solar

Plucka

I have a Ford Transit cab chassis with a caravan mounted on it and a enclosed box trailer which I tow.The van is setup for 240 volt with 12 volt solar but the problem is I have to park in the sun to charge my batteries and in Australia with day after day of 40c heat the van gets too hot .I thinking of putting extra panels on the roof of the box trailer  so I can park it in the sun and the van in the shade with cables to the van. Often I drive without the trailer to do my shopping etc so I need to able to switch from one set of panels to the other so the the batteries are being charged in the van.My question is there special switches needed to switch from one set of panels to the other [ just adding extra panels would overload my setup] and how would cable length affect  output .

BB.

Details matter (Vmp-array, Imp-array, Vmp&Imp of each panel, how many panels on truck vs trailer, brand/model of charge controller, MPPT or PWM, etc.).

How many hours a day of sun, amount of loads, battery bank would be helpful--But not really needed to answer the physical setup (a "complete discussion, vs just question asked".

Specifically, the charge controller get their power for their computer from the battery bank (in general). You can disconnect the solar panels from the Vpanel input of the charge controller(s) without issue. The battery bank connections for charge controllers should be made first/broken last (you don't want to connect Vpanels first--The controllers may not boot correctly, or at all, or may do some other screwy things).

In general, you want the charge controller(s) close to the battery bank (short heavy cables from Vbatt of controller to battery bank +/- Bus so you do not have much voltage drop (for 12 volts, around 0.05 to 0.10 volts is about the maximum drop you would like to see at maximum charging current).

So... Batteries in Van. Charge controller(s) in Van. One array on Van properly configured to run charge controller.

Array on trailer, wiring from trailer through good quality DC connectors rated for current, to input of charge controller in Van.

If 1x charge controller can handle the power from both arrays, then the second array should be in parallel with the first array (Vmp-array for Van and Trailer should be the same). The currents from the two arrays do not have to be the "same" (Imp-array). 

One issue may be... DC current sustains arcs much better than AC current. Unplugging and plugging the trailer array to the truck could cause arcing and damage the plug/receptacle. Using a switch to turn off the trailer array (either in the van or in the trailer) can help prevent the arcing.

However, if the trailer array is plugged into the van and the van array is working/some light on it, my guess is the arcing of the trailer solar power plug will not be an issue... To have arcing, you generally need >12 VDC to sustain an arc. If the trailer array is in parallel with the van array, then both will be pretty close to the same voltage and should not be an issue.

If, however, you have (for example) to solar charge controllers... One for the van and a second connected to the trailer wiring, you probably will want to install a switch in the wiring from the solar array on the trailer... Turn power on/off at the switch (switch should be off when unplugging or plugging in the trailer array). It is probably not a huge issue, but would not hurt.

If you need to disconnect one array because it is too big for your present controller, then simply put a switch (or a switch rated breaker) going to each array.

Longer term, in might be nice to have a 2nd charge controller, or replace the first with a larger charge controller so you can get all the energy you can harvest (depending on your power needs).

And, if you get a MPPT controller to connect to the trailer (higher end controller rated at 140+ VDC), you can run an array with Vmp-array~100 VDC maximum (+/-) and use much smaller diameter cabling and/or much longer distances from trailer to van (i.e, park the trailer >10-100 meters from the Van, and run a reasonable size "extension cord" from the trailer to the Van--And you can optimally place the trailer while keeping the van in deep shade.

As I said, details matter... A PWM type charge controller is hard pressed to go out to 10 meters for cabling to the array (need heavy/expensive copper cabling for any good sized array with Vmp-array~18 volts). A higher end MPPT controller, you can go upwards of Vmp-array~100 volts (again, details matter) and remote park your trailer much easier.

And having two controllers on one battery bank is not usually a big issue--As long as they are properly wired and configured.

I am not sure how you spec your wire diameter/gauge in Australia... But using AWG (American wire gauge), it can be the difference between 6-4 AWG or heavier cables for short distances (PWM controller with Vmp~18 volts) and 14-10 AWG cables for long distances (Vmp-array upwards of 100 VDC) from trailer to van (i.e., regular outdoor rated 240 VAC extension cords).

-Bill

Plucka

Thanks Bill, I do use a mppt controller and most times the distance between trailer and van would be less than 10 meters and as I don't want to be undoing wiring connections I'll look into switches, Would it be a problem if I ran a cable from the trailer via a BMS [ lithium batteries]  direct to my batteries with a switch to disconnect before unplugging . I'm 74 and things that were easy once now seem to be difficult to work out.        John

Estragon

I'd turn off the charge controller, and use a DC rated breaker(s) to de-energize the circuit(s). With that done, plugging/unplugging a cable should be okay.

If "direct from the trailer via BMS" means without a charge controller, I wouldn't.  A BMS is likely just bleeding off a bit of power from individual cells for balance, but not designed for direct connection to an array.

Plucka

The BMS would be after the charge controller  and where should dc breakers go-after the charge controller or after the BMS  

Estragon

There should be a properly sized breaker or fuse on the controller output positive in any case to protect the output wiring in the event of a controller short. 

For your disconnect wiring, a breaker (or breakers) would be added between the controller input positive and the trailer pv.  Turning the controller off and opening the controller input breaker means no current flows/arcs across the trailer disconnect plug when plugging/unplugging.

I didn't see a string configuration above, but generally with more than two panels or strings in parallel, you should have a breaker on each parallel connection.

BB.

I am not sure where the BMS and Lithium batteries are (both mounted in van or one in van and one in trailer)? Do you have two Li Ion battery banks, or one Li Ion in the trailer and one Lead Acid in the van, or what?

Need to understand the details to make sure everything is safe...

-Bill