#### DBK

Now it's all hooked up, tye voltage is down to 12.6 in the roof, and 12.6 at the input to the controller, my understanding is now that the open circuit voltage (Voc) is that voltage when it's not connected. The maximum power voltage (Vmp) will be the biggest it can give to the controller and the controller sets the voltage at any point in time to just above that of the batteries, thus creating a charge. The MPPT, would do the same just with a few more amps as it does something a pwm doesn't to up the amps as it drops the volts.

Anyway, we have a working 100w panel and PWM controller again, which I will rewire when I move it to make space for the new panel. Thanks again for all your input, I wouldn't have had a clue without you all. You're stars!

Now, I could get the laptop out and get on with my tax return (which is what I was going to do this morning, but needed power), or could have a beer, hmmmmm, decisions!
What the PWM does is effectively couple the solar panel directly across the battery so the battery drags the voltage down. The graph below shows a typical curve for current against voltage.

Power is volts times amps and for your panel maximum power is at 17.7 volts when it is generating 5.7 amps. An MPPT controller will take that 100W of power and generate say 14.4 volts and 7 amps from it. A PWM controller can't do that and the current it can generate could be read from the graph above - if it had numbers on it of course. But at 12.6 volts it might be generating about 6 amps which is about 75W or 25% less power than the MPPT can generate.

OP

#### Em and Tim

Thanks DBK, that makes sense of it all.

Any ideas as to why a cable may corrode like that?

#### DBK

Any ideas as to why a cable may corrode like that?
Damp I guess, copper does corrode. This is why you can get copper cable where the conductors are all tinned, they resist corrosion.

OP

#### Em and Tim

Just to start a new tangent on what has been one if tye most useful threads on here fir me (once again, thank you all for you advice and insight).

I am looking at aluminium profile to attach it to the roof. My thought is T profile is probable the best/strongest option, and I was thinking if going with 40mm by 40mm, and that gives a good surface area for glueing, riveting hinges etc, but as the panel is 40mm deep that gives no air space beneath, in which case 50mm by 50mmm tempts me. That comes in 2mm or 5mm thickness, which I suspect will vary quite substantially in price.
Is 2mm up to the task?
Should I go up to 60mm x 60mm (3mm thick) or even 70mm x either 50mm or 70mm (back to 2mm thick?
Is Sikaflex 292i the right adhesive?
How should I prepare the roof for glueing?

#### HandyAndy

The taller the angle the more rigid it would be so you could get away with the 2mm. Don't forget the solar panel itself is aluminium angle / extrusion so once bolted to the "frame" you make it will increase it's stiffness further.
I'm a classic belt and braces, thicker is better kinda guy (welding is my trade) and Even I would go with 2mm in this application I think.
Also be aware that the internal corners are sometimes not a square edge but radiused, so you might have to allow a little room for that when you want to bolt to a flat face.
Sometimes after making stuff nice and sturdy I stand back and look at it and think fuuuuu that looks agricultural

#### DBK

292 is the strongest, marine grade I believe but a lot of folk have used 291 including myself. I bought their activator to prepare the surface, which I roughened with fine wet & dry paper. I don't think the activator is essential, you can use meths as many have.

An account of my experience fitting a solar panel is here, although it doesn't really start until post #27!

But there are other threads on here which may be more useful.