Solar battery saver - 100W panel is perfect fit for JSW roof rails

[Diesl]

Due to retirement and the pandemic, I'm not driving much anymore. I already used up one battery (which was already eight years old at that point) early last year, by letting it sit without disconnecting from the car. After sitting completely discharged for the second time, it was dead.

To keep the new battery alive, I attached a solar charger. Initially I used a 25W solar panel behind the windshield, but the battery state of charge was still degrading. I switched to a 100W peak power panel, which is about as large as will fit behind the windshield of a JSW, but after a few weeks with the solar charge controller connected to the battery 24/7, the battery would again be at 12.2V, which is about half charged (12.8V = 100%, ~11.6V = 0%). I could probably make this work by connecting the controller only when the sun is shining, but that seems like a lot of work. A 100W panel should be plenty big, so something odd was going on (in addition to the solar charge controller parasitic current draw).
Today I finally measured the charge current with the panel behind the windshield, and directly exposed to the sun: it was more than a factor two difference in current! So I moved the panel to the roof. It turns out it fits almost perfectly between the JSW roof rails. The charge controller is zip tied to the left rear passenger 'panic handle', so that I can check the readings from the outside.

I'm using a 25A 'uxcell' solar charge controller https://www.amazon.com/gp/product/B071VVRWCD for about $20, and an
' HQST 100 Watt 12 Volt Monocrystalline Solar Panel' https://www.amazon.com/gp/product/B018BMGTTO, currently $80.

JSW_100W_panel_roof_2.jpg

• Diesl
• Apr 23, 2021

100W solar panel between the JSW roof rails

JSW_100W_panel_side_view - Copy.jpg

• Diesl
• Apr 23, 2021

JSW roof mounted 100W panel - side view

JSW_100W_panel_front_view - Copy.jpg

• Diesl
• Apr 23, 2021

JSW roof mounted 100W solar panel - front view

panel dimensions: https://images-na.ssl-images-amazon.com/images/I/51ERFNvRykL._AC_SL1000_.jpg

 

[Mongler98]

usually i hate on stuff like this... but that's actually a really clever and not to omuch more than the cost as a proper tender and an extension cord... and you don't have to run an extension cord out?...... really nice good idea that's practical and (darte i say it) green.... lol its not... but it looks the part, and on a TDI VW... everyone will now know how cool of a hippy you are for saving the earth LOL thats all joking aside... nice job man...
only thing i have to add is that you might want to move that cord around every week or so, so you don't develop a spot on the weather stripping. not sure if it would but that would be in the back of my mind!
how many watts are you pulling on average on a clear day over the entire peak daylight time? ive wanted to do this for my array of various batteries and my mini fridge at my shop because i'm sick of a 10$ a month utility charge for just that!

 

[ticaf]

Cool. 100W is more than overkill for a battery tender though. 25W should have worked.

 

[turbodieseldyke]

The 100w is probably hiding the near-death of your battery. I had a 2W panel on my Mk3 one summer while working out of town, and -- lacking a charge controller -- it severely F-worded my cluster and battery. The battery can now only hold a 6v charge. The cluster is 90% functional, at least. Only the fuel gauge is wonky, and I have a work-around for that.

I hope you live in that one part of that one street in Chicongo where it won't get ripped off. Per Mongler's concern about your weather strip, maybe you can find another throwaway strip to wedge between the open window space.

 

[Mongler98]

Cool. 100W is more than overkill for a battery tender though. 25W should have worked.

for tending.... 10 watts is more than enough... from a good source. i have about 12 different batteries all on tenders... and that's more than enough... 9 years strong on my marine deep AGMs... and still hold nearly the same amps as the day i got them... trolling motors every year.
i would highly recommend you put a solid state maintainer on that or something.... if you have that right to the battery... your going to be killing the battery. i thought you had this connected to something that tends the batteries.... like another battery.....

 

[gearheadgrrrl]

Early TDIs through 2003 at least were shipped with a small solar panel with suction cups to keep the battery from draining while in transit and on dealer lots. Was only about a foot square but produced enough to do the job, I have one laying around here somewhere.

 

[Lightflyer1]

Like this.

VW ICO915 687 Solar Panel/Battery electrical for car with plugs suction cups | eBay

<p dir="ltr" style="margin-top:0; margin-bottom:0;">Vw 1CO 915 687 solar pannel foe car with plug . Condition is "Used". Shipped with Priority Mail . Please look at all pictures it is pre-owned once again please look at all pictures thank you and have a wonderful day</p>

www.ebay.com

Are you sure your lighter plug works when off?

 

[Diesl]

Early TDIs through 2003 at least were shipped with a small solar panel with suction cups to keep the battery from draining while in transit and on dealer lots. Was only about a foot square but produced enough to do the job, I have one laying around here somewhere.

That's interesting. I think the problem with using the 25W panel is my solar charge controller that sucks too much energy when not charging, in addition to whatever load the car is putting on the battery. I also thought the 25W panel should be more than enough. It's certainly plenty to keep my 18Ah motorcycle battery alive, but in that application the panel is out in the open, plus I put a disconnect between the battery and the fuse box.

Anyway, my main point is that there are panels that fit exactly between the Sportwagen rails. And second, that the windshield apparently absorbs 50%-75% of the solar power for silicon solar panels.

TDD, the current battery is new.

 

[JohnTF]

1st off - find the problem , what your doing in covering up a problem .
If there is a problem , not sure about your post , 8 yrs. on a battery is above average .
Then 100 watt panel is overkill to keep a battery charged - check out batteryuniversity.com batteries --- self-discharge-rate [ that rate is gotten from the battery manufacture ] .
Then check many solar / alternative energy sources to learn about solar - and what it can / and cannot do .

 

[Diesl]

John, the problem is me not driving. The car is just sitting there. I think it has been a pretty common issue here in the city for the last year; several of my neighbors are dealing with the same problem. Hooking up a charger once in a while is another solution, but that is easier if you have an electrical outlet nearby.

I checked today, and the panel finally seems to be working beautifully after switching to the roof mount. The battery is back up to 12.7V. The controller is programmed to kick in when the battery drops to 12.6V, and to stop charging when the battery (while charging) reaches 14V. For comparison, the alternator is putting the battery at 14.2V, so this should be pretty safe.

For cars without rails, there are thinner 'flexible' solar arrays; has anybody tried those? It would also allow to fit a smaller array than 100W. I'm not sure though how I would feel about taping or gluing something to the roof.

 

[JohnTF]

Your stilling missing the point , battery should not discharge [ you also did not give any info on how long it sat --- to discharge enough to not start ] .

So unless your letting the car sit for many months [ maybe 6 or so ] the car should not discharge enough to not start !

There is an " excessive draw " discharging the battery .

One of the small dash panels are enough to keep the self-discharge-rate , then you add what the car draws , typically memory for radio .

Those small panels about 5" x 12" are so low of a charge rate - that no charge controller is needed .

 

[turbodieseldyke]

Those small panels about 5" x 12" are so low of a charge rate - that no charge controller is needed .

Considering what one of those tiny panels did to my Mk3 one hot summer, i'd keep a controller on there. Way too many computers in his car he doesn't want fried.

Another option could be to just disconnect the Negative battery cable to drastically slow down the discharge.

 

[Diesl]

John, I appreciate your interest and input, but I assumed 'not driving due to retirement and pandemic' was enough info. I drove on only a few days during that whole year. Three months of sitting, twice, was enough to kill off the eight year old battery. Neighbors have even killed new batteries; didn't want that to happen to mine. I'm sure there are other solutions (like, getting rid of the car...). I could also try to find a solar controller that is less of a drag on the battery.

And disconnecting the battery should absolutely work. Not sure what the radio says after a long disconnect though; doesn't one need to enter some code? When I swapped the new battery in, the radio was ok with that.

Edit: I checked the drain currents:

• According to https://pics.tdiclub.com/data/500/TSB_27_08_04_Matrix.pdf, the car should draw 40mA (give or take); that would drain a fully charged 72Ah battery in 2.5 months; so the battery being empty after a few months of just sitting there is actually expected.
• I measured 75mA, but I did not wait the full five minutes.
• The solar charge controller draws 8mA, so it is not the main culprit.

If car and charge controller together really draw 83mA, then just to keep the battery at its current state would need 24 Wh per day, or four hours of full sun with the 25W panel behind the windshield (worst case factor four reduction; my measurements suffered a bit from varying cloud cover, but were between a factor 3 and 4). This assumes a 100% efficient solar charge controller, which mine is definitely not. I think it wastes the excess voltage, and just delivers the panel current.
So it's no wonder the 25W panel didn't do much to recharge the battery.
The 100W panel mounted outside, which can deliver up to ~5A, should fully charge an empty battery in about 15 hours of full sun.