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When I designed and installed my solar system (two 100 watt roof panels and a 60 watt portable folding panel), I saw that given the narrow voltage range within each of the bulk, absorption and float charging cycles, that two competeing controllers would not work. The first thing I did with my portable is drill out the rivets attaching the PWM controller and remove it.
AM Solar, who sold me my two roof panels, were nice enough to review the voltage of my portable and confirm that it could be wired in parallel to the roof panels and all controlled by my single MPPT controller.
Remember the obvious: that when wiring panels in parallel, the amperages combine (are additive) but the voltage of the combined system will be that of the panel that produces the lowest voltage. So if combining roof panels and a portable, you do not want a portable that produces significantly less voltage than your roof panels or you drag the power produced of your entire system down (volts x amps = watts).
Since most panels generally produce similar voltages, one other important aspect of having a portable, is to minimize the voltage drop from the panel to your common controller. Your portable may be rated for a similar voltage as your roof panels, but if you have 20, 30 or more feet of #10 or worse, #12 wire between your portable and your controller, the voltage drop in the cable will make the combined voltage of all panels that much lower and you’ll have less power (watts) from the entire system.
I use marine duplex #8 cable to get from my portable to my system. Due to the finer stands and higher quality of the insulation, the marine #8 is super flexible and voltage drop is minimized. Being duplex, the red and black cables are in a common white sheath to keep things tidy. I use Anderson connectors for an easy, safe, and foolproof method of connecting the cables. The portable can be just 15 feet from my rig or if in a grove of trees, I have a 25 foot extension I can insert to get it even further out of the shade into the sun.
For a combiner, I used mini battery switches with a 1, 2, BOTH or OFF function. So I can use either the roof panels (1) or the portable (2); or combine them BOTH; or turn OFF all sources to the controller. Makes it safe to work on any part of the system and assure myself that the controller is drawing on exactly the panel(s) I want it to.