12V Community Garden Sign - Need help with calculations.

[stevehull]

OK - the PV panel puts out 16-17 volts at 6-7 amps and the 50 A (30 A?) charge controller will keep the car battery charged.

Hook it up!

Steve

[stevef22]

10-4 will hook it all up.

thanks for your help! will keep you posted.

[ldjessee]

Hello,

I am curious if your sign is working well and staying lit as long as you had hoped (5 hours)?

Thanks,

[creeky]

60 watts / hr X 5 hours = 300 watts.

It'll take your solar charger (pwm, and acceptable but not awesome) and panel about 2.5 hour of full sun to charge the battery. Your solar setup is okay.

I'd change the battery.

Your as described battery will probably last a year or two with that set up.

A small lead deepcycle at 100 amp hrs would be 1270 watts. You use 300, should last 3-5 years. Don't forget to water. And if you have an equalize setting on the controller use it. The battery will last longer.

A small lithium battery at 40-50 amp hrs would be 600 watts. You'd use 300 which is 50% dod. Hopefully last 10-15 years. Make sure you don't overcharge the battery. Pick a charge setting that charges only to 14.1 is best. No need to equalize/water.

Good luck.

[Sirius]

Steve,

You need to look at Ohm's law. Voltage = Watts X Resistance. Or Watts = Resistance / Voltage. You already know you don't need an alternator to run the LED lights. They run off of DC voltage and your battery is DC. Your charge controller is there to make sure you don't damage the battery. A car battery has smooth cathodes inside which give it a higher "cold cranking amperage". That's because a car battery needs to turn a lot of metal in the car before the engine starts up and makes the alternator run to recharge the battery. A deep cycle marine battery has perforations in the cathodes, which give it more surface area. It's not able to handle the work that a car battery has to put up with every day, but can handle frequent charging and discharging.

Once you know how much "work" your led system needs to perform, i.e. the number of Amps, pick a battery that can handle those Amps. Look at the Watts that the lights are going to need to work. Once you have the numbers for Amps and Watts, you can figure out how much of a solar panel you will need to produce those Volts.

You can probably do with one good solar panel from Harbor Freight, but you might need more than one marine battery. Don't forget about cloudy days. Solar panels will produce electricity even in low light, but if you're having bad weather for a week or so, you'll need more storage capacity to keep the lights on.

[Sirius]

Oh, and I forgot to mention this, the thicker gauge the wire is from the battery to the lights, will decrease the resistance. So use a thick extension cord and keep the length down to the minimum you need to get the job done.

[ham789]

Interesting project.
Please post more pictures to show how you dealt with some issues...
Looks like sheet metal with the "sign" cut out. Very nice job...

How did you address the issue of having something behind the cutout
to defuse the light so it doesn't look like a bunch of led dots?
Ditto for the issue of being opaque enough so that it looks nice
when front illuminated by the sun?
Would also be interesting to see how you mount it all to look aesthetic
yet not be a target for vandalism, theft, weather damage etc.
Looks like a public space, probably owned by the city. What kind
of permits did this require? Did they make you buy liability
insurance in case the solar panel falls on somebody?

I helped with some mountain-top solar-powered radio repeater sites.
They had to provide proof of liability insurance even though the
site was behind locked gates on a mountain top.
The logistic and liability issues were far more hassle than the actual
installation. The wind generator was a popular target for hunters
and hikers. Fortunately, they didn't seem to want to shoot the solar
panels.

How are you going to handle the on/off of the LEDs? Low voltage
disconnect to avoid damaging the battery over a week of cloudy days?

I did some back-of-the-envelope calculations based on info provided
here, what I could glean from the data sheets and a liberal amount
of pulling guesses out of my numbers orifice ;-) Someone will
surely check my math.

I did the math for the 5630 case, but the concept scales.
Looks like the modules want 60ma at 12V. If they're what they
look like, there's gonna be a 52ish ohm resister in series with the
string of 3. About 30% of the power is in the resistor at 12V and
not providing any light.
Depending on the setting of your charge controller and the depth
of discharge you allow, the actual voltage may vary between 11
and 14V. That's 41 to 99ma...over 2:1 in brightness and may exceed
the LED power limit at 14V. How you deal with that may have
significant impact on the battery capacity you need.

At nominal 12V 60ma, you get about 180mW per led.
600 leds is 108W of power actually producing light. The other
36W is just heat.
Nominal light output is 124 lumens/watt x 108W = 13392 lumens.
Divide that by 800 and you get 16.74.

So, the light output at 12V is equivalent to about 17 60W incandescent lamps.
That puts it in terms with which we're all familiar.

And that will vary 2:1 depending on the output voltage of the battery
over the time it's being discharged.

You probably lose half of that in the medium between the LEDs and the
sign that makes it look nice both night and day. And more in the overlap
behind the metal part of the sign, depending on how the size of the
array lines up with the "sign" part of the metal sheet.

Almost all of that 144W shows up as heat. Don't forget to measure
the temperature of the modules at max voltage to see if you need
to heat sink them.

If you choose the dimmer LEDs, I think everything just about halves,
but I haven't done the calculations to prove it.

Are we having fun yet?
Post more pix please...

[jjackstone]

Originally Posted by Sirius

Steve,

You need to look at Ohm's law. Voltage = Watts X Resistance. Or Watts = Resistance / Voltage.

Sirius,
I may be mistaken, but I believe Voltage = the square root of W x R. Then Power = V squared/ R.

JJ

[Sirius]

JJ,

Nope. I double checked before I posted. I wanted to make sure. It's been a few months since I got my HAM license and I had a surgery in that time so we're talking about a fair amount of pain killers in between to deaden the brain cells. Although my math may be off. I know there's no squaring going on, but I may have lumped up the math on what multiplied by what or divided by what. That's why I suggested Steve check it out for himself.

[jjackstone]

Not to be a pain but,,