Vehicle solar install

[@rt]

Hi

I have a 4WD vehicle, 12 Volt fridge and some other loads such as LED lighting, MPPT charge controller, SLA battery, and a solar panel to be mounted on roof racks.

The atypical thing is I don’t want to buy or use a battery isolator. The vehicle battery and charging system should have nothing at all to do with the solar system.
It’s possible I will investigate if there’s a need for one of those small panels to trickle charge the vehicle battery if I’m camping stationery for a prolonged period.

So... question is.. Solar panel mounted to roof racks. I see no reason why the solar panel negative can’t connect to the vehicle roof rack mounting point,
so that only one insulated conductor has to be lead into the cabin to connect to the charge controller. ie. Solar panel and vehicle share common ground.

AFAIK, when the charge controller wants to stop the panel charging the battery, it shorts the panel... I still see no problem for the vehicle there,
but maybe I’m missing something when it comes to the SLA solar battery. Is it’s negative terminal also permanently common to the PV panel negative through the controller?

What about the load switched by the charge controller (usually lighting)? If this happened to be negative switched, a common negative for the load might prevent it’s functionality?

Basically I’m asking if anyone knows of a reason it wouldn’t be safe.. for everything in the solar system to potentially touch the vehicle chassis.

[Clear as mud]

The main problem I forsee is having a constant DC current through the roof-rack mounting point, which could lead to corrosion over the long term.  But is ultimately no different than other grounding points on the vehicle, just maybe more important to keep waterproof, since it's the roof.

[Renate]

Something to watch out for:
Both MPPT and PWM controllers might have a common positive instead of a common negative.
No, I'm not talking about some wacky reverse polarity vehicle. I'm talking about standard negative chassis.
But sometimes they have the plus lead of the solar panel connected directly to battery plus and they regulate the connection between chassis and the minus lead of the solar panel.
That means that the minus lead of the solar panel goes negative with respect to ground.
You need to check that.

I think you need a real connection on the minus lead of the solar panel even if it is grounded.
On my system if the minus lead broke it would be showing -60V with a potential for 10 A!

AFAIK, when the charge controller wants to stop the panel charging the battery, it shorts the panel.

No, it just stops drawing current.

[wizard69]

Hi

I have a 4WD vehicle, 12 Volt fridge and some other loads such as LED lighting, MPPT charge controller, SLA battery, and a solar panel to be mounted on roof racks.

Cool i'm interested in such a configuration on my pickup.   Finding the ideal cap to do this isn't easy though.
[quote[

The atypical thing is I don’t want to buy or use a battery isolator. The vehicle battery and charging system should have nothing at all to do with the solar system.
It’s possible I will investigate if there’s a need for one of those small panels to trickle charge the vehicle battery if I’m camping stationery for a prolonged period.
[/quote]
You wouldn't want to connect the two systems anyways.   It might make sense to have an emergency switching option to charge the vehicle battery in a pinch.  The other thing here is that keeping things separate allows for freedom in voltage selection for your camper.   That is a 24 VDC or higher storage system could make sense.

So... question is.. Solar panel mounted to roof racks. I see no reason why the solar panel negative can’t connect to the vehicle roof rack mounting point,
so that only one insulated conductor has to be lead into the cabin to connect to the charge controller. ie. Solar panel and vehicle share common ground.

It might make sense to ground to the vehicle but I suspect there is a big potential for reliability and voltage drop issues.   Reliability of the grounding structure is a common issue with vehicles.   Beyond that with low voltage solar you always need to consider voltage drop across the wiring.  So run both the + & - directly to your charge controller.

AFAIK, when the charge controller wants to stop the panel charging the battery, it shorts the panel... I still see no problem for the vehicle there,
but maybe I’m missing something when it comes to the SLA solar battery. Is it’s negative terminal also permanently common to the PV panel negative through the controller?

I don't believe that is true.   If the charge controller actually tops off the battery  though, it would make more sense if it could dump excess capacity someplace else.   That might be a more expensive charge controller though.   

What about the load switched by the charge controller (usually lighting)? If this happened to be negative switched, a common negative for the load might prevent it’s functionality?

I'm not sure what you mean by this.   The charge controller shouldn't normally be switching loads unless it is a dump load.

Basically I’m asking if anyone knows of a reason it wouldn’t be safe.. for everything in the solar system to potentially touch the vehicle chassis.

I can't see an issue, in fact it may be a good idea to keep the solar charge storage referenced to the vehicle system.   Just make sure everything is designed for a negative ground reference.


Now for an aside.  One of the reasons I mentioned the possibility of a higher voltage system than 12VDC is that I'm now seeing vehicle lighting solutions that take a wide DC voltage as an a acceptable input.   In fact a recent lighting system is capable of somewhere around 12 to 60 VDC max.   The obvious advantage here is being able to use lighter gauge wiring.   So it is worth considering DC lighting that can run at a higher voltage if you can find what you want fixture wise.

[@rt]

Thanks for the replies folks I’ll answer the questions I can, and comment randomly.

@wizard69,
The solar charge controllers usually provide a load controller specifically intended for lighting.
In fact, I’ve owned and seen many of them, and all of them had/have a lighting load output, usually smart controlled to turn on lights at night.
The current crop of controllers now often have USB ports for charging mobile devices.
High current loads are connected across the battery and fused of course.

If the lighting load control was negative switched, it would be an issue for it to share common ground, even with the PV panel or battery.

@ Renate,
I have more than one charge controller of reasonable quality. Perhaps I could check if the negative is common for those with simple continuity test?
One is sealed, the other can be opened to physically check inside.

@ all,
The reason I thought the controllers disable the PV by shorting it is the they usually warn not to connect to any other source than PV.
If a controller shorted a wind turbine for example, that would brake the turbine, and then break the turbine and maybe the controller too.

The reason I find it an attractive proposition to only run a single conductor from the PV panel to the cabin is because that’s the most difficult of the whole install.
A flat insulated ground strap might be enough to poke in anywhere.. through a door sill, etc...
but two conductor thick cable, I haven’t even thought of a neat way to get that inside the vehicle yet.

[Renate]

I have more than one charge controller of reasonable quality. Perhaps I could check if the negative is common for those with simple continuity test?
One is sealed, the other can be opened to physically check inside.

You don't need to take anything apart.
A DVM will show either a dead short between PV+ and Batt+ or else PV- and Batt- (PV=photovoltaic).
If you have the PV+ to Batt+ that is so that they can use an N MOSFET on the PV- and they have at least +12 V gate drive without generating any voltage.
Still, it's more normal and expected to have the PV- be ground.

I had an EPEver Tracer A 20 Ampere that had PV+ to Batt+
My newer EPEver Traver B 40 Ampere has PV- to Batt-

The load switch could have the same issues as the PV, they might switch either leg.
The load switch is kind of pointless, the current rating is usually (?) the same as the charge current.
I don't need that limitation, or the switching, or the current monitor drop.
I always wire loads directly to the battery.
The only situation where I can see this load switch making sense would be a PV/battery/security lighting at an unattended location.