# How to automotive/boat devices limit voltage to 12 volts?

I live on a boat and get all my electricity from 12 volt battery arrays. The batteries are charged using either 1) alternator on the diesel engine or 2) charger/ inverter powered from shore power at a dock or a portable generator. Yeah, some 120v power while on a dock or running the generator but not usually.

Most everything onboard is powered 12 volt DC, but in reality it's actually fluctuating from 12.4v to 14.6v. So how do electronics like chart plotters, SSB radio, am/fm car radio tolerate these changes? Obviously they do it, and hopefully efficiently.

Is there a simple device to do this?

So far all I could find is " consider a simple PWM driver with mosfets controlled by a microcontroller, and figure out the correct pwm voltage correction empirically and be done with it. " which sounds interesting but doesn't do it for me.

• Look for a 12V voltage regulator. Mar 23, 2017 at 16:36
• There are literally millions of variations to regulate voltage if you look for every application, depending on power and current. Often a current source to a voltage reference diode is used then output is scaled down and compared to reference. Many linear IC's also use "controlled current sources" with R ratios to amplify so as to inherently designed to reject supply variation. Also known as power supply Rejection Ratio PSRR. Yet many still are sensitive to voltage somewhat. Mar 23, 2017 at 17:02

Devices designed for automotive environments have a wide input range that covers the typical automotive 12V rail. They spec the input at 15V on average. Most cars have a nominal voltage of 14.6V while on and charging via the alt.

They work fine at these raw fluctuating voltages. Some like motors or fans do not need regulation. Others, like radios or computers, require cleaner signals so they internally regulate down/up and filter the noisy 12V rail as best they can.

If you need a regulated 12V input, use a low dropout voltage regulator, or a switching regulator.

• LDO isn't going to work when the battery voltage falls to 12.0V. In situation where regulated 12V is needed, he'd need a proper buck/boost switching DC-DC converter. Mar 23, 2017 at 17:03
• @volo if the car falls to 12V while on, you have bigger issues to worry about. Mar 23, 2017 at 17:10
• While cranking in cold weather the battery voltage may drop to 8V or so. Automotive electronics is designed to work over the range of app 8-16V. Mar 23, 2017 at 17:36
• @Kevin while cranking the only thing that should be getting power is the starter. Which is why most radios cut out while cranking. And at 8V, I expect your battery to be dead. 10V won't start my little Toyota in good weather let alone winter. Mar 23, 2017 at 17:51
• @Passerby In a modern car a great deal of circuitry still has to operate while cranking. All of the computerized engine controls for example. 10V may not start your car but that is probably not because the electronics have stopped working. Mar 23, 2017 at 17:55

Automotive voltage is often referred to as 12 Volts DC when it is actually 13.2 volts DC. The voltage is determined by the chemical make of each cell. The chemical make-up produces 2.2 volts DC per cell multiplied by 6 cells. Many auto alternators charge at the rate of 15 volts. Equipment designed for automotive operation is also designed to deal with these variations. A bigger problem to consider would be the damage that can be caused when a battery is allowed to fall below 12 volts DC.

There are multiple ways you can achieve 12V regulation:

1. Buck-Boost Converter
2. LDO (Voltage Regulators)
3. Voltage divider (That is never used... Very inefficient)

Let's focus on the buck-boost converter since this is probably the best way to supply 12V continuously over a range of time. A buck converter will convert high input voltages to a fix output voltage whereas a boost converter will convert low input voltages to a fix output voltage. A buck-boost converter is a combination of both converters described in the past couple sentences.

As you can see in the circuit above, there is a switch controlled by PWM that will control the voltage on the load. Here is how the current will flow depending whether or not the switch is open or close in the circuit.

Note that the 12V can also be regulated using a LDO. In this case, it can be useful since there is not a major voltage drop across Vin and Vout. Therefore, the power dissipation through the LDO is much less. Sometimes, dropping the voltage from 12V to 5V, care must be taken since the LDO may overheat and destroy itself.

• While useful, op question is really about automotive devices, which you don't address at all. Mar 23, 2017 at 17:52
• @Passerby The storage batteries have nothing to do with the "automotive" devices. It's more like the deep discharge batteries used in a wind power system. Which are nothing like the engine's starter battery. The battery charger could be powered by a generator driven by the engine. Mar 23, 2017 at 19:04

This question is easy to mis-understand since there are so many possible devices.

For the parts of the devices that care (mostly the digital electronics) the internal circuits will be operating at typically 5 volts, or maybe even 3.3 volts for modern self-contained circuits. You might also find some parts of the circuits running off 9 volts.

All of these are easy to generate from your battery bank (nominally 11V to 16V) with either linear or switching regulators. Since the regulators are inefficient, it makes sense to only regulate the parts of the circuit which are sensitive. This means that the backlights, etc, might run off the unregulated inputs.

Amplifiers (and to some extent radio transmitters) can effectively act as regulators themselves so might not require such a carefully controlled supply rail.