Bike lighting using 18V battery pack - Help!

Thread Starter

Zorch

Joined Jul 14, 2008
4
I am putting together a lighting system for my bike using some 18V rechargeable batteries I have.

I am running a pair of 12V LED lights and a small power inverter that runs on a pair of AA batteries.

Sorry for the quality of the diagram (its been a few years since my last electronics class) Can someone tell me what the resistors at A & B should be?



Any other tips are more than welcome too.

Thanks!
 

SgtWookie

Joined Jul 17, 2007
22,220
That isn't enough information, I'm afraid.

What is the rating of the LEDs? Forward voltage @ current?

Your inverter running on two AA batteries probably won't last more than a few minutes unless your 12v LEDs have very low current draw.
 

SgtWookie

Joined Jul 17, 2007
22,220
OK, so about 83mA @ 12v.

Looks like you're trying to run the LEDs in series, but your inverter isn't hooked up in series with the battery.

If you're running the LEDs in series, you will need > 24v.

What is the output voltage and current of your mini-inverter?
 

Thread Starter

Zorch

Joined Jul 14, 2008
4
I figured that I would be able to run the LED lights directly off the DC current (bypassing any sort of inverter). The inverter that is there is made for my 15' of El wire - i don't know if i would trust it to run anything else!

I'm not opposed the adding one if it is necessary/more efficient - but for time and equipment cost sake I would rather keep it as simple as possible
 

SgtWookie

Joined Jul 17, 2007
22,220
OK, have a look at the attached.

The LM317's are adjustable voltage regulators rigged as constant current regulators. You might get by using 1.5 Ohm resistors, but you'll be pushing the limits of the LEDs, shortening their lives.
Note that the resistors are 2 Watts. That's a bit more than the amount of power to be wasted in them, a hazard of using linear regulators. But, this is the easiest way to power your LEDs, and ensure good brightness untill your battery is discharged to around 15v - by that time it's really dead anyways.

For the 1.6 Ohm resistors, you could use several smaller-wattage higher-resistance resistors in parallel, such as four 6.2 Ohm 1/2 Watt resistors. That would give you 1.55 Ohms, which would be close enough.
 

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