use Solar Sunroof as a trickle charger for battery

[notorious]

I'm going to install solar sunroof soon into my D2 and I was thinking to install an additional switch somewhere in a car to choose between 'normal fan mode' and 'trickle charge' mode.

That additional switch will allow me to to connect solar battery directly to the following device manufactured by CTEK that basically converts any DC power source including solar cell power into a relevant charging voltage.

http://www.ctek.com/gb/en/chargers/D250S%20DUAL
http://www.ctek.com/Archive/ProductP...%20DUAL_EN.pdf
http://www.ctek.com/Archive/ProductM...%20DUAL_EN.pdf

What you guys think?
What can be wrong with this idea?

P.S. Power rating of FL D2 sunroof with 4 courses of cells is 33,6 Watt.

[Delboy]

I'm not familiar with the solar panel sunroof but from what i ahve seen on other threads its not a complicated device and combined with the charger you shown i don't foresee any problems with it working.

[PsYcHe]

If it puts out enough power, then I don't see why you couldn't splice a charger in with a switch.

[ainarssems]

I have been thinking about this a s well. Couple things to consider is that panel wattage is rated as maximum so it will be for direct sunshine in middle of hot clear summer day when the sun is shining perpendicular to the panel and panel needs to be clean as well. Also panel itself will be deteriorating as well depending on the panel it will loose 0.1-0.8% a year.

So today I decided to measure actual volts and amps going to the fan. It is a clear day with lots of sun but it is at a fair angle and cold. So my measurements were 2.7 Amps and 3.7V which is roughly 10W You would need DC-DC converter to take it up to 14V which makes it 0.64 amps at 90% efficiency. It should be enough to keep the battery charged but unrealistic to charge it up from flat. This was measured on PF 3 row panel so for FL you might get 30% more power.

Also fan is not connected directly to the solar panel, there is DC-DC converter already in the sunroof so to increase efficiency you could hook up battery charger directly to the panel but would be a bit tricky to run extra wiring.

[notorious]

Thanks Anairs. Very useful information.

[ainarssems]

And now couple of hours later sun is a bit lower and angle not so good it measures 1.9A and 2.6V or approximately 5W

[ainarssems]

And now at 16:40 the car is in the shade out of direct sunlight and it is 1.25A and 0.8V

[ainarssems]

Last reading of the day 17:50, slightly cloudy and sun almost set, not directly visible behind the houses, 0.15V & 0.15A

[Goran]

Thanks for those real-life measurements. I no longer have a sunroof but am interested in solar cells generally. That's a really low output voltage, I imagined they had more cells wired up to produce close to 12V, but I guess not, never occurred to me they step up the voltage from the panel.

[ainarssems]

I am not sure how the cells are wired up in the panel and if the DC-DC converter is step up or step down.

Converter is in the sunroof itself and would require trim removal which I have not investigated how it comes off to measure output directly from the cells themselves. I did the measurements at the relay.

Usually solar cells produce about 0.5V each and are wired 36 in series for 18V output . Like I said I do not now how the A8 sunroof is wired but I would not bee surprised if it is also giving 18V from the panel and then using stepdown DC-DC converter to boost output amps but at lower voltage.

My guess is that if they wired fan directly to the solar panel of 18V it would be trying to draw too much amps because of low resistance of the fan and therefore dramatically reducing voltage and overall power output. Ideally you want to balance current and voltage near the max voltage where it produces most power in the region that i have marked with yellow circle in picture below, then use DC-DC converter which would then convert to optimal amps and volts for the consumer.