Flexible panel installation on aluminum roof rack

[Herve]

Hi,

I bought a second solar panel for my roof, the renogy 160 watt flexible panel:
https://smile.amazon.com/Renogy-Extremely-Flexible-Monocrystalline-Solar/dp/B079HJQBVW/

I want to install it on the rear aluminum roof rack (the one tha FWC builds).
I want to install it in a manner that I can remove it in the rare occasion that I will carry something on the roof.

My idea:
- buy aluminum plates so that it create a frame around the panel but leave the center empty (not sure how thick, enough to be rigid, any recommendations?). I am under the impression that it's better that the bottom of the panel is not completely covered by the plate, right?
- glue the panel to the aluminum (any recommendations?)
- drill several vertical holes in the aluminum roof rack and in the plate and blot the plate to the rack (so I can remove the bolts)

What do you think? Would you do it completely differently?

Thanks!

Herve

 

[PaulT]

I mounted a regular frame solar panel to the factory rear aluminum rack.
Because the aluminum tubing is very thin walled, I used two 1.5” wide by 1/8” thick aluminum angle to bridge from front to rear. The panel is mounted to the aluminum angle.

To mount the aluminum angle to the rack tubing, I used long bolts vertically through the tubing with large washers and nylock nuts. It seemed like lock washers and standard would require tightening the bolts and nuts so much that it would distort the tubing. Learned that on the first bolt.

For the flexible panel, I would likely use 3 or 4 cross supports of aluminum angle and use Velcro strips underneath the panel to control any flapping. And bolt the panel’s long sides through some flat aluminum stock.

Just my 2 cents worth.
Paul

 

[ckent323]

Why not use a thin solid sheet of Aluminum and fasten at the corners and edges to the support crossbars?

It seems to me that the Aluminum sheet will provide some support to the panel allowing you to mount the panel with an air gap under it as well as protect the underside of the panel.

If you use adhesive under the panel make sure it is thermally conductive adhesive (many adhesives are not thermally conductive).

 

[Herve]

So is it fine if the whole underside of the panel is blocked by the aluminum sheet?

 

[ckent323]

Sure. Why not? Aluminum is a very good thermal conductor. Air flowing under the Solar panel is good. Solar panels get hot. The panel output tends to drop a little as a panel gets heats up. Further, high temperatures can degrade the materials that solar panels are made from.

The lifetimes of flexible panels has been disappointing so mounting a flexible panel in a way to make it easier to remove is probably a good idea.

Related:

 

[DesertDave]

I had a factory aluminum rear rack on my first FWC.

Initially I mounted solar panels to a plywood sheet and put rubber furniture feet on the bottom of the plywood, and then placed this on the roof when I was camped. It worked, but was awkward.

Later I clamped those panels to the roof rack with some stainless plates I had made up, using the existing mounting holes in the panel frames, and not doing any drilling into the FWC rack frame. That was the install could be removed without trace. It was still on the rack when I sold the camper. I liked the idea of something being reversible without trace. Sometimes you change your mind, sometimes you wish to sell something and buyers usually have a different opinion of modifications.

 

[WillTheThri11]

I like the idea of using c-channel to slide the panel into a slot and then maybe some pins with cotter pins to make it easy to remove temporarily.

I'm curious, why did you decide you needed another panel and what other panel do you have already? I'm looking at getting the same panel you mention, but was actually hoping it would meet all my needs.

 

[Herve]

I'm curious, why did you decide you needed another panel and what other panel do you have already? I'm looking at getting the same panel you mention, but was actually hoping it would meet all my needs.

Today I have:
- 180W panel (not flexible, solarblvd 2 years old)
- CTEK D250SA MPPT charge controller 1.5 years old https://smartercharger.com/collections/on-board/products/ctek-d250sa
- Victron BMV712 battery monitor 1.5 years old https://www.victronenergy.com/battery-monitors/bmv-712-smart
- Two VMAXTanks SLR125 batteries so 250Ah, they are 2 years old https://www.vmaxtanks.com/SLR125-12Volts-125AH-Deep-Cycle-Solar-AGM-Battery_p_38.html
- Iota 120V charger with IQ4
- CTEK also does the alternator charging

Main draw is Isotherm 130L fridge, plus the usual stuff.

I had a single occasion when I drew too much on the batteries: BMV712 says deepest discharge 210Ah 11.32V.
Otherwise average discharge 107Ah and it stays above 12V.

I have no problem with weekend trips but on the 3rd day I start to not have enough battery.

And it takes a long time to charge from say 75% battery left to 100% full:
- about 4 days of solar parked in my driveway!
- about 2 days hooked to 120V!
This seems odd.

I'm still struggling to find the weak link. Trying to add another panel.
But I wonder if:
- the battery are the problem?
- the CTEK is the problem? (would not explain why it's slow to charge with 120V)
- the solar panel isn't producing as much current as it used to (would not explain why it's slow to charge with 120V)

On that second point, the CTEK charging has 5 stages and the first one is desulphation where the voltage and current oscillate wildly. I do see this oscillation in the BMV-712. But I'm wondering if it stays at that stage too long and delays bulk charging too much and because of that doesn't charge rapidely. page 11 https://www.autoelec.com.au/assets/brochures/D250SA-manual-UK-EN.pdf

Any feedback on what could be wrong would be greatly appreciated.

I might try swapping the old solar charge controller to see if it charges faster...
Otherwise I worry I have to replace the batteries already...

Herve

 

[PaulT]

Herve,
With Lead acid batteries, the first 80% charges rapidly in bulk mode. After moving to absorption mode, the battery accepts a charge more slowly and tapers off as the voltage rises with the amps it accepts until the the amps accepted are low enough to meet the fully charged specification when it switches to float mode.

A characteristic of LiFePO4 batteries is that bulk mode continues to a much higher state of charge than lead acid so a 100 aH lead acid battery doesn’t reach as full a charge as a LiFePO4 100aH in the same solar conditions and time.

In other words, your battery behavior isn’t unexpected or unusual, especially since you only get full use of your solar panel output from 75% to 80% and then it takes the charge slower. Adding more solar would likely help only from 75% to 80% state of charge.

You could jokingly say the first 80% of the charge takes 80% of the time and the last 20% takes the other 80% of the time. This is one reason to switch to LiFePO4. Another is that it is commonly recommended to discharge lead acid batteries only to 50% while some LiFePO4 batteries tolerate 100% discharge without too much reduction in life cycles.

Paul

 

[ckent323]

Herve,

If you are routinely discharging your AGM deep cycle batteries to less than 12.3 V (~50% SOC) you are probably shortening their life (sulfation begins to occur around 12.4v according to reports I have read).

However, that lifetime shortening may not be a big issue depending on how often you have discharge/charge cycles every year. Suppose you routinely discharge the batteries to 25% of charge and you only get 300 cycles of life out of your batteries but suppose you only charge cycle them 30 times a year. You will still get many years of use from them (albeit with some degradation in capacity over time). So routine discharge to between 50% SOC and 20% SOC is probably fine as long as the battery is immediately fully recharged before discharging again.


Chronic undercharging of batteries (even AGM deep cycle) leads to sulfation and will also shorten battery life. If you are routinely deeply discharging and chronically undercharging your batteries they may not last very long.


For AGM deep cycle state of charge at 12v is about 25%. Below 11.8v is 0% state of charge.

PaulT's comment on charging time is reasonable. According to Battery University - "During the constant-current charge, the battery charges to about 70 percent in 5–8 hours; the remaining 30 percent is filled with the slower topping charge that lasts another 7–10 hours."

Ref:

https://www.batterystuff.com/kb/articles/battery-articles/battery-basics.html

https://www.emarineinc.com/agm-batteries-marine-battery-and-agm-battery-charger

https://www.vmaxtanks.com/FAQ_ep_42.html

https://batteryuniversity.com/learn/article/charging_the_lead_acid_battery