Solar panel to offset alternator load
 

I was interesting in solar panels some time ago, but the 1-5Watt ones never peaked my interest as 5w divided by 12V equals about 1/2 an amp and they were costly 20-50 dollars a panel. No point.

I recently found about peltier coolers, but those dont generate much. I though about hooking up 10 to the top of the radiator, enough heat (~180*F), though I am not sure if it will work with out a heat sink on the other side, will it?

Recently I found a flexible 100W solar panel on amazon for $200 bucks, almost twice the cost of the conventional flat ones. I need it flexible to place it on the trunk of the car, plus its only a few MM thick. Bonus! and not aero penalties. 100W is about 8amps. I am unsure how many my car eats, but I estimate a good 10. And I've been reading the constant drain/no alternator thread, so I am guessing this will be a 10% improvement/ 3mpg increase. What do you guys think?

Okay calculated that if mpg goes from 30 to 33. It would take 16.6K miles to pay back the 200 bucks. Or 3 yrs. (by the way, wasn't there a calc around here that would calculate when a mod would break even?)

Would you get 10% even by totally removing the alternator? Thats a pretty large chunk of power.

No way you will get 10% better FE by eliminating the electrical load.

No way you will get the full 100W by placing the panel on your trunk.

The 100W rating assumes the panel is facing directly at the sun, and it's a bright and cloud free day.

If a 100W panel is added to the car, you will want it to switch off with the ignition, otherwise you will fry your battery when the car is parked and not loading the solar panel.

It's not so easy.

I tired a 40W panel for about six months. Most I ever saw out of it was 1.1A (facing it directly at the sun at midday) or about 15W. On the vehicle, the most I ever got was 0.8A or 11W.

I used an MPPT charge controller to make the most of the available current. Problem is as soon as you start the engine, the voltage jumps to ~14v and the controller detects this as a full battery and shuts off the panel while driving.

In the end all it did was help keep the votage up during EOC, and it took about 30mins on a sunny day to top up the battery (to float charge) after the engine was shut off.

I was planning on fitting a total of 400W of panels (I had the space) and deleting the alt, but the head gasket blew and the MB100 projects got canned.

The other issue is solar panels get extremely hot and transfer heat to the paintwork of the car (paint doesn't like being constantly baked) and onto the interior. Even with a 10mm airgap the underside of the roof was much hotter than the areas adjacent. A shame as solar panels work best in hot climates.

My 40W solar panel now lives on the garage roof and is used as a maintenace charger. A small ~10W panel wouldn't be a bad idea, pariculartly if it can be mounted in free air (eg a rear spoiler or kammback), but it would be mostly to prolong battery life.

The controller would have also prevented overcharging & ruining the battery when parked.

But you could have switched your system to bypass the controller when the alternator was running - you would have lost the efficiency of MPPT, but avoided the problem of it detecting 14v and sitting on the sidelines.

Had the project contiued that's what I would have done along with an alt. kill switch to turn off the alt. just prior to parking it (so the solar panel would have had more to do).

I understand your concerns, but I have read a review on the specific solar panel which a user a state north of me -placed the panel on the ground and got 70w and I think at lease 20w when cloudy. Seems like a new gen one and not one of those boxy ones. Did I say it was 3mm thick? Let me link everyone and we can further discuss. And lastly, reply to your last point, I am. I know how batteries, especially my AGM doesnt need 18V to charge it. My batt charger charges at 15.5v, my car does 14.9 (which I think is a tad high), a honda does 14.3 (pretty darn good), spec for AGM is about 13.4volts. So first I'm trying to find how to step down the voltage. Thanks for your input.

Some of this was answered above and I chose the 100W flexible because it's practically the size of my trunk lid. 40 inches by about 26 inches. THe thing about heat...hm. Let me think about that. It's an external type aka not hatchback, so it shouldnt affect me, and most of the year its 40-65*F, so I dont see another 10*F being a problem as the paint probs gets up to 90 in standing sun (correct me if I'm wrong). Did I say its not a conventional one aka glass frame, so I dont know what your 10mm will do.

Aerodynamically its nil. My stock cd is of .37. Put the top down it goes to .44, but sq ft is 18, but still 3mm wont upset the terrible transition the convertible top has against the trunk, this is not kamback. =)

Ive seen flexible 100 watt panels on ebay, however they are in rolls that are 16 ft long, a foot across. Id have to run that length wise across my car bumper to bumper.

I saw go for it, panel to battery using blocking diodes if they arent already built in. Im using a power supply to supplement my charging system, 150 watts and got 6 mpg in my honda. I recharge the batteries at home.

https://www.youtube.com/watch?v=XmZ4EfGJRqU

I also used the semi flexible type panel. I never saw anywhere near the rated output, I live at 32* latitude. Mounting angle is not going to be in your favour on a boot lid, and cuts your output by around 30% right there compared to angled house roof mounting.

The paint temp will depend largely on the car colour. On my white roof the temp difference was about 30*C on a cool day. I was really surprised by this and it basically killed the idea of solar roofs for me :( I had a 10mm gap because the roof had ridges in it. This heat build up also reduces your panel's output.

Keep in mind that elevating the boot lid should improve aero, so you could build a height extension for the boot lid to mount the panel on. If you can find a boot lid cheaply I'd get one and mod it. Keep the original for returning to stock. Again, couldn't do this with a roof!

So what you're actually saying is based on the likely actual output of a 100W panel (never mind the lower MPG of a Miata), silvermazda should expect a 1MPG gain.

I'd like to see him do it, but he should have realistic expectations going in.