Thanks JohnUnless your van has pre installed wiring and controller then I would wire in my new mppt controller to feed the power to both batteries.
Generally the in built controllers are not the greatest so always best to fit the new panel and controller as a separate unit.
I have an older van that has built in pre wiring but couldn’t even find it when I had a look for it as was going to use it as a quick fix until I had all the parts I wanted.
Only if the fitted wiring is of the correct gauge, longer the cable run greater the voltage drop.
Sounds like you made a better job than me although I did the voltage loss calculation and thought it wasn't worth the hassle of running bigger cables unless I thought it was likely I'd fit a bigger panel later.On the 510/520/530 models the solar pre wiring is normally found in the wiring loom that's accessed by the removable panel in the lockers above the hob.
Located in the 'rats nest' of wiring will be a red positive wire ( approx 4mm2) with a spring clip connector. There is no dedicated spare negative, you have to find a suitable one from the available spring connectors in the loom.
I'm almost sure there is also a limit on the size of panel the pre wiring can take. I recall it's 100Wh.
There is no solar controller pre fitted.
To get the solar controller as near as possible to the leisure battery, in my opinion, it's best to run new 6mm2 cables using the cable box on the roof, run through the lockers, along behind the bed roller cover, and down the passenger side B pillar trim. All cables are hidden and you only need a short piece of cable conduit between the locker and bed roller cover.
I did the above on my 530 and it took me a full morning to run the new cables and install the solar controller. Unfortunately I can't take pictures the moment as the motorhome is in storage until next week.
Hope this helps.
Maybe not a better job but I've fitted 300Wh of solar on the roof so didn't want to take any chances with the pre installed wiring and to be honest I like tinkeringSounds like you made a better job than me although I did the voltage loss calculation and thought it wasn't worth the hassle of running bigger cables unless I thought it was likely I'd fit a bigger panel later.
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I never had a problem without the solar so I suppose I like tinkering too it was actually lockdown boredom that tipped me over the edge!. I think the preinstalled wire was 4mm2 so about 2.5% loss I thought that was okMaybe not a better job but I've fitted 300Wh of solar on the roof so didn't want to take any chances with the pre installed wiring and to be honest I like tinkering