1 or 2 12V deep cycle and LED lights?

[JoanieBonzo]

Hi everyone-

I hope some one can help! I have a ton (300' +) of LED lights. They are the old variety which look like strips of 3-4 lights on a plastic backing- some look like dominoes. Last year we covered our art cart in them and ran them off a 12 V deep cycle. This year we are doubling the amount of lights. I was wondering if it was necessary to include another battery or will one do just fine. I also have red LED lights. I read on the net that red are different watts than white???? Can I still connect them to the white ones without problems? I really don't feel comfortable calculating Ohm's Law or the like. Any advice would be greatly appreciated!

Thanks- see you on the Playa






______________________________
LED Fluorescent
led ceiling lights
led strip

[Shadow Catcher]

TLI too little information. With out some additional information there is no way an answer can be given. What is the current draw, a VOM from our friends at Harbor Freight or elsewhere can tell you that, we then need to know the amp hour rating of the battery(s).

[kludge]

Correct, you need to know your total current draw and how long you need to run your lights to answer that question.

Ohms law is easy, so don't be afraid.

Volts = Amps * Ohms
Amps = Volts / Ohms
Ohms = Volts / Amps

You will also need to know your power draw (measured in Watts)

Watts = Amps^2 * Ohms
Watts = Volts * Amps

So in your situation...

The current to run your lights in Amps multiplied by how long you want to run your lights in hours is the number of amp hours you need for your battery, generally speaking.

Amps x hours = Amp*hours

There are a couple things to keep in mind when comparing "Amp hour" ratings and "Reserve Capacity" ratings:

The "Amp hour rating" is a low load/low current draw rating and the "reserve capacity rating" is used for high load/high current draw rating.

The Amp hour rating is based on a 20-hour discharge time, and the reserve capacity rating is based on a 25 Amp current draw.

A 100 A-h rated battery will last 20 hours with a current draw of 5 Amps. 100 Amp*hours / 20 hours = 5 Amps.

5 Amps x 12 Volts = 60 Watts

This is what you might expect from a Group 27 battery.

But remember this: under low load the battery will deliver more Amp-Hours than when at high load. So not only do you need to the Amp-hour capacity of the battery you need to know your load when you determine how many batteries you need.

Say you have a higher load of 25 Amps...

25 Amps x 12 Volts = 300 Watts

100 Amp*hours / 25 amps = 4 hours, BUT NOT REALLY

At this kind of high load pay attention to the reserve capacity number, it may only a fraction of the 240 minutes you might expect from the 100 A-h rating.

[kludge]

I also have red LED lights. I read on the net that red are different watts than white???? Can I still connect them to the white ones without problems?

Sorry missed this question the first time through...

White (and Blue) LEDs have a different voltage than red (and green and orange and yellow) LEDs. White LEDs run at ~4 Volts and Red LEDs run at ~2 Volts.

The Watts of an LED depends on how much current you let flow through it... I'll get to that later...

If you have an LED "module", this should have already been taken care of by the manufacturer, as long as you are using it at the voltage rating the manufacturer specifies.

If you have component LEDs you have to put a resistor (or a more sophisticated circuit) in series with the LED to control the current or you will burn them out.

So here's an example... warning math to follow...

The data sheet for a red LED rates it at 2V (nominal) and 40 mA max current. It's a good idea to run things under max and 10-15mA is a good number for LEDs.

So we have a 12V battery. That will have a max charge voltage of about 14.4 V and a float voltage of about 13.4-13.6V, so we want to use the worst case of 14.4 Volts here.

14.4 Volts for the battery minus
2 Volts for the LED equals
12.4 Volts on the resistor

We want 15mA or 0.015 Amps on the LED so...

12.4 Volts divided by
0.015 Amps equals
826.67 Ohms

The closest 5% resistor value is 820 Ohms, so that is the resistor we will use. Now we need to know what size resistor.

12.4 Volts times
0.015 Amps equals
0.186 Watts

So we will use a 1/4 Watt resistor.

The total power used by each LED and resistor will be
14.4 Volts * 0.015 Amps = .216 Watts (approx, to get the real number I have to plug 820 Ohms back in to the equation)

If I have five LEDs then thats a total of 1.08 Watts

BUT there is a more efficient way...

Let's use five LEDs in series

2 Volts * 5 = 10 Volts

14.4 Volts minus
10 volts for the LEDs equals
4.4 volts

4.4 volts divided by
0.015 Amps equals
293.33 Ohms - or the closest resistor value of 300 Ohms

4.4 volts * 0.015 Amps = 0.066 Watts, so I can use a smaller 1/8 Watt resistor

My total power is still

14.4 Volts * 0.015 Amps = 0.216 Watts

BUT now I have five times the light.

Here's a primer on LEDs...

http://www.kpsec.freeuk.com/components/led.htm