Fitting Spotlights

[His Dudeness]

Apparently 20w (2 x 10w spots) pulls about 1.7 volts from a 12v battery when running, which is sod all.


I thought i'd give my battery a little booster charge last week seeing as the weather was getting very nippy, but when i first took it off the bike and tested the voltage straight away on my bench it was showing as 12.97v (which would be slowly dropping), and once connected to a charger it showed as fully charged in less than 30 seconds, so it was good to know that the electrical circuit and battery was all fine and was coping very well with the spots, headlights and heated grips on a high setting in very cold conditions.

That 1.7V is the forward voltage of the LED or a diode in general. A LED is just a diode that gives off light when current flows through it. You know the way a diode allows current to flow in one direction but blocks it in the other direction? The diode doesn't allow current to flow straight away like a piece of wire does, it needs a certain amount of Voltage drop across it before it allows current to flow through it. That's the 1.7V you are talking about. They call it the forward voltage of the led or diode. That's not the number you should be looking at. The LED spots are designed for 12V so the LEDs where arranged to drop 12V across them. The number you're interested in is the current that they use. They usually give you power consumption in Watts rather than current in Amps but Power(Watts) = Volts x Amps so you can work out the amps by rearranging  Amps = Watts/Volts.  So for the spots they use 20 Watts/12 Volt = 1.67 Amps. That's the number you're interested in. From that you can see how much of a current draw the spots are on your battery, reg/reg and wiring. Or you can size the new wiring, relay, switch and fuse from it. Wiring is rated in how many amps can safely flow through it before it starts to over heat. The wider the wire diameter the more amps can safely flow through it, that's why things that use a lot of power like an electric heater have a thicker wire than say a lamp. If you're tapping into a wire to power extra accessories you have to make sure the wire diameter is large enough to take the extra amps or it could over heat or blow a fuse