Solar panel battery charging experiment

[EcoCivic]

Hello fellow hypermilers, I have decided to do an experiment with my K24A2 swapped 04 Civic to see how well I can charge my Odyssey AGM battery just using a practical solar panel setup without any help from the alternator or an external charger. I also plan to experiment with using the solar to assist the alternator instead of disabling it entirely.

I have calculated that I can theoretically get around 330 watts of power from the roof and around another 80 watts from the trunk deck under ideal conditions with currently available solar panel technology, but since I'm limited to commercially available sizes and shapes and the roof panel would have to be flexible to not significantly affect aerodynamics, about the most power I can practically get would be around 50 watts from the trunk and 200 watts from the roof. Not bad!

For now, I have ordered a 50 watt Newpowa solar panel to put on the trunk, which I'm going to connect to a very nice Victron 20 amp fully adjustable MPPT charge controller I got from a friend. Although very overkill and maybe not ideal for a single 50 watt panel with a short circuit current of under 3 amps, if I decide to add a 200 watt panel to the roof in the future, I won't have to worry about my charge controller being capable enough.

As for weight and aerodynamics, I don't think the effects would be significant if set up properly. The 50 watt rigid panel I'm putting on the trunk apparently weighs around 6 pounds and I don't think the effect on the aerodynamics will be significant at all with the panel only being about an inch tall and mounted flush. A flexible panel flat on the roof shouldn't affect aerodynamics significantly either and although I haven't been able to find any exact weights, a 200 watt flexible panel seems to weigh under 10 pounds. So factoring in 1-2 pounds for a charge controller, wiring, and mounting hardware, an entire 250 watt setup would still weigh under 20 pounds.

Anyways, I'll post an update as soon as the 50 watt panel arrives and I have time to get something set up. In the meantime, thoughts and suggestions are welcome as always!

[freebeard]

Why would you not settle for a solar panel/Bonneville spoiler and have it improve the aerodynamics?

[EcoCivic]

Originally Posted by freebeard

Why would you not settle for a solar panel/Bonneville spoiler and have it improve the aerodynamics?

I'm not sure what exactly you mean by that? I assume you mean make a spoiler out of solar panels? That would be cool, but almost certainly impossible with the sizes of solar panels I have seen and surface area would be quite limited.

The 50 watt panel I got is about 14 X 30 and cost about $50, which is about as close of a match as I could find for my approximately 15 X 45 trunk deck.

The only option I saw that was closer would be using two 13.5 X 22 inch 30 watt panels, but that setup would have cost about twice as much and weighed about 6 pounds more for only 10 watts more, which didn't seem like a good trade off to me compared to adding a 200 watt flexible panel to the roof later on if I end up wanting more power.

At current prices and rated specs, a 50 watt panel on the trunk and a 200 watt panel on the roof would provide up to 250 watts for about $289 and weigh around 16 pounds. That ends up being around $1.16 per watt and 15.6 watts per pound.

Dual 30W panels on the trunk and a 200W panel on the roof would cost about $339 and weigh about 22 pounds for up to 260 watts of power. That's about $1.30 per watt and only 11.8 watts per pound.

[freebeard]

I'm not sure what exactly you mean by that?

Here's a whole thread...

https://ecomodder.com/forum/showthre...ler-14946.html

....but it's hard to find a pickture that doesn't have a watermark.

[Piotrsko]

I would think a comsumption survey would be handy here. If the device producing watts hours is significantly lower than the watts hours consumed, eventually the electical system shuts down to undervoltage

[Blacktree]

This is an interesting project. But there are a couple things you may need to consider.

First of all, 250 Watts won't be enough to completely replace the alternator. But you probably know that. You might need 4-5 times that much Wattage, depending on your car's power requirements.

Also, while reducing the load on the alternator sounds good in theory, there is a downside. The alternator has a "sweet spot" where it's most efficient. If it goes out of that zone, its efficiency drops. So ironically, giving the alternator a helping hand might actually waste power. Personally, I wouldn't bother unless the alternator is being overloaded.

The exact numbers may differ from one unit to the next. But I read a scientific paper where the alternator they tested was most efficient at 30-40% of its max load, spinning at 2000-2500 RPM (that's alternator RPM, not engine RPM).

Instead of "helping" the alternator, maybe you could turn it off when it isn't needed. Sort of like pulse and glide. I think some modern cars do that already.

[EcoCivic]

Originally Posted by Piotrsko

I would think a comsumption survey would be handy here. If the device producing watts hours is significantly lower than the watts hours consumed, eventually the electical system shuts down to undervoltage

I already did that. With the engine running and no accessories on, the car uses around 8 amps, about 5 of which is the fuel pump and the remaining 3 is the ECU, sensors, ignition system, gauge cluster, etc. With no accessories on, I should have around 8 hours of range on a full charge just off the battery.

For most of my driving, I have nothing on but the stock radio, which only draws another couple amps. Turning on my LED low beams adds about another 3 amps, bringing the total to around 13 amps for engine running with radio and low beams on. With a 13 amp load, I should have around 5 hours of range just off the battery.

Assuming a 10 amp load and rated output from the solar panel, I should be getting around 3.5 amps from the 50 watt panel, effectively only drawing 6.5 amps from the battery. At that rate, for every hour I drive, the car would need to sit in the sun for about 2 hours to fully recharge. Real world output will obviously be somewhat less, but without testing, it's hard to say how much less.

The good news is since I don't drive a lot at the moment, I rarely take long trips, I have a very large deep cycle battery, and the car sits outside 24/7, taking a few hours or a day to fully recharge after a trip won't be a problem for me. And worst case scenario, I don't mind connecting my AC charger and/or turning on the alternator, which I will probably wire to a switch on the dash for convenience.

[EcoCivic]

Originally Posted by Blacktree

This is an interesting project. But there are a couple things you may need to consider.

First of all, 250 Watts won't be enough to completely replace the alternator. But you probably know that. You might need 4-5 times that much Wattage, depending on your car's power requirements.

Also, while reducing the load on the alternator sounds good in theory, there is a downside. The alternator has a "sweet spot" where it's most efficient. If it goes out of that zone, its efficiency drops. So ironically, giving the alternator a helping hand might actually waste power. Personally, I wouldn't bother unless the alternator is being overloaded.

The exact numbers may differ from one unit to the next. But I read a scientific paper where the alternator they tested was most efficient at 30-40% of its max load, spinning at 2000-2500 RPM (that's alternator RPM, not engine RPM).

Instead of "helping" the alternator, maybe you could turn it off when it isn't needed. Sort of like pulse and glide. I think some modern cars do that already.

Great info, thanks! With everything turned on, my car's power consumption is around 65 amps and 250 watts is only around 18 amps at 14 volts, so that definitely wouldn't be nearly enough to entirely replace the alternator under all conditions. But for how I drive and my typical consumption of around 10 amps, that should be plenty for almost all conditions. As little as I drive, just the 50 watt panel may end up being plenty since I can count on one hand the amount of trips I have taken in the last several years long enough that significantly draining my 74 amp hour battery would be a possibility.

Interesting, I didn't know alternators have an efficiency curve but it makes total sense. My alternator is rated at 105 amps since the engine was from an Acura TSX, so it definitely has a very easy life powering my Civic with all LED lighting and comparatively very few electronics. I'm likely going to wire it to a switch on the dash so I can turn it on if needed such as for the very occasional road trip and keep it off the rest of time except when engine braking. Completely removing it is not an option on this engine because the same belt also drives the water pump.

[EcoCivic]

I finally got my solar panel and charge controller and wired it up temporarily to test everything and it seems to be working great. As soon as I have time, I will be mounting the panel and charge controller and permanently wiring it in.

Despite the very cloudy and rainy weather we have been getting all week, the 50 watt panel is more than adequate for topping off and float charging the battery all day at whichever voltage I choose. I'll have to wait for a sunny day to test it's peak output and actual charging capacity though.

The Victron charge controller has been working amazingly well too, it was very easy to set up using their app and it seems to be doing exactly what it should. The one thing I'm still fine tuning is the float voltage temperature compensation because it seems like Victron has used a baseline temperature other than the industry standard 25 degrees C since setting the float voltage to 13.5 with the Odyssey recommended +/- 24mV per degree C offset results in a float voltage of around 13.2 in a 25 degree C interior, so I had to set the float voltage to 13.8 to get a float voltage of around 13.5 at 25 degrees C. Time will tell how well the voltage changes with temperature, but so far it's looking good.

[ttrainxl]

Couple things. First, 250 watts at 14 volts is about 18 amps, as stated by someone else here. If your consumption is 13 amps, I’m not sure how you figure you’ll get five hours of range. Maybe adding the peak output of the panels and the reserve capacity of the battery? Second, you will never get peak output from your panels on a car. Driving in shade aside, a panel needs an absolutely perfect angle with the sun to get peak output. It just won’t happen without a sun tracker that physically angles the panels, which would cause major drag. I’d be very surprised if you ever get 10-12 amps.

On my off grid 33 foot sailboat that I live on for four to five months during the winter, I have three 100 watt panels, and while it’s enough to run the fridge and lights, I never get more than 10-12 amps from them, and I’m able to angle one of the panels. And two of them are new.

Also, a 74Ah battery is pretty darn small. My house batteries are group 31s that have a 190 amp reserve capacity. A group 24 is about the size of a small pickup truck battery and their reserve capacities are over 100 amps. And if you’re not using a Li-Ion battery, 50% discharge is a completely dead battery. So you don’t have 74 amps, you have 37 amps. A Li-Ion battery would significantly add to your range because you can discharge them to about 85-90%. For more range, add a couple more batteries.

Also, a solar array puts out amperage at any given time. When a cloud passes over, you’ll see amperage drop by 2/3 or 3/4 very often. It’s very difficult to estimate how many amp hours you’ll be putting into your battery bank. A general rule with boats is to figure your power consumption and build a system that puts out triple what you need. On my boat, on paper, one solar panel should be able to run my fridge, and one battery should keep up when it’s cloudy. In reality, three panels and 570 amps of battery power doesn’t keep up with the fridge when it’s been cloudy for four days, with the temp turned as low as it will go. And I’ve tested the duty cycle and it’s less than 50%. You’ll always have a 100% duty cycle in a car with the engine running.

I think this is a cool experiment and I wish you luck and hope I’m wrong, but I think you’ll get about 30 min of drive time in real world scenarios and have to wait all day to drive another 20 min. Unless it’s cloudy. Then you wait days on end if you can’t plug in.

Still, now I want to try it with my civic too! Haha.