# Can an alternator cause components to run at a higher-than-rated voltage?

Given that the voltage regulator on my motorbike is set to 14.3 volts (so that it charges the 12 volt battery effectively) and given that the alternator is what provides power for the electrical system while the battery is charging, does this mean that the components in the system rated at 12 volts are being run above spec (ie. at 14.3v)?

Is this safe or could it shorten the life of the components (which are mainly regular incandescent bulbs and LED bulbs)?

Part of me thinks that the designers are relying on the voltage drop caused by the long runs of cable in the wiring loom to effectively reduce the voltage to near 12v by the time it reaches any components.

As Kevin said, anything intended for automotive use will have this taken into account. These systems may be commonly described as "12V" to the public, but it's well known the voltage will be higher.

Designing electronics for automotive "12V" power can be a challenge. The fact that the battery charging voltage, and therefore the running voltage most of the time will be up to nearly 15V is no big deal. The electronics does have to be designed for a wide enough average voltage range, like 9-15 Volts if you want it to operate over most conditions.

The much tougher part is that spikes of 10s of volts can happen regularly. For example, you might be tempted to use a ordinary 7805 regulator to make a low current 5V supply from automotive 12V power, but it might not last very long. There are special regulators specifically designed to tolerate the high voltage spikes.

Another issue is that even the 14.3V you measure may only be a average over 100s of ms. Old fashioned alternator regulators were just a thermal voltage switch that turned parts or sometimes all of the field windings on/off. They might only vibrate a few times per second. You got average 14.3V, but the peak to peak variations could be substantial. Modern regulators switch faster and react faster, so this is less of a problem with newer vehicles.

Yet another issues is the extreme temmperature range. The same circuit has to work in Phoenix after being parked in the sun for 8 hours and northern Minnesota when starting on a winter morning.

As for the ordinary LEBs (light emitting bulbs) you ask about, this doesn't shorten their life because they were designed for this environment and the quoted lifetime already takes this into account. You should get the life it says on the package. However, you should get substantially longer life from a car headlamp if you were to only run it in a lab with a well regulated 12V power supply. Of course that's not much use in lighting up the road in front of where you are driving.

This is safe, and will not shorten the lifetime of those components. This is not because a component rated for 12 V will not be damaged by 14.3 V, but because it's actually designed for that rating.

I'm not sure why your bike charges at 14.3 V, but the automotive standard is 13.8 V. Neither of these is 12 V, that's just a nominal value.

The real question is whether an automotive part designed for 13.8 V will be damaged by 14.3 V. The answer is "No", the 13.8 V power supply that automotive parts expect is very noisy and fluctuates a lot, and any well-made part will tolerate the difference.

• This is correct. When designing electrical components for automotive use, the nominal voltage is considered to be 13.8V, and brief spikes up to 40V are expected. Sep 20, 2011 at 10:56

The voltage drop in the cables is going to be negligible or the cables are very likely going to be melting. For example in a common household $0.75 \mbox{ } mm^2$ cable, you'll need around 24 meters of cable to produce 1 V drop at current of 1 A. Automotive cables are generally thicker and will have much lower resistance.

The main thing here is to take a look at the component tolerances. The 14.3 V for lead-acid battery charging looks good, if the battery has 6 cells.

For the light bulbs and LEDs (there's no such thing as LED bulb!) things get more complicated. The over-voltage is going to shorten the life of those light sources, but the main problem is estimating by how much. Usually filament bulbs can sustain higher voltages and the impact on it should be minimal.

The impact on the LED could be higher, but that mainly depends on the LED circuit. The simplest calculation used needed current for LED and the diode's voltage drop to get the resistor needed to limit the current. If the diode is set to work at its maximum current on 12 V, then you could have problems with its lifetime. The good thing is that in most cases that won't happen. Precise resistors are more expensive than low precision resistors and the tolerances of diodes in each batch are a bit different, so often there is some room left to maneuver. Unfortunately, we can't provide you precise information unless you post pictures of the LED circuit. The other option is that there is a dedicated LED driver which will provide constant current to the diodes regardless of input voltage (if it is within tolerances). In that case, diodes themselves won't be damaged by higher input voltage, but the driver will dissipate more heat which will negatively impact its lifetime. How big impact will be depends on the driver itself and I can't help you without the model number.

Another point worth mentioning here is to double check the regulator. Most alternators will produce around 13 V 15 V at idle and there's a high chance that the regulator will go out of regulation. Are you sure that the regulator is providing 14.3 V?

One more point worth mentioning is that on automotive systems, voltages are very problematic and it's not uncommon for them to quickly change. Are you sure that the devices you use are actually 12 V rated or are they designed for 12 V systems? There are other popular reference voltage systems, such as 24 V and the 12 V on the automotive component may very likely mean 12 V system. In such case, the components will actually be rated at values higher than 12 V and should have no problems with higher voltages.

• It is not uncommon for a 12V automotive system to have spikes up to 50 or so volts. Regulators and drivers have to be able to cope with these voltage spikes, so in general, items designed for 12V automotive will have internal regulators that can remove these spikes and regulate down to the voltage they need. Jun 14, 2011 at 23:14