I was having alternator problems a week ago,so i turned off my stereo so my battery would last longer,I put in a new alternator on the expedition and now I've been trying to turn on my stereo and it won't what can it be?

It is possible that you have a faulty alternator. You do get rare occasions where a faulty unit slips through the system and gets sold. Please read this all the way through as it give more than one way to check for where the problem could be.
How old is the battery? They are considered 'Old' once they are over 5 years old. If you know how to use a multimeter try this.
You'll need to check both the alternator and the battery. For the battery - With the engine OFF test the DC voltage of the battery. It should be somewhere between 12.6 and 13.2 volts. Anything lower than 10 volts and it could be a faulty battery. Even a fully charged battery can struggle to supply power if it is old or worn out.
For the alternator - Start the vehicle and retest the battery voltage. It should read somewhere between 13.7 and 14.7 volts. If it's noticeably lower (or higher) it is most likely the alternator.
If you don't have a multimeter you can try a basic test. Get a helper to stand in front of the vehicle. With the park brake On turn on your head lights THEN start the engine. Your helper is watching for a significant dimming in the headlights. The lights should dim as you crank the engine then brighten again. Next turn everything else on, including the stereo, A/C, courtesy light. This is to put the battery and alternator under heavy load. If the headlights dim noticeably It is possibly the alternator.
Most companies often have a guarantee or a warranty on the alternator so a replacement shouldn't cost you anything.

First, a bad battery doesn't turn the light on. Just A bad alternator or ECM. Start the car. put a volt meter on the + and - sides of the battery. If it reads 12 to 15 volts (closer to 15 for a new alternator). The alternator is good. And is charging. The problem is the wiring or the ECM. It can be bypassed to turn the light off. I just did it a week ago. Problem is , I don't have the wiring diagram for your car. Don't get a book from the parts store, it won't tell you. Go to alldata or Mitchell and purchase a manual. It's online and lasts a year. But they give you good electrical schematics...

Wouldn't be a fuse, but it seems as if the alternator is still not putting out normal power, or else your battery has a problem taking a charge. Better have the battery tested for a dead cell. also see what a voltmeter says about alternator output now. If you have an electrical problem in the wiring somewhere or in the alt. charging controls, then possibly one or more of the diodes in the alternator may got burnt out. That means the alternator puts out less power.
On some newer cars, the engine computer, pcm, controls the rate of charge of the alternator. So you have to be careful diagnosing -know how your system operates. That's why mechanics always ask for year, make, model, and engine size if applicable.

If you have a drain on your battery , it would not take 18 months for the battery to go dead . What do you mean The battery gauge in the dash shows only about 1/2 power, When ? When the vehicle is running ? Not charging ! The battery gauge as you call it has nothing to do with the Battery . That's showing how much the charging system is putting .The volt's indicator is controlled by the PCM - engine computer . With the engine running should show 14.1 volts .if your only seeing 7 the charging system isn't working. The alternator is controlled by the PCM as well ! Have it check by a GM dealer .

check the wire from alternator to the battery and make sure its not loose on alternator. happened to me a few years ago, forgot to tighten wire to alternator. car lights on dash came on and car quit while driving. just make sure you are getting at least 14 volts at battery while its running, good-day !

Try your headlights. If they are real dim or don't come on at all, the battery is dead. Since the battery is new, I would think the alternator is not charging the battery while the engine is running, You can have the alternator checked,

i know all about the vegas heat. i live in henderson. the alternator is going out. the regulator is internal so you have to replace the whole alternator.

You need to get someone who knows how to use a Multimeter. It is a device that registers how much voltage, is present at your battery when your alternator is charging. Anyone can use one of these tools and they are cheap to buy, just add some AA batteries. Put the negative lead on the negative battery terminal and the positive lead on the positibe battery terminal, set the device to read Voltage and read the display that tells you how much voltage is present when the car is off, then on, and when you turn on different accessories like head lights, AC, etc. your alternator should produce more Voltage with each new thing you turn on. You will see this voltage go up as you read the multimeter on the battery terminals. If you don't see the Voltage go up the alternator is bad.

Now, likely a new alternator should be good, and you said you had the batter tested, hopefully they knew what they were doing. Lastly, I would unhook the aftermarket stereo for a week or two and see if that solves the problem. Also you should check all connections to the battery and to the alternator to make sure there isn't anything loose, that was the problem on my car, the alternator conector was loose and wiggeling and caused the battery light to come on intermittently. Hope this helps you out!!!

My 2003 325xi died on the highway a few weeks ago, and I thought it was the battery. It turned out to be the alternator, which was an expensive problem to repair. If your battery is new, it could be the alternator, or a problem with the electrical system. I find the BMWs to be too complicated electrically, which often leads to problems! I'd check your alternator...

Yes this sounds like the problem may be the alternator has low output voltage.