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How does the electrical system work?

I participate in an online forum for the Mitsubishi I MiEV electric car. There is a debate raging about how the 12V side of the electrical system works. My side is that all accessories like the radio and headlights run off of the 12V battery and is recharged by the DC/DC convertor. This is just like a gas car where all the accessories run off of the 12V battery and the battery is recharged by the alternator. The opposing side says no, this is incorrect. In a gas car all of the accessories run off of the alternator and the 12V battery is only used to start the car and afterwards its totally useless, the same goes for the 12v in the electric car. In the electric car, this opposing side says the accessories are directly hooked up to the DC/DC convertor and the 12V battery is only used to engage the drive system.

I’ve worked on cars and motorcycle for a long time and feel I know how the system works. Are they any experts that can clarify this and explain how the electrical systems work, in either the gas or electric car?

In a non-hybrid car, all accessories, the battery, and the alternator are connect to the same circuit. When the engine is running, the alternator raises the voltage above that of the battery’s voltage so that the accessories’s load is met by the alternator. Residual charging goes into the battery as well.

In the unlikely (but not unheard of) event that the alternator can’t supply the current demands of the accessories, then that current will come from the battery. This is more prone to happen when someone installs an ‘over the top’ megawatt stereo amplifier. In that case, they can be driving down the road draining their battery as they’re driving.

I’ve also seen some of the higher wattage inverters used on cars result in dead batteries. If the inverter draws more than the alternator can produce, the rest of the current demand comes from the battery.

I can’t speak for how hybrid or MiEV vehicles work, but I suspect it’s similar.

Hi Joe,

Okay, so it sounds like the battery and alternator run in concert to provide the necessary amperage since voltage should be relatively constant right? So, powering that megawatt stereo would take 12v (+/-) X the Amperage to get the Watts (Ohms Law). Anything over what the alternator puts out would be absorbed by the battery.

So, if the Alternator goes out then the accessories are running off of the battery until it is depleted and that is when we start to see brown headlights and dim dashboards? The battery acts as a buffer between the electrical system and the alternator plus the amp needed for firing the starter or in my case the charger/inverter and dc/dc convertor.

Thanks for the clarification.


Not sure how it is with hybrids but on a conventional car the battery acts as a filter when the car is running.

The alternator puts out a pulsating signal that well exceeds 14V in the peaks of those pulses. In the valleys, the signal may very well be close to zero - certainly way less than 12V.
The mean value of that wave is more in line with 14-ish volts we’re used to seeing.
The battery absorbs (“integrates”) those peaks and valleys to average it out to about 14-ish volts. That’s why it is never a good idea to take the battery out with the car running to see if the alternator is good, a practice that was common on old cars: the new cars having electronic circuits are designed to only withstand a certain supply voltage. The peaks on a generator/alternator far exceeds those voltages. Because the rise times are fairly fast, you get something called ‘ringing’ on the peaks as well, which basically is a signal riding on top of that wave that can be in excess of several hundred volts. The ringing is fast so quite brief but can damage circuits or certainly age them.
So that’s why you need a battery. It filters all that mess out.

Of course, it is a little more complex with there being a regulator in the circuit, ect, but that’s the basic operation.

@MLucas - your last post does a good job of summarizing how the electrical system works, except that I’d rephrase one sentence like this: "Anything over what the alternator puts out would be supplied by the battery.

You’re right, as are the responses above. @RemcoW does a good job of explaining it, I’d use the word “buffer” as opposed to the word “filter”.

If they say the battery is useless after starting the car, have your friends try an experiment to demonstrate. Take any late model car, start it and turn on the electrical accessories, remove the battery and drive around for a while. See what happens.

RemcoW, there is one slight misstatement in your post. The alternator is a three phase AC generator with a full wave rectifier, that is the reason there are so many diodes. And for that reason, the output from the alternator is actually fairly smooth, even if the battery was not in the circuit. There is only about an 13.4% difference between the valleys and the peaks. This is why they are so much more efficient than the old DC generators.

Each coil of the generator is 120° apart, so there is always at least one coil that is at least at half voltage, COS of 60° is .5. After full wave rectification, there is only 30° between the peaks so the minimum voltage is about 86.6% of peak. COS of 30° is .866

Other than that very minor but common misconception, very good post.

What @keith said!

@Keith, yup you are correct. I was more thinking of a generator, I guess.

The current from the alternator is smooth but on some vehicles the field control/voltage regulator response is too slow to maintain a steady voltage.

In addition to the 3 phase output, the claw pole design further reduces output variation/ripple by producing a more square than sinusoidal waveform.

Alternator output also varies with RPM (and heat). At engine idle, with enough load, an alternator may be operating in deficit to the overall demand. The battery provides power during these situations as well. I had to go to an undersized pulley on one of my hot rods to get enough power from the alternator at idle to keep from draining the battery too much. It had to be sized right so it didn’t over rev the alternator at WOT, a balancing act…

Hot rods? Nice! What do you have, Twin?

Newer Crown Vic cop cars have a 200 amp alternator that will deliver almost 100 amps at idle. It’s not very big…It has a free-wheeling clutch built into the pulley to prevent snapping the serpentine belt should the driver lift his foot off the gas suddenly at high RPM…If you rev the engine up a little and turn off the key, you can hear the alternator spooling down like a turbo!

Thanks everyone for all of this help. This is great.

So, it does sound like everything is connected to the battery. When I’ve had my hands in the innards of a few cars and some motorcycles, this is the way it seems to be. This is also why we can run accessories in the ACCY key position when the engine is off and listen to the radio (Yes/No?).

To clear up another point. I’ve read that the Alternator is only running 50% of the time, to reduce the wear. Is this true as well? Or just Internet mumbo-jumbo?

From my readings in the Owners Manual, Mitsubishi states that all of the accessories are running off of the Starter Battery. I don’t recall reading anything about everything being connected directly to the DC/DC convertor. It seems like for efficiency purposes, that the 12V starter battery would only need to draw power from the convertor as it is depleted. Not be constantly drawing power from the convertor.

Again, thanks for everyone’s replies and help with my question.

“To clear up another point. I’ve read that the Alternator is only running 50% of the time, to reduce the wear. Is this true as well? Or just Internet mumbo-jumbo?”

The alternator is always running when the engine is running. The regulator keeps the alternator output at what the system is demanding, which is usually far less than the alternator’s maximum output. Running an alternator at more than 50% of rated output will kill it in short order. Maximum output is only for recharging the battery after a starter crank or a heavy electrical load.

Look at it this way. You can certainly run up a flight of stairs in 5 seconds, right? You could probably do it 2 or 3 times. But how far up the Empire State Building could you run before you just couldn’t go anymore? An alternator may be rated at 140 amps, but it can’t do it for very long.

MLucas, ask your friends this: if all the acessories in a gas powered car are driven off the alternator and the battery is only used to start the car, how is it that you can turn the key to ON without starting the engine and the accessories will all still operate?

Post back with their responses. The verbal acrobatics should be interesting enough to make even John Stephens proud.

I just find it amusing that someone named Lucas is posting a question about electrical systems;-)

@doubleclutch Lucas has not really treated me any worse than delco etc, but yes that is irony.

I (but who am I) would have to agree with akk the above. The idea that the battery is ther just to start the car is Incorrect.

Hey, I just thot of something. How would a radio, lights, door locks, power windows, ETC work if the engine wasn’t running, if they ran off the alternator???
Excuse me, I now see that this has already been addressed. I had read only the first page.