use Solar Sunroof as a trickle charger for battery

[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.

[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?

[27litres]

When I read this I suspected the power output would be too low for charging.
The interior fan runs at a significantly slower speed when on solar power than on internal power.

Maintenance is a good idea, though 0.65amps isn't much. The CTek maintenance chargers operate at 0.8amps, and like ainarssems said, that kind of amperage will only maintain a battery, it is totally insufficient for charging (unless you have about 6 months to wait, and even then I doubt it'll get you far!).
Even a standard 4amp charger will only get you to about 60% as the batteries internal resistance increases as it charges.

It'll be interesting to see how the maintenance charger mod goes, though as my car spends about 80% of its life in a garage, I'm not sure if it'd be worth while for me!

[Goran]

Originally Posted by 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.


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.

Thanks, so isolating diodes should do the trick. Yes re-reading the page it looks like the rest of the circuit has to be built around the LT3652 module, and the module looks like it has adjustable current and maintenance voltage, with a LiFePo4 battery I wouldn't want it to switch to 13.5V maintenance voltage as that would not charge the battery at all.

Originally Posted by 27litres

It'll be interesting to see how the maintenance charger mod goes, though as my car spends about 80% of its life in a garage, I'm not sure if it'd be worth while for me!

Very good point! I am so silly, since I'm planning to move to somewhere with a garage, maybe this wont work for me. Unless I make a static unit on the outside of the garage to plug into the battery when the car is going to be stationary for a while.

[ainarssems]

I did change relay for switching over to fan when hot with 5V version and it does work reasonably well. Sometimes it makes couple of clicks when switching to fan but it does work. It has been 1 week now and I have been monitoring voltages on the remote control from aftermarket alarm to avoid drain from battery by central locking pump and lights in doors and boot when they are open and so that readings are consistent with car being left for a week untouched. From the past experience I know know that fully charged battery drops from 12.6V to 12.4V in the car if left on it's own for a week. Good news are that now the battery is still at 12.6V after 1 week. I have been checking voltages early in the morning when I come home after work slightly after 6 am so that battery have had a rest during night and settled. The reading has been 12.6 V every morning. By 8 am it raises to 12.8V and is 13V on a sunny day, I have even seen it at 13.2V couple of days. On a cloudy day it takes a bit longer than 8am to reach 12.8V and it remains at 12.8V, does not go any higher.

So all in all the system does work I just need to find a good place for the temperature switch to make it work effectively. I the past I have been putting car on the charger if it does not get used for 2 weeks and voltage drops to 12.2V, looks like with the solar panel connected to keep battery topped up I should get away without need to use external charger.

I will put together a small guide, parts I used and diagram for anybody who would like to replicate it shortly.

Here is a quick picture of remote control showing voltage 13V taken this morning at 09:20, it was 12.6V at 06:30 but has gone up to 13V now.

Even a standard 4amp charger will only get you to about 60% as the batteries internal resistance increases as it charges

There is no reason why 4 amp charger will not fully charge battery to 100% as long as it can provide voltage over 13.2V( normally you need 14V to effectively reach 100% charge level but you can do it with 13.2V as bare minimum it will just take very long time). Pretty much any charger can reach voltages in excess of 15V but most will be limited to 14.8V. In fact most smart chargers including Ctek will limit charging current to 0.8A when battery reaches 80% to reduce water loss from battery.

module looks like it has adjustable current and maintenance voltage, with a LiFePo4 battery I wouldn't want it to switch to 13.5V maintenance voltage as that would not charge the battery at al

It all depends how you configure your circuit, things like maintenance charging, PVM and low current mode are optional extras and voltages are adjustable using different value resistors. The only limitation is that input voltage has to be at least 2.6V higher then charging voltage as it does not do step up. So for a regular 18V panel you can get output voltages up to 15.4V but it will not work with 8V solar panel from OEM sunroof. LT8490 would be more suitable choice for OEM panel but I cannot be bothered to spend extra time and money trying to figure out how to build my own circuit around it.

Unless I make a static unit on the outside of the garage to plug into the battery when the car is going to be stationary for a while.

What's the point of that, you still need to connect cables to the car, might as well just use regular battery charger and save the extra expense and hassle for solar panel on the garage.