LED strip lights

[tiggen]

I have a problem that I hope somebody can help with. Let me preface this by saying I have an extremely rudimentary understanding of how electricity works.

I bout some LED strip lights at the local Pep Boys. They come attached to a battery box with a bunch of button cell batteries; I don't remember the quantity or the voltage. Here's a link to a similar product.

These were put on a kart for a 24 hour race so that we could identify the kart as it circulated at night. Before doing so I replaced the button cell batteries with 2 12v batteries, like these, because that's what fit in the little battery box. I also hooked the strips up to a switch so that we could switch them on easily when it got dark.

They worked like a charm for the first hour or two, but over time they faded to the point that you could barely see them, even if you were sitting in the kart.

I chalked it up to low batteries, but later, days after I returned home, I turned on the lights, and to my surprise they immediately went to full brightness and stayed there for over an hour.

So my questions: why do they fade over time if it's not the batteries and how can I rig it so that they don't fade over a 12 hour period?

Hope I explained this clearly, and thanks for any help.

 

[BFBOB]

FIRST, find out the voltage these lights are designed for. It should be marked on the light strips, battery box, or both. If not, load up the battery box with the batteries it is designed for and measure the voltage output. Don't just count the batteries and multiply by the individual voltage. They might not all be in series.
Then, either find in the specs or measure the current draw of the lights. Multiply by the number of hours you need the lights to function, and this is the necessary Amp-hour or Milliamp-hour rating of the battery you need. Say the output of the battery pack is 12V, the lights draw 150 mA and you want them to stay lit for 24 hours. 24 x 150 = 3600 mAh, or 3.6 Ah. You'd be in luck in that case because a very common battery size, used in many alarm systems and easily obtainable is 12V 4Ah.
I guessing that you have just witnessed the phenomenon of battery recovery. Many types of battery will seem to recover some charge after resting a while. They don't really; what they do recover is the ability to output their charge at a high rate.

But first, determine the proper voltage. Too-high voltage will toast an LED just as surely as an incandescent.

 

[tiggen]

They are def rated for 12V DC, b/c they are supposed to be hooked up to a car battery. Am I correct in assuming that the lights likely wouldn't fade when hooked up to a car battery b/c the alternator would be constantly recharging the battery?

 

[BFBOB]

Read previous post. car battery holds lots of Ah's. Enough? Do the arithmetic, get the right size battery.