Cheap Led Lights Fed From Car Battery

April 3, 2011

I am fitting footwell lighting to my Aygo and I am using cheap LED lights available from cut price stores.

I am powering these from the car's Battery and just wanted to share with others who may be using LEDs but continually buying batteries. The LED lights I have bought can be connected to the car's 12volt supply providing a 2k resistor is connected in series with light.

Caution the supply must come through a fuse!

The light shown comes powered by three 1.5v batteries. The batteries lie in a triangular formation.

The power from the car's Battery has to be inserted into an adjacent + and - terminal, by soldering in wires. The negative wire is connected to the car's body.

There are three pairs of + and - terminals. To find the correct ones you will need to measure continuity between each pair.

Two of the pairs are shorted together the other pair is where to insert the supply.

Alternatively three of these lights could be connected in series when no resistor is required.

See later post - use 500 ohm reststor with these LEDs

Good luck.

April 4, 2011

Wiring in series is not recommended, as then you'll lose all light when the on-board voltage drops little below 11V or so...

I think 2k is a bot much to be honest. I usually run single LEDs on 12V with just 1k in series. These white ones will probably

have a higher voltage-drop so you could use even less resistance, especially as they can also have more current usually...

April 4, 2011

Wiring in series is not recommended, as then you'll lose all light when the on-board voltage drops little below 11V or so...
I think 2k is a bot much to be honest. I usually run single LEDs on 12V with just 1k in series. These white ones will probably
have a higher voltage-drop so you could use even less resistance, especially as they can also have more current usually...

I did this excersise properly by measuring the current being drawn by the LEDs as bought.

I then calculated the resistance required to limit the current.

The LEDs now pass the same current on car Battery as they did on their own original batteries!

And incidently I checked your theory with the lights in series (not that I am wiring them in series, but just a passing thought)

I wired the three sets in series and switched on. Started the engine,lights remain on - start a/c, lights remain on - switched on headlights,lights remain on!!!

April 4, 2011

Car usually runs above 13v anyway when on. I run my interior lights from a connector block on an extra set of iso leads on my stereo connectors.

April 4, 2011

A car with a working chargingsystem will run above 14V with

the engine on. But with the engine off it can drop as low a 10V.

I'm not sure how the LEDs inside the lights are wired, but if they're

in series (probably not) you WILL get problems on the long run.

I made some 12V LED-clusters myself, with 3 white LEDs in series with a 120 ohm

resistor and they work fine as long as on-board voltage is above 11V, below

they start flickering, as LEDs don't light below their threshold-voltage...

April 5, 2011

A car with a working chargingsystem will run above 14V with
the engine on. But with the engine off it can drop as low a 10V.
I'm not sure how the LEDs inside the lights are wired, but if they're
in series (probably not) you WILL get problems on the long run.
I made some 12V LED-clusters myself, with 3 white LEDs in series with a 120 ohm
resistor and they work fine as long as on-board voltage is above 11V, below
they start flickering, as LEDs don't light below their threshold-voltage...

Sounds as if you need a new Battery Jan!

April 5, 2011

I have replaced the Battery once on my 5 year old aygo

April 6, 2011

they work fine as long as on-board voltage is above 11V, below
they start flickering, as LEDs don't light below their threshold-voltage...
Sounds as if you need a new Battery Jan!

No I don't :P My car only goes below 11V while starting, once the engine is on I'm allways

running 14.1V or higher... The Battery IS allmost 4,5 years old, but it gets regular exercise:

I make allmost no short runs with my car, so after starting it allways gets charged while driving.

Had a stress-test done at a batteryshop last november and my Battery

measures out just fine, so I'm keeping it as long as my car keeps starting :P

I did a bit of math by the way: are your lights really using less than 7mA while on?

This 7mA is on the high side, as I used 14V in my calculation and only used the value

of the resistor. With the LEDs in series and lower voltage you get WAY less than 7mA,

which I personally find kind of hard to believe.

I'll measure one of my own when I get the time...

April 6, 2011

I think my Battery died because at the time i was running 2 amps plus all my extras on the inside. I hated those 6x9's anyway.

April 6, 2011

they work fine as long as on-board voltage is above 11V, below
they start flickering, as LEDs don't light below their threshold-voltage...
Sounds as if you need a new Battery Jan!
No I don't :P My car only goes below 11V while starting, once the engine is on I'm allways
running 14.1V or higher... The Battery IS allmost 4,5 years old, but it gets regular exercise:
I make allmost no short runs with my car, so after starting it allways gets charged while driving.
Had a stress-test done at a batteryshop last november and my Battery
measures out just fine, so I'm keeping it as long as my car keeps starting :P
I did a bit of math by the way: are your lights really using less than 7mA while on?
This 7mA is on the high side, as I used 14V in my calculation and only used the value
of the resistor. With the LEDs in series and lower voltage you get WAY less than 7mA,
which I personally find kind of hard to believe.
I'll measure one of my own when I get the time...
First photo show LEDs running through 2k resistor with a current of 4.8ma. Experiment showed it could run on even less, but of course we don't have the spec on these LEDs so without that I can only run them at the status quo.
However I have just checked and found the triple A batteries with which I did the original test were a bit flat I suppose I should have thought of that, although in my defence which I put before the court, they had never been used but were found lying in a drawer!
So, having procured some fresh new batteries I find a current of around 19ma being drawn and a much brighter light a quick recalculation finds me inserting a 500 ohm resistor and bingo a current of circa 19ma being drawn. Amen