Solar charging to 12 V battery?

[Jyden]

Having done some extra reading on the HSD system in Toyotas hybrids, I got the thought, that I could use solar modules to feed power to the auxillery 12 V battery, which - as far as I understand it - provides the power for lights, fans heated seats etc, but not the AC-system.

It seems to me that the 12 v battery gets charged form either the HV battey or the ICE depending on conditions.

So charging the 12 v battery via roofmounted solarmodules and a 12 v solar charge controller should get some power to the 12 v battry, that the ICE or HV battery then shall not deliver, thus save some energy, thus get a bit better milage dues to running the ICE less for charging HV battery?

Any thoughts?

Links on the HSD system:
Hybrid Synergy Drive - Wikipedia, the free encyclopedia

http://techno-fandom.org/~hobbit/car...f/thsiicon.pdf

[Cobb]

I think phack has done this on the hood of his prius.

[RedDevil]

I got a 9W solar panel just for that.
Turns out my 12V battery was bad. Hooking up the panel would cure it slightly, but not completely.

It got replaced under warranty, and now my poanel is too big for the battery. It would gently overcharge if I left the panel on.
In the northern center of Europe you'd get about as many kWh per year from a solar panel as its peak production in Watts. So my 9Wp panel can theoretically produce 9 kWh per year over here.
That may save me, maybe, 6 or 7 liters of gas a year. Ideally.
Of course the panel is not light. And solar energy does not distribute evenly over the year. The net yield would be less. Even if the battery could store all the panel provides.

I believe a solar panel for the 12V battery does have benefits, but you won't be able to prove it by fuel consumption measurement; it is too insignificant.

[redpoint5]

The fuel savings will never offset the cost of the PV, and never give a measurable increase in fuel economy. There are many problems with this:

1. Cost of PV
2. Engineering the PV to the contour of the roof
3. Reduced aero efficiency due to solar modules on roof
4. Increased weight
5. How much solar radiation do you even get in Denmark?

I bet there would be a greater fuel savings by replacing the heavy and inefficient lead acid battery (50% charge-discharge efficiency) with a lightweight LiFePo4 battery (90% charge-discharge efficiency).

[Cobb]

Red, you hadnt tried driving with the panel plugged in vs not?

The insight uses a dc to dc converter that maintains the 12 volt system and more loads causes less mpg, less assist and more regen to take place.

[oldtamiyaphile]

I have a 40w panel on the MB100 roof. In reality, if I aim it directly at the midday sun I get 16.8w. On the flat roof I've never seen more than 11w out of it (roughly enough to run the stereo).

With the alternator connected, the panel only takes 30mins to fully recharge the battery after the engine is turned off. I would really need to swtich off the alt some time before parking so that the panel would actually have some work to do. The amount of time I could run without the alt would depend on how long I plan on being parked, but if I then have to park near a tree, power pole etc that could cause a problem.

Currently, the panel is mostly wasted as the alt keeps the battery very close to full, but it should be very good for battery life.

[RedDevil]

Originally Posted by Cobb

Red, you hadnt tried driving with the panel plugged in vs not?

The insight uses a dc to dc converter that maintains the 12 volt system and more loads causes less mpg, less assist and more regen to take place.

I have not hooked it up at all. I have a dedicated lead-acid charger for that.
It could charge the old battery all night at 0.8 Amps, twice the panel's output.
A mppt solar charger could help but it really is more expensive than the panel was.

The other reason is that I have not found out yet whether the panel has a diode to prevent it from draining the battery at night. And I'm not sure where to fit it.
Maybe I'll take it apart and rearrange the cells in a long strip at the base of the windscreen.

I could get a small 12V (~ 13.4 V) LiFePO4 battery and charge that with the panel, hook it up with a diode to the battery.
I see the voltage switch between around 13.85 Volt and 12.65 Volt, depending on what the DC converter is doing. In the latter case the extra pack would aid the 12V battery.
It would help, marginally. We're talking a few Watts here while the power consumed is around 10 kW average. You'd need kiloWatts to get a noticeable difference.

If the LiFePO4 pack nears empty its voltage will sag, but it cannot sink below the 12V battery so it won't ruin itself.
I wil put cell overcharge protection in just in case.

[Jyden]

Due to losses in the ICE and system in general, I would think that power produced on the solar panel directly to the 12 v battery would be worth the double or more as the power would be produced without any losses.

So it seems, that the system and controllers ECU etc. CAN handle power beeing fed "backwards in to the system? No problems with that experinced?

Not overcharging the 12 volt batrery would be taken care of, by putting in a chargecontroller between PV panel and battery. This also prevents power going back from battery to PV panel.

As the leght the Prius can cover purely in EV mode depends among other things on how much power is at hand in the EV battery and also what amount of power goes to drive systems fed from the 12 volt battery, it seems to me, that feeding power to the 12 v battery from PV will stretch the amount of pure EV driving I can do.
For instance if I can cover the power drawn from the driving lights more or less via PV, the ICE stays off longer?
A controller can be had for about 10 bucks and small flexible solar modules kan be bought for 10 buck's each, and I would need 56 of them to make a 28 watt's roof mounted thinn laminated module bringing 20 volts to the controller.
0.5W Flexible Solar Cells of Amorphous Silicon Can Foldable Very Slim Solar Panel 2V 250MA For Diy Phone Charger Free Shipping-in Solar Cells, Solar Panel from Electrical Equipment & Supplies on Aliexpress.com

If we assume, that it can produce 28 kWh a year, and that it replaces a fuel amount of twice that, (50% effeciency in ICE to 12v battery) it is 56 kW, with 7 kW in a liter of gas that amounts to 8 liters a year....
If I assume 15.000 km's driving a year at avarage 22 km/l => 682 liters.
It would give me a 1,17% improvement in milage ?
If 25 km/l it would give me 1,33% better milage

Maybee time to look at a bellypan for more bang for the bucks?

[RedDevil]

If you want flexible solar cells take a look here: EV-Power | Solar Panel GWL/Sunny Flexible Mono 60 Wp (MPPT 18V)
That would do

A liter contains about 9 kWh of raw power, but the engine typically wastes more than 75% in heat. So a liter yields 2 kWh, tops. Need to convert that to electricity, another loss. Electric power from the engine costs almost an euro per kWh in Europe.

You may actually regain the investment by feeding the battery with solar power... but I think it is best to feed a piggyback battery, like I described. And you can charge that from the mains before you leave in the morning, so you use the battery capacity to the max.

(I want to do a high-voltage parallel pack to boost the hybrid pack, but the 12V setup may be a safer testing ground)

[Jyden]

Thanks for the link. That panel would be perfect, and could easyly be glued to the roof and cable feed in the bach hatch to a controller mounted in the trunk with leads going to the battery.
60 watt's should do the trick.
Theres not 9 kW in a liter of gas. You are thinking diesel. Less energy in gas than diesel. 11.8 kWh/kg x 0,7 (density) = 8,26 kWh/liter (even less with alcohol in it) Assuming a 25% effeciency due to all the systemlosses as you say. It starts to look as a 2% imporvement in milage, if one can use all the power produced by the panel, and that probably not possible.

Not much for the piggyback thing. Too much trouble in daily life. As the car is still new, I don't want to work on the big EV battery. Would like to be feeding in power as I drive to extend range in EV mode. For the time beeing, I belive this is better done to the 12v battery.

But grounds for an experiment I think. Would look cool and stock with that panel glued on to the roof