From the moment it was installed, the radio has a fairly loud humming noise, in addition to the music.
When the car isn’t driven for 2-3 days, the battery goes dead. It will get started with a jump pretty easily, and after running for ~1 hour is fine. I would expect this to happen if the car was left along for over a week, not just a few days. This has happened three times.
In that time, I’ve had the car battery tested 2-3 times at AutoZone. Each time, the readings are fine and the connections and installation looks fine. The last time, the theory was that there was a short somewhere that was leeching more electricity than usual, and that this could have been caused by installing the battery with the wrong process (negative before positive), though there would be nothing visible.
Does this make sense? Any other good theories, or what should I look for?
With the battery going dead in a few days, this is quite likely. A competent auto repair shop can test for excessive electrical drain on the battery. Some parasitic drain is normal, to maintain computer memory, radio memory, clock function etc., but will usually take weeks or even months to cause a dead battery. If the drain tests within normal specs (usually 20-80 millliamperes), then the battery is faulty in not being able to hold a charge. I doubt the AutoZone test addresses this.
Absolutely not. This comes from someone who does not know what he’s talking about, misinterpreting and significantly distorting something he’s heard. The advice to connect the positive cable before the negative one is a precaution against an accidental momentary short circuit should someone ALSO clumsily let their wrench touch the positive terminal AND the car body or engine simultaneously. Doing it the other way 'round cannot affect the battery or any part of the car’s electrical system in any way. Creating the short circuit could, but the sequence of hookup, by itself, cannot.
The humming noise might be due to something accidentally disconnected (e.g. a noise suppressor), but I think it’s most likely coming from the alternator. If so, it might have been caused by an installation error such as causing a short circuit as described above, but possibly is related a battery fault that doesn’t show up on AutoZone’s limited testing ability.
A good auto repair or auto electrical shop can solve your problems.
You likely have a stuck relay, is your horn working? A stuck relay takes about 2 days to discharge a battery. Feel your relays after the car has been sitting for a couple of hours when not in use and see if one of them is warm. This will quickly tell you which relay is stuck on. Could even be a window switch relay.
There are numerous causes of the high current draw and it could possibly be a cheap brand car radio that has become damaged…
Be aware that a cheap brand car radio usually has a warning saying “INSTALL EARTH FIRST”. I don’t know how they can be so bad, but I guess they are so cheap that they install 5 volt max capacitors (and semicondutors ?) in a 12 volt circuit… (proper way… use 12 volt rated devices even if the intended function is for lower voltages.)
This is a clue, too. It indicates something electrical that is generating RF noise in the vicinity. Maybe a stuck relay, like HoneyBadgerDC said, or a chattering one. But it’s not likely to be the battery itself. (Though if you don’t find this, and keep letting it be completely discharged every day or two, that battery will have a very short life.)
First, be certain it isn’t simply that the radio is malfunctioning, or on an AM frequency with the volume up.
If the radio didn’t have a problem before it should not have after. Is it possible that that battery was briefly installed backwards? Who installed it? Have you even opened the hood? Are the terminals corroded or loose? They should not wiggle or twist at all. also be sure that something isn’t making contact with the terminals, and that the battery is secured to prevent making contact with the hood if boucing over a bump.
The horn works fine. It gets frequent use. I don’t know what the relays are, how do I find them? Are they accessible to a automotive know-nothing?
Factory.
I believe I had let the car sit idle for two weeks while traveling - and that battery was fairly old and somewhat corroded IIR. Something fairly innocuous.
The battery was purchased, installed, and checked a few different times by AutoZone. It’s one of their housebrand (DuraLast?) Gold. They have confirmed that it is installed correctly, is properly charged - all the standard electrical things they check for free. I do not see any significant corrosion, though there is a bit.
You should have an owners manual that will direct you to the relay box or boxes. Most likley it will be in your fuse compartment but you may have more than one. I have seen relays that gut stuck and produced a buzz, not very common but it can happen.
Check the alternator. This charges the battery like a cell phone charger charges a cell phone battery. If it is not working, it will not recharge the battery. Also it can have a problem where it makes “bad” (noisy) electricity.
Try using the radio without the car started. No humming? If you then start the car and get humming, then I would suspect the alternator.
-You can also have the voltages checked prior to starting the vehicle and after starting the vehicle. (Volt meter)
-You can check the amperage draw on the battery when the car is off. (Amp meter)
-And you can check for electrical noise coming from the alternator with an oscilloscope.
(Above done by someone who knows automotive electrical - not auto parts guys or your typical auto repair guy.)
True, the alternator can be really noisy because its three phase, and to recitfiy it to DC it has 6 diodes, one for each lump of the three phase (two for each phase.) So in fact it can be missing one or two lumps… which produces noisy DC …
Now I suppose it might be able to generate something just over twelve volts and put a little bit of charge into the battery, but such a tiny amount it discharges over two or three days.
The simple test is put a volt meter onto the battery then its running, its got to be 13.7 or more volts to charge it properly… if its only 12 something, its not really charging properly, its stopping just after a little tiny bit of chemical is converted.
The bad alternator may have wrecked the first battery, because when an old battery gets fully discharged, pollution generated from all the times when it was used before and it formed a new layer there’s some sort of polution created ( one layer when it was drained to 1/4 and then 1/3 and then a half, etc,) and due to acid loss when it is fully discharged… (its not meant to be fully discharged. One cell with less electrolyte is driven into reverse and polutes itself. … )
I switched over from automotive to heavy duty trucks about 30 years ago. But a simple way to check for an alternator drawing down the battery when sitting was to see if the nut holding the pulley was magnetic with the key turned off. It would indicate a bad diode. Most alternators I have seen in recent years this would not be an issue as they no longer us the exciter wire.
I don’t know what the relays are, how do I find them? Are they accessible to a automotive know-nothing?