A bit of theory #
As you may know the law of ohms exists, U = I * R (this law is needed to understand the problem). The wiring of a Fiat 500 is quite thin and has some transitions here and there that increase the ohmic resistance. This greatly affects the voltage ‘left’ for the light bulb.
A 12V light bulb obviously works best when fed with 12V. When the voltage is 80% (9.6V) of the required voltage, the bulb gives about half the light. This can cause a poor light display and poor visibility in the dark. On the other hand, the lifespan of the bulb increases a lot.
Nowadays, headlights for the Fiat 500 are also available that contain a 55W (60/55) halogen bulb instead of the standard 40W (45/40). The voltage drop over the wiring is then even greater, more current means more voltage drop.
A simple calculation (theoretical) #
Assumption: cabling and transitions have a resistance of 1 ohm (which is pretty good for a Fiat 500)
A 40W light bulb is about 3.6 ohms.
Total resistance is therefore 4.6 ohms. So the current passing through the bulb is I = U / R = 12V / 4.6ohm = 2.6A. So the voltage across the bulb is U = I * R = 2.6A * 3.6ohm = 9.4V
A 55W light bulb is about 2.6 ohms.
Total resistance is therefore 3.6 ohms. So the current passing through the bulb is I = U / R = 12V / 3.6ohm = 3.3A. So the voltage across the bulb is U = I * R = 3.3A * 2.6ohm = 8.7V
So it is important to bring down the resistance of the wiring and the transitions. How can we do that?
Relay and thicker wiring #
One solution is not to use the wiring from the Fiat 500 and use thicker wire directly from the battery. To still be able to switch the headlights on and off with the standard switches, we need a relay. A relay is actually an electromagnetic switch, you can then switch high power with low power. Exactly what we need.
Fiat van der Laan Headlight Power Upgrade #
Fiat van der Laan has created a solution where you don’t have to cut into the original wiring harness. You can place the relay between the wiring harness and bulb via a plug and socket. What remains is to attach the ‘thick’ wire to the battery.
When you turn on the headlight switch, the relay is energised. When the relay is energised, the current goes via the thick wire, through the relay to the lamp (and bypassing the original thin wires). This solution ensures that there is a resistance between the lamp and battery of 0.1ohm or less. Here is another calculation with the new resistance for the wiring.
A 40W light bulb is still about 3.6 ohms.
Total resistance is therefore 3.7 ohms (3.6 + 0.1). So the current passing through the bulb is I = U / R = 12V / 3.7ohm = 3.2A. So the voltage across the bulb is U = I * R = 3.2A * 3.6ohm = 11.7V
Below you see the measurements we have done at Fiat van der Laan. 9.8V without relay (using the normal wiring) and 12.3V with relay. A charger was attached to the battery in both cases. Much brighter light and a better light image were the results.
Schematic #
Below you will find the schematic of the Fiat van der Laan Headlight Power Upgrade. The 0V is attached with a short wire to the chassis. So there is not much resistance over there.
Final product #
Below you’ll see the product we have made. The product can change a bit over time, but the concept remains the same. A 10A fuse is included.
You can order it here: LP7200 (you need two pieces per car)
The difference #
On the first picture you’ll see the headlights without a relay installed. The second picture is with a relay installed. When driving with the Fiat van der Laan Headlight Power Upgrade you’ll notice more brightness and sharper edges on walls (light image).
What customers say #
- “Installed and what a difference!”
- “Much brighter!”
- “Very good idea and it works great!!!”
- “A must have for people who drive in the dark”
- “My halogen is finally working properly now”
- “Without cutting wires, very nice idea”
