Solar assist: Vehicle roof top solar panels

[oil pan 4]

Yesterday was overcast all day and the solar panels with MPPT charger still put some charge into the battery.
Have 50 watts installed now.

I can only install 1 or 2 panels at a time because the weight of the lead shot I secure the panels with is just too much for the roof. If I install them in a stair step formation I should be able to do 3 at once.

This is the general wiring scheme.
The 3 amp automotive blade type fuses are actually located close to the terminal board. The are to protect the genasun from input short circuits. I read the manual and this charge controller can not tolerate input short circuits.
The solar panels could care less if they are shorted.
The 10 and 15 amp input and out fuses put are also blade type automotive fuses and are located at the input and output terminals on the charge controller.



I drove home last night then by 1100L the next morning the charge controller was showing full charge with only 50 watts worth of panels installed.
Looks like I need to get that grid tie going.
The suburban after the alt delete is using less power than I originally figured and the solar panels with MPPT charge controller are producing more power than expected.

[freebeard]

Lead shot? Roof?

If the panels are installed horizontally do you have an opportunity park next to a South-facing white wall? Or could you have hinged, tilt-up reflectors?

[oil pan 4]

I have 90 watts installed as of today, which given the dreadful weather forecast for Monday and Tuesday is 40 more watts then I was planning on adding.
I am thinking that 50 watts is all really need for just going to and from work.
With the MPPT charger and over paneling I don't need to worry about not getting enough power.

These solar panels are sitting flat, just by them selves.
In the summer while at work I plan on parking next to a building to block the sun later in the day.

The bags of lead shot squish the RTV that holds the panels on nice and flat, also ensuring a good seal. Only problem is 3 bags weigh up to 75lb.
Setting 3 panels at once requires around 200lb of shot placed on the roof (some of the bags are no longer 100% full).

You don't have to use heavy lead shot. A sand bag should work just as well.

[oil pan 4]

After a little 40 mile drive last night I checked my solar system while it was charging at noon.

90 watts worth of panels were generating 17.7 volts and producing 2.3 amps.
That is 45% of rated, not bad for laying flat with some dust on them.

The genasun charge controller is doing its MPPT thing, with that input power of 17.7v at 2.3 amps it is putting out 13 volts and 3.1 amps.

That's almost 41 watts going in and just over 40 watts coming out. If you try to argue with that, you will lose.

If this were a cheaper PWM controller I would only be getting 2.3 amps or less.
So if I were using a PWM controller I would have lets say best case scenario, 13 volts coming out amps 2.3 amps. Or 30 watts.

With flat dusty panels running at 45% of rated, having a MPPT controller instead of PWM controller is like having PWM controller and 4 to 5 additional panels.

I would have to say having an MPPT charge controller makes a huge difference if you only have limited solar panels on your vehicle.

So with an MPPT controller I only need 90 watts worth of panels to get 40 watts of power.
With a PWM controller I would have needed 130 to 140 watts worth of panels to get 40 watts.
Most vehicles don't have room for 90 watts, so that makes a lot of decisions for you as to what your panel and charge controller configuration will look like.

So at this time, if you put solar panels on a car, SUV or most vehicles, the 5 amp genasun LiFePO4 controller for around $100 is the way to go.
Yes I know it says its for lithium iron phosphate, but I am using an optima Absorbed glass mat battery. I have LiFePO4 batteries, I just have not installed them yet.
The 14.2volt LiFePO4 charge controller will obviously work with LiFePO4, it works flawlessly with lead acid AGM and it should work well with conventional flooded, sealed lead acid and gel lead acid.

[freebeard]

Thanks. That's good to know.

[oil pan 4]

Just incase I am able to install more solar capacity then what the 10 amp genasun can handle I am experimenting with a way to rig a 15 amp Morningstar MPPT lead acid controller for LiFePO4.
You might be thinking "I should just use this 15 amp Morningstar for my lead acid battery instead of the lithium specific 5 amp genasun". True, only thing is the price difference between the 5 amp LiFePO4 genasun and the Morningstar sunsaver tristar is at least $140. The price difference alone would pay for most if not all the solar panels you could put on your car.

The main reasons you do not want to use a lead acid charge controller on a LiFePO4 battery is the cell equalization and temperature compensation.
Equalization is when the battery voltage goes up to like 15.2 Volts or more once every 4 weeks or when battery voltage drops below 11 volts at rest.
Temperature compensation is where more volts are added to the charging voltage for lower temperatures. Lead acid batteries benefit from this, LiFePO4 batteries only ever want steady 14.2 volts all the time, hot or cold.

There are 2 ways I am going to try and accomplish this.
#1 this Morningstar has a switch to disable equalization, so leaving it off solves half the problem right there.
#2 get the charging voltage right. I may be able to just set it for gel batteries (14.0v even) or use the sealed batteries (14.1 volts) setting and forget it.
But then there is the pesky temperature compensation.

Lucky for me this Morningstar has an optional remote temperature probe for the battery.
I believe this temperature sensing element is a thermistor. Which means it has a fairly high resistance value that changes a lot with temperature.

Temperature compensation as stated by Morningstar on this unit is about 2 millivolt per degree F. Doesn't seem like much.
But this part of New Mexico has a about 120'F temperature difference between the coldest winter nights and hottest summer days.
This charge controller shouldn't hurt a LiFePO4 battery with active temperature compensation, but I like to be thorough. Plus the gel and sealed voltage wont quite get the battery up to full charge on a hot day with temperature compensation.

So I what I am going to do is get the remote temperature sensor, measure its resistance at 70'F and stick a resistor equal to that value in the remote temperature sensor terminals. Then resell the probe on ebay, since I wont need it for anything else.
If I can lock in the 14.1 volt sealed battery setting all the time that would be perfect.

[oil pan 4]

Using solar to replace all electrical power for an alt delete defiantly not practical for everyone.
My little 40 mile night drive took the better part of 2 days to recharge the battery.
Most of us will still have to use an external charger at night to keep the battery up and use the solar during the day to at the very least lessen the depth of discharge for the return trip.
With my short 12 mile round trip the electrical system was easily getting a full charge off only 50 watts worth of panels.

I have 90 watts installed, have 50 more watts to still put on and then I may expand up to anywhere from 180 to 220 watts.

[freebeard]

How many watts per square foot of collector?

[oil pan 4]

These panels I found are rated for about 13 to 14 watts per square foot.
Actual amount of power going to the power grid will be about 1/3 of that.

[vasonline]

Hi,

The Epsolar Tracer solar mppt charge controller has a menu option
to disable the temparature compensation.
(I do not use it in my car )
Maybe yours has that option too?

I switched temp compensation off because I wanted to automatically switch on
a load at 14.5V I was kinda difficult to design an electronic circuit
vith the voltage changing a little ever so often (measured it).
Just to give you an idea.