How many solar panels do I need?

[M & J]

I threw my back out last night Rick. But I had to go to work today........   lol  Glad youre better.

[gadgetman]

Solar panes will shade the roof when mounter 2 in off the roof. My back 3 panels are on the luggage rack which is about 6 to 8 in off the roof. You can feel a real difference in the heat of the inside ceiling where the panels are.


I see no need to tilt the panels if you have enough of them flat. Flat works just fine for me, but i dont camp a lot in the winter time either.


The secret is having as many watts as will for on top, more is always better. Also always use poly panels as they tolerate shade better than mono panels do. shade one section of a mono panel and the whole panel goes weak. Do that with a poly panel and it still charges.

[Rickf1985]

Gadgetman, Good idea with the luggage rack. I always just consider that another place for leaks to happen and not really good for anything because who is going to stack stuff 12 feet in the air? Can you say slightly low bridge and it is gone!? Do you have them all the way across and if so then how do you access the roof?

[gadgetman]

They are all the way across and leave me a 14 in walkway, Thats 3 on the rack. The one forward of that I have it set to tilt up so I can walk to the ft when needed. The ft has 2 more.


A third panel was added to the rear. These are Kyocera 130w poly panels

[Rickf1985]

Ok, Got one of the signs stripped down and tested and I am a bit disappointed. I am only getting .6 amp into the battery. This is the original battery for the sign and it is dead, the voltage going into the battery is right around 19 volts so I will need a charge controller. Dave, the last picture is a close up of the board just for you.  I know how you love electronics. This is the controller that flashes the sign, the connections at the bottom that are hidden by the wires are from left to right, Solar panel, battery, switch.

The voltage from the panel going into the board is 21 volts and into the battery is 19.5 volts so there is no control there. I can't believe the batteries can handle that for 10-12 hours on a sunny summer day. I shaded half the panel with my fat head and only lost 2 volts. Here is the kicker, when I noticed how low the amps were, .4 at the time I cleaned the panel and brought it up to .6. The panel was not even visibly dirty.

Caught it with the lights in the flash position.


What I started with.



With the flashing lights turned off, it would drop a quarter volt every time the lights flashed on and recover to this.


Moral support


Here is all that is left.




And here is the close up of the control board for Dave to peruse.


Now my question to you solar gurus is this, do you hook these up in series or parallel and then to a controller? I am thinking that series will give me 120 volts DC and .6 amps which when stepped down by ten would be 12 volts and 6 amps. Real rough figures there, I realize I need 14 volts for charging. My question would be how much would be lost to heat in the step down process?

[M & J]

Is that around your place Rick? You have alot of.......... Stuff. 

[Rickf1985]

You are only seeing a tiny bit of the "Stuff"!

[gadgetman]

Wire them in parallel so the voltage stays at the panels rated voltage and the watts add up. Then buy a 12v charge controller that will handle the wattage. This is the cheapest way to do it. If you wire in series you will need a mppt controller which cost 10 times as much as a small wattage pwm controller. I usually fuse each panel before the parallel connection. The fuse whould be rated at 2 times the panels amp rating. The reason for the fuse on each panel before the parallel connection is for easy testing of each panel when needed w/o the need to open a harness someplace, and any voltage output should always be fused.

Controllers have 2 main ratings, the wattage/amps they can handle for output and the panel voltage input max. Mppt controllers can handle a high voltage in. PWM controllers handle a much lower voltage , usually around 30v max.

Have fun !

[gadgetman]

Here is a idea for you. Take each panel and add hindges so they can fold "flip flop" and all be in one nice stack that unfolds into a straight line. Add a fold out leg to each end panel that lays inside the back of the frame.


Add 1 strip fuse block with enough fuses for each panel, Napa sells those fuse boxes. Now wire each output pos from each panel to the fuse block.  Joun all grounds and install a #10 length of wire as long as you need for the ground connection.  Use a #10 wire from the pos common leg of the fuse block to a controller and then anothe lenght of #10 wire  to the controller output that matches the ground wire length.


Now you have a portable charge station you can hook to anything and all is concealed on the back of the panels  i.e.  fuse block, controller and the rolled up wire. They will all fold and be one nice portable stack.

[Rickf1985]

Now I see where you get your username!
As a side bonus I am getting 8 DOT round Rear recessed LED marker lights and two really bright four LED clear lights out of each of these signs. That will be 48 rear markers and 12 clear lights. I can have fun with this. Hm? W%

Oh, and 6 sealed 12v-22 Ah batteries.

[DRMousseau]

These batteries typically have a usable advertised life of about 3-5yrs and I've never really seen them last that long in a satisfactory manner.

How did you measure .6A here??? I hate coming up with amperage readings without a good inline meter. I noticed your measuring WITH the battery in circuit. Have you taken measures WITHOUT the battery??? Straight panel output??? Careful here,.... panels in the sun are "HOT" with voltage and current! As long as light is hitting them,... they are puttin' it out!!! That .6 would disappoint me too, IF this were the panel no load output.

And remember, this is all DC current. Your not gonna "step down" the voltage with any amperage gain. These are much like battery banks,... and like the Gadgetman noted, if all components are the same, a series circuit will multiply voltage output with same amperage,... and a parallel circuit will multiply amperage output with the same voltage.

A PWM charge controller is usually the best choice, but high voltage and complex systems will require a MPPT controller. FORTUNATLY, an PWM controller can be had far more economically and this is all you really need.

I think these would make really nice USB charge stations. You'd have to control output voltage to 5V and install some USB outlets. Most phone chargers are only 1amp and USB chargers for small laptops and tablets are about 2amps, so a little fold-up bank could have a few outlets for small electronics charging.

[Rickf1985]

I measured the amperage inline between the ground wire and the battery terminal on an almost dead battery. By the way, in three hours it fully charged that battery. I have two of these batteries in a solar bus stop light that have been there for 7 years so far and have never been touched. They are in a metal enclosure that get as high as 160 plus degrees in the summer and as low as -10 in the winter. I would say they have lasted a bit longer than 3-5 years. When I took over the maintenance of one of the buildings I found emergency lights that had never had the batteries changed out in 11 years and more than half of them were still functioning fine. The battery in the pictures is three years old and I just tested it at 14.5 volts, pulled it down to 12 volts to remove the surface charge and waited ten minutes and tested it again and it reads 13.1 volts. Excellent! I can assure you that these sealed batteries last longer than 3-5 years under normal use. Even extreme use.

[DaveVA78Chieftain]

Here is basically what you want to do: http://www.instructables.com/id/portable-20w-solar-charger/

Given you already have the solar panels and batteries, you can put this together for less that $25.

EBAY Controller

As Gadget said, connect the panels together in parallel to increase amperage

FWI: 
The solar panel and battery neg leads are connected together on that board.
The pos leads are only separated by a diode (D3) which is used to prevent the solar panel from discharging the battery overnight.
So yes, full panel voltage is being applied across the battery during the day.

[Rickf1985]

You mean connect the panels in parallel? And I don't see anywhere in the description on the controller whether it regulates the voltage from the 21 volts down to 12? Something this cheap worries me, I have always figured you get what you pay for. Will these cheap controllers prevent overcharging?

[gadgetman]

Yes Rick he ment parallel not series

The cheap controllers will work fine with the wattage you are dealing with. They are ment to take a solar voltage of about 30v and output a 14.5 volt charging voltage.

[Rickf1985]

Thanks, Now I have to decide how I want to mount them. I like both your ideas, the luggage rack and the folding setup. I like the folding setup the best since it can be set up to face the sun no matter which way you are pointed and it can be angled as needed.

[DaveVA78Chieftain]

See, magic!  I did say parallel