller to the batteries. This means the controller should also be wired directly to the batteries by suitably sized positive and negative cables, so the controller also can accurately monitor the actual battery state as part of the charging process. It’s unavoidable to “ground” the controller’s “ground” (not negative) terminal to the vehicle chassis “ground”, the same “ground” as the batteries are connected to. But that’s not the same as running your panel output directly through the vehicle’s “ground”. That’s my view of it, based just on how my system works.

3 comments Share
10 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

I think my 75V blue solars don't have a separate ground at all.

As long as the connection is good I don't see why measurements would be false. Each negative terminal (PV, battery, load) connected via a solid cable to the same point on the chassis.

Can you explain what exactly would be wrong?

The MPPT will go into error, I don't know if you can damage it but it won't work.

You should ground the panel frames to the vehicle, not the output wires.

Thanks. Good to know.

Problem is in a vehicle normally the negative of the battery is ground.

WKirby avatar image
WKirby answered ·

It would be to prevent a ground loop from being created.

3 comments Share
10 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Thanks. Studied the concept of ground loop a bit...

I wonder how this can be relevant for charge controllers and if there really is a loop in my case:

All three minus from the controller would go straight to chassis ground - next to the battery clamp.

There is no grounding for the controller's case, right?

I guess a loop would be created in case I ground say the panel and the battery AND pull a negative wire.

So if you don't run a negative wire from the PV then you'll probably be fine since you only have the positive wire going to the MPPT. If the controller's input were isolated from the output then it would be a bad idea.
Do you have short links from the MPPT to the grounding point?

The false measurements that DanC speaks of are valid in the case the there is a shunt in the negative of the circuit somewhere. The shunt may be bypassed via the chassis, but I don't think this applies to your setup.

Having said that, I'd still prefer to have the circuits separated with their own runs to the charge controller. Is there a reason why you can't or wish not to do it that way?

I guess the only shunts involved at this stage would be in the MPPT itself. If I later choose to add measurement equipment it would be connected right at the battery poles. Should be fine I presume.

My plan is to put at least three BlueSolars into the van. Each one will have its own (maybe) tiltable array of panels.

The batteries are sitting in different places in the van. They already share a common ground.

I want to use the controllers to switch different loads - also distributed throughout the vehicle. That is a lot of cable. If possible to avoid half of that it would be great.

I already have some holes in the roof ... I'd rather avoid drilling more.

My idea is to connect all three negative terminals of the controller just outside of it (probably superfluous?) and go shortest way possible to chassis.

If I miss something please let me know. I'm by no means an expert. :D