I generally work with 12V standard and simple DC wiring. Awhile back I put a bunch of LED lights on a bicycle for a Youtube video, though I ran them off of a single 9V battery. I would like to run two 9V batteries, but seeing as how this will result in 18V, I'd rather not blow out my lights.

Where and what size resister should I use so that everything works as it should?

Here is pretty much how the bike is set up now, if this gives you any idea: https://www.youtube.com/watch?v=iNgIvaooP88

  • \$\begingroup\$ LED datasheet(s) and schematic would be more helpful than a Youtube video. \$\endgroup\$ – Null Nov 10 '14 at 17:12
  • \$\begingroup\$ There's no benefit to raising the voltage and then wasting the extra power in a resistor. \$\endgroup\$ – Dave Tweed Nov 10 '14 at 17:15
  • \$\begingroup\$ If you have a system that works for 9V and want to increase how long it will last putting more 9V batteries in parallel would be the way to go. \$\endgroup\$ – Gorloth Nov 10 '14 at 17:16
  • \$\begingroup\$ Even better - use a battery designed for higher drain devices - typical 9V batteries have very little power for their size/weight, even compared to cheap alkaline AA batteries. \$\endgroup\$ – Grant Nov 11 '14 at 0:02
  • \$\begingroup\$ well, here is my next question. in light of what you have mentioned about 9 volt batteries, is there like a box that I can buy (no idea what this would be called) where I could hook double A batteries up to and make 12 volts, and this item would then have a positive and a negative lead that I could wire into my bike's existing wiring? that almost seems like a better option overall? \$\endgroup\$ – ThunderHead289 Nov 11 '14 at 0:56

If the LED strips work from 9 volts, just connect the two 9 volt batteries in parallel - this will give you 9 volts, with twice the usable running time.

If you connect the batteries in series, then use a resistor to drop the voltage to 12, you will be wasting a third of the battery power in the resistor.

To determine the appropriate resistor, you will need to know what current the LED strips draw. With that information, you can use Ohm's law ( V = I x R ) to determine the required resistance, and the power formula ( P = V x I ) to determine the power rating of the resistor. (I in both calculations is current, in Amps.)

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  • \$\begingroup\$ can I use some tool of measurement to determine the led light strip draw? also if light sections were to burn out, would it alter this value? if this is the case, I should most likely stick to 9 volt. I just know they can be brighter than they are since I run a lot off 12. \$\endgroup\$ – ThunderHead289 Nov 10 '14 at 17:39
  • \$\begingroup\$ Use a multimeter (DVM = Digital Voltmeter) to measure the current. Yes, the current will depend on the number of LED strips that are working (and even on the number of LEDs in a strip). \$\endgroup\$ – Peter Bennett Nov 10 '14 at 17:45

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