# LED install in r/c car

Discussion in 'LEDs and Optoelectronics' started by Noobster, Oct 28, 2013.

1. ### Noobster

18
1
Oct 28, 2013
Hi,

I'm trying to fit 8 led lights in my R/c car. I want to power them from a separate battery. I'm not too clear on electrical matters but I've managed to get some of the way using this website

http://led.linear1.org/led.wiz

My leds have a forward voltage of about 2.3volts, forward current of 20mA, and I want 8 of them. So using the website calculator and having a source battery of 9v I have a current draw of 80mA. I don't know how long that will make a PP9 battery last? I would like to use a smaller 3v coin battery, again plugging the numbers into the website it gives me a 160mA draw, but again I have no idea how long the battery will last.

Also I'll probably want to put an on/off switch in the circuit so I guess that will affect the resistance to some extent?

11,141
2,542
Nov 17, 2011
3. ### Noobster

18
1
Oct 28, 2013
Ok. But PP9 or coin batteries don't state their capacity. Is there a way to work this out?

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,480
2,826
Jan 21, 2010
If you know the manufacturer and model you can probably get a datasheet from the manufacturer's web site.

In the absence of that, you may be able to find a list of typical values.

18
1
Oct 28, 2013
Thank you.

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,480
2,826
Jan 21, 2010
Actually, another point that's worth making is that you can estimate the relative capacity of batteries from their relative volumes or weights.

Two batteries using the same chemistry, one twice as heavy and twice the volume of the other is likely to have twice the capacity in Wh. If the battery voltages are the same, the capacity in Ah will be in the same ratio.

This breaks down with very small batteries because the weight and volume of the packaging are a significant portion of their size.