I have a 12v string of leds that has a power rating of 7W. If I have a 12v 35W battery will it power my leds for 5 hours?

10/15/2012 11:14:42 AM

What's the amp-hour rating on the battery? 35W means your battery is good for ~2.9A at 12V, which is more than enough for your 7W LED string (which should require ~0.6A at 12V), but it has no bearing on how long the battery will last. The battery should indicate somewhere a rating along the lines of milli-amp hours (mAh) or amp-hours (Ah) or something similar.

Based on a draw of ~0.6A (per your LED specs) and a life of 5 hours, you'll want your 12V battery to have a minimum 3000mAh (3Ah).

10/15/2012 12:50:07 PM

If the 12v is constant, isn't 35W the same a 2.91A since P=VI?

[Edited on October 15, 2012 at 1:12 PM. Reason : .]

10/15/2012 1:06:26 PM

That's correct, and that's where I got my numbers, but that's what the battery is rated at for a maximum - if you throw a 1,000 ohm resistor across the leads, you'll only get 12 milliamps out of the battery, with a power consumption of 0.144W. Thus, the 35W is a rated value - typically, if you get higher than the rated values, the internal resistance of the battery can cause excessive heating within the battery itself.

10/15/2012 1:23:40 PM

^ Exactly. LEDs aren't resistive loads, so you'll want to use a current limiting resistor between your battery and the LED string (else you'll burn out the LEDs or melt your battery).

Here's a tutorial:

http://www.sparkfun.com/tutorials/219

Basically you just calculate the resistor value via Ohm's Law so that you're only delivering the rated wattage to the LEDs. Pretty simple.


Also, the 12V isn't constant . . . . batteries have a discharge curve. AKA the voltage will go from 12V down to 10ish as the battery is used. For LEDs it really doesn't matter much, unless you're using lithium-polymer or similar rechargeable batteries that can be damaged if you let them discharge too low.

10/15/2012 3:50:40 PM

Not trying to be a dick, but considering the question, I'd imagine the LED string was prefab and probably already has a resistor built in.

10/15/2012 4:26:44 PM

^true.
You can get a 3000 MaH 3 cell (11.1 volt) Lipo battery at an RC Hobby shop. You will need a cheap charger too.
11.1 is the nominal voltage. When fully charged it will start at 12.6v and slowly bleed down. Just don't get the lipo down below 9V because you will damage the battery. If your LEDs go out then you went well below 9v.
http://www.raleighhobby.com/ will be able to hook you up.

10/16/2012 1:21:24 AM