use Solar Sunroof as a trickle charger for battery

[Goran]

Ah, yes, that makes more sense, a higher voltage stepped down. I didn't know about the peak power curve, hmm, wiring up these cells is more complex than I thought.

[ainarssems]

Managed to move sunroof trim out of the way so could do a bit more measurements today. It's not a good day as it's fairly cloudy and no direct sunshine so the output from solar panel is rather weak. DC-DC controller in the sunroof says 8.2-8.4V and 30W max on the label. The voltage from the panel was 11.4V without load and 8V with the load. Voltage from the controller was 8.4V without load and 1.5V with the load. Did not measure any amps as there was not much sense without nice sunshine it would be very low anyway.

[Goran]

That's a fairly decent voltage coming fresh from the cells, is it really necessary for the controller to reduce it so low to 1.5V?

[ainarssems]

I think it is because of low resistance of fan motor. I do not know what is the exact value but fan has 40A fuse so allowing for some reserve margin say if the fan draws 30A at 12V it would be 0.4 ohms. This resistance will vary depending on rpm as the inductive resistance will be added as well but for sake at argument lets assume resistance will be 0.5 ohm on stationary motor at the moment when power is first applied.

I still need to verify the voltages coming from the panel at good sunshine but lets say panel produces 8V load voltage and gives 3A of current for a total of 24W at best case scenario. If you put 0.5 ohm conductor at the output of panel it will try to pull 16A from it but as the panel can supply only 3A max the voltage will drop dramatically and overall output will be reduced.

I have modified the previous picture to show what I mean.
You would go from point A producing 24W to point B producing only 4.5W with 3A @ 1.5V. DC-DC converter with 90% efficiency allows to take that 8V 3A input to convert to 3.29V and 6.58A (21.6W which is a lot better than 4.5W when hooked up directly). Or if the sunshine can only produce 10W of power at 8V and 1.25A it would go down to 1.25A and 0.62V at 0.5 ohm resistance for 0.77W, DC-DC converter can convert it to 2.12V and 4.24A for 9W.

This is very simplified version with fan motor stationary with the least electrical resistance. As it starts to spin up inductive resistance will get higher so DC-DC converter output voltage will go up and current down maintaining the power.

[ainarssems]

Did some more testing.

Bright sunlight 12:10 when sun is highest at 40 degree angle, output from panel 8.2V under load and 4.2V and 3A from DC-DC converter with the fan connected and running. So it is definitely 8V panel, output from converter is 8.4 V without load but voltage drops with load to optimal voltage/amps depending on power available from the panel and load at the same time keeping output from panel at least 8V at all times to maintain highest possible power. I have tested it with varying load and highest I got out of it was 2.2 A at 7.7V =16.94W.Not to forget that sun was at 40 degree angle with solar panel being near horizontal position which means that it was only getting 64% of solar radiation from what it would get at perpendicular position (so would be 26W if it was perpendicular which seems plausible for 3 row panel).

For comparison in mid June midday sun would be at 60 degree angle and get 86%, midday Christmas it would be 14 degree angle and 24% of radiation. And this is the most efficient time midday and with clear sky. With the clear sky about 70% of solar radiation reach ground level, on a cloudy day it is about 10% so it is about 7x difference between clear day and cloudy day.

Bad news is you cannot use off the shelf solar chargers as they are made for the most common 18V panels so they try to keep voltage above 17-18V in the similar fashion like the DC-DC converter in sunroof keeps it above 8V. So they will always think panel is overloaded and reduce duty cycle expecting voltage to go above 17V until there will be no current flowing. There are couple of solutions,- modify off the shelf solar charger to maintain 8V rather than 18V, design your own charger or just use simple variable DC-DC converter in series with existing converter. I will look into modifying off the shelf charger like this one: http://www.ebay.co.uk/itm/1214502709...%3AMEBIDX%3AIT
But for testing purposes I have used this DC-DC converter: http://www.ebay.co.uk/itm/2009739061...%3AMEBIDX%3AIT
I was able to get it to supply peak 0.6A @ 14.4V output from converter but I had a big voltage drop to 12.5V in the wiring as I just twisted wires together. Will do proper soldered connections and thicker wires for less voltage drop for more testing. 0.6A peak was at very bright sunshine but as the sun reduces a bit input current increases at the converter and it goes out of efficient band, starts to heat up very quickly and output drops to 0.3A and it does not recover by itself even if power supplied from panel increases again, it still remains 0.3A going to the car. I did attach heatsink to the converter to keep to cool but that does not help to recover power. If you disconnect it for a second it does recover to 0.6A. I think to keep the efficiency it is best to adjust it to 0.3A. I had a fairy flat battery with 12.2V resting voltage, after connecting solar panel it quickly raise to 12.4V and to 12.5V after 2.5hrs and later when the charger was disconnected it remained 12.3V so there was some charge going in the battery. In the afternoon about 16:00 car was in the shadow behind house and I only had 0.05A going in the car.

I think i will use thermal switch like this one http://www.ebay.co.uk/itm/KSD-9700-T...item418f872329 with relay similar to this one: http://www.ebay.co.uk/itm/2718146544...%3AMEBIDX%3AIT
so that it is charging battery by default but if temperature in car gets above 35C it will switch from charging to operating fan, then switch back to charging when temperature drops below 20C. Will have to test temperature and possibly go for different value temperature switch. I will also be using isolating diode to prevent from current going back in DC-Dc charger at nighttime and discharging battery.

There is still plenty of of testing to do to determine best way do to this but the whole concept seems to be viable even if you can only get 50mA going to the car on cloudy day while it will not charge the battery it might just keep the car from drawing current from the battery and slow it's discharge. This is probably pointless for somebody who drives car every day and does more then 5 miles on the journey as the alternator will do the job keeping battery charged. But for me where S8 is only 'Sunday' car it should relive myself from the duty to put car on charger every 2 weeks or so. Like I said there is more testing to do but it looks promising and should be even better with the later 4 row sunroofs.

I think at this point I will proceed with more testing and will update when I have final solution. In the meantime if anybody have some ideas I will be happy to hear them and discuss them.

[brasiliangringo]

Ok i need a solar powered aerator for some carp i will be putting in the pond - anyone have any experience ?!

[notorious]

Originally Posted by ainarssems

so that it is charging battery by default but if temperature in car gets above 35C it will switch from charging to operating fan, then switch back to charging when temperature drops below 20C.

Excellent. Following thread with interest !

[ainarssems]

So finally connected everything as I wanted today, it was 17:00 hrs when I finished so very little power generated by panel. I did run into issue while testing temperature switch. I heated switch with hair dryer for it to switch over the relay to fan but as there was very little power delivered when fan is connected voltage drops too low and relay switches back, voltage raises again and relay engages again so it keeps clicking all the time. Normally temperature will not rise if there is no decent amount of sunshine so hopefully it will work OK with plenty of sun. If not then it is not dealbraker issue, it will just wear out relay sooner and it is making noise. If it does not work I will try either 5V relay or maybe go for transistor to do the switching.

The real test is to see if it will keep battery charged. I have Yuasa 88Ah 730CCA battery which I bought 6 years ago from local parts shop for just under £70 at the time, conductance tester shows battery being 61% health and 630 CCA at current state. Fully charged battery at current condition just taken off charger shows 13.2V which drops to 12.6V overnight, then to 12.4V after week and 12.2V after 2 weeks without driving car. At this point it takes 2 days to fully charge battery with automatic charger from Lidl which delivers 3.8A up to 80% charge and then 0.8A to 100%. I do have OEM phone, Satnav and CD changer which I believe all are getting power when ignition is switched off but using very little, I also have aftermarket alarm. I did charge battery for last 2 days and ideally I would like my current arrangement to keep battery fully charged at 12.6V. I don't think I will need to use car in near future so I will try to leave it on it's own for 2 weeks without starting and see what the results are compared to 12.2V what it would normally be if there was no solar charger connected.

[Goran]

Very impressive! I no longer have a sunroof but I would still like to install some sort of solar panel on the rear hat shelf to keep my battery topped up.
I just came accross this, I'm not sure if its a complete ready-to go charger which just requires a solar panel, and what happens if the solar panel cant supply 2A.
http://www.electronicsweekly.com/new...ently-2014-03/

The circuit itself seems fairly cheap.
http://www.futurlec.com/LinearTechno...52EMSEpr.shtml

Forgive my ignorance, what happens when the car is running, do you disconnect the solar panel from the battery charging mode?
How do solar cells react to having the voltage of the alternator applied accross them?

[ainarssems]

If the input drops below 2 A output will drop as well, it should provide maximum available amps up to 2A output. So if you have 0.5A and 17 V coming from panel, that's 8.5W with 90% efficiency it will be 7.65W available at output =0.53A @14.4V. If there is 2A @18V available from panel= 36W x0.9 efficiency = 32.4W /14.4V= 2.25A but will be limited to 2A by charger.It's a good find but if I understand correcttly it is only IC for sale, you need to build the circuit yourself and the whole schematics are there just for reference. Resistor values R4 and R5 are for regular 18V panel but if you change those values you could get it to work with 8V panel on A8 sunroof as well.

Forgive my ignorance, what happens when the car is running, do you disconnect the solar panel from the battery charging mode

I have used isolating diodes, not so much to stop power from alternator going to the panel as the DC-DC converter only works in one direction anyway but more to stop it from draining battery at night time when there is no sunshine as the power smoothing capacitor on output side on DC-DC converter would consume little amount of power from battery without isolating diodes. But they provide extra isolation against power going back to panel as well.