OK, EE's help me out on this one

I never turn my car radio off when I cut off the engine. This causes a lot of problems with my lady friend when I have borrowed her vehicle. She says that she has always been told that when the ignition is started, that accessories that have been left switched “on” “leech power” from the battery, and as a result shortens battery life.

I don’t agree with this, but I don’t know enough about electricity and batteries to argue the point. It seems to me that there might be a slightly higher spike, but the generator is going to keep the battery charged.

BTW, I am not talking about any accessories that are actually using electricity while the engine is off (like headlights)–just stuff like the radio, heater fan, etc.

The car battery will drain slightly even when your car is off (see http://www.autolife.com/freds/fredfaq.html)..)

However, I couldn’t find anything that suggested that having accessories off after you have turned the car off will drain the batteries. Of note, the headlights on on a different circuit than the radio (for example), so it seems to me that this is not true.

It’s bernard, just under new management

The vast majority (never say “all”) of new cars are set up so that when the starter is engaged most non-essential electrical equipment is supressed. Your radio will not work while the key is turned full right (or wherever the ‘start’ position is in your vehicle). Neither will the air conditioner/heater. The headlights will burn, but will dim considerably due to power consuption at the battery. Electrical equipment begins to work once the key is returned to the ‘operate’ position.

The overwhelming majority of people have more than the average (mean) number of legs. – E. Grebenik

In a very old car, you’d want to shut off the headlights before starting the engine. This is because the lights are a big enough draw that, if your battery is borderline anyway, the starter motor might not get enough voltage.

However, cars nowadays don’t have this problem. When you turn the key to “Start”, the car knows enough to shut the lights and the accessories, leaving as much juice as possible for the starter.

On the one hand, therefore, your ladyfriend’s information made sense at one time, but is now out of date.

On the other hand, however, the radio requires a lot less energy than the headlights. Its requirements are so small that even an old car would barely notice.

Regardless of which way you think about it, you needn’t worry that your car is being hurt by your leaving the radio on.

Of course I don’t fit in; I’m part of a better puzzle.

I witnessed a common and very annoying problem with really expensive cars. Often, someone who has a high-line car also has a preference for high tech. I was once the unfortunate stablekeeper for a Porsche 928 S4 that had front and rear in-dash radar detectors, a woofer in addition to the stock twelve speaker system, two cell phones, a police scanner, an accelerometer, and a whole bunch of other shit, all installed aftermarket.

The vehicle could sulfate an oversized Interstate battery in four days, just sitting in the parking lot. Since the battery is located in the trunk, under the woofer and all that other crap, I was less than enthused about this car’s habit. Try changing a battery in a plush white carpeted trunk without getting it dirty–it’s not easy or fun.

I figured the problem was “leakage” from all those gadgets. Aftermarket installers almost invariably wire electronic devices straight off the battery. So I made a removable cap that I could use to disconnect the toys while the car wasn’t in use.

That worked for about two weeks, then it died again. It turned out that this three hundred horsepower monster already had such a large electrical demand on it that its alternator could not keep up with the battery drainage even when the vehicle was driven well above highway speeds for hours at a time (woo hoo! That was the best “test” I ever performed!). Solution: yank all the shit.

So the moral of the story is if you’re going to have a bunch of trinkets added to your car, find someone really, really good to do it for you. I don’t know that guy.

In every vehicle I have worked on (quite a few) the ignition switch’s “start” position completes the circuits for only the starter and ignition (and occasionally emissions controls). The “run” position enables all other circuits. Lighting circuits are usually not run through this switch.

On the other hand, I have also seen more than one car w/ an electrical fault/modification that will let some accessories draw power anyway.

So, both positions are valid. But unless you have noticed a problem in the past, I wouldn’t worry about it.

Did you try a smaller alt pulley (spins faster) or a high output alt? Aftermarket alternators are readily available in the 210-300A range. These have worked for me in similar situations.


Changing just the pulley could be a bad idea. True, it would charge the battery more quickly. But when driving fast, you could make the armature spin at a higher RPM than it can handle. Having your alternator fly to bits in the middle of the interstate is considered a Bad Thing.

If you’re going to swap anything out, spend the extra bucks and upgrade your alternator. It could save a heck of a lot of trouble later on.

Of course I don’t fit in; I’m part of a better puzzle.


You are correct in theory. IRL no need to worry.

The pully swap is the solution if your current alt is almost enough. It will spin the alt only marginally faster. In fact, for most OEM alts the pulley diameter is the only difference between different output ratings.

True, a faster spinning alt will wear faster , but “fly(ing) to bits” is exceedingly rare (read: “unheard of”).

In practice, the belt will slip more for smaller pulleys, defining the lower size limit (diminishing returns).

Finally, well before you overspin the alt it will throw the belt (admittedly a bad thing, but not a Bad Thing).


how about an alternator bursting into flames as you cruise on the highway? It happened once when my wife was driving the car; when we asked what would have caused it, the mechanic just shrugged and said, “It happens sometimes.”

seized bearing, worn brushes, shorted diode, bad regulator, …
Any of a thousand things to cause excess friction or electrical arc. In short:


Concerning Mjollnir’s original question, I think there’s less of a problem with the battery than the radio itself. If you only operate the radio while the car is running, it always gets 14 volts or so. If you leave it on when you turn the car on and off, it gets a wide range of voltages–0, then 12 when you switch to the accessory position, then probably 10 or 11 when you switch to “run” (before starting) then to either 0 again or 7 to 9 when the starter runs, then back to 14 when the car is running.

If I were designing car stereos, I would make sure they didn’t break when exposed to this range of voltages. Unfortunately, not everybody agrees with me, particularly car stereo designers. Most will be fine, but some may fail.

Hey, aren’t you supposed to be at work?

pmh: It can’t be unheard of; I heard of it! Those guys on Car Talk deal with really bizarre problems sometimes.

I once wired a tape deck/car radio directly off of the battery in my first truck. One time, I had turned the volume way down, but not off. The next morning, the battery was dead. Of course, this truck’s battery also died when I forgot to close my door completely, and the dome light killed the battery too.

I’ve had two VWs where the orginal owner wired the car stereo directly to the car battery so you didn’t need the keys in the ignition to play the radio. Had the battery drain twice because I left a tape in the stereo and it was at a blank (no music) part leading me to think that the car stereo was off.

My family once relied on a '50 Chrysler with overdrive; I remember someone leaving the radio on overnight and killing the battery. As a possible solution, depending on where you regularly park your car, you could obtain (from Harbor Freight or J.C. Whitney or the like) a dashboard solar battery charger that plugs into the cigarette lighter socket. Runs $15-$20.

Looking at all of the above, in summary, if the radio (and other accessories) is/(are) properly wired up such that the ignition key, in its (totally) off position disconnects it/(them), it/(they) can’t run the battery down at all. As to radios alone, they don’t draw enough current to anywhere near affect starting, on even a low battery, even if the ignition switch doesn’t cut them out during starting. The cut-out on starting, as it relates to radios, must be for the purpose of protecting the radios (or simply a convenience in wiring the switching off of all accessories (except for clocks and memory settings in radios, etc.)). Furthermore, even a radio left at moderate volume, which radio wasn’t wired to switch out with the ignition-key switch turned completely off, I wouldn’t expect to run down a halfway decent battery over night. What I hate though is certain small lights left with their own switches on, e.g. trunk lights, in cars I’ve had, that are not wired to switch off with the key, will easily run down a battery in reasonable condition overnight. AFAIK, the only current drains left in effect on unmodified modern cars, when their ignitions are totally switched off, are clocks and memories holding settings in radios or other accessories. Such remaining drains are, for all practical purposes, negligible.

Ray (EE, but not a car expert. I defer to car experts on this subject. However, though I know there are many car experts, I have never run into one who was in the business of reparing cars. :0 )