Alternator problem alternator putting out 13.2 volts new battery. a mechanic said it needs 13.8 volts just to run the computer system. Wont't start unless i put a charge on battery

what make , model an year vehicle ? 18 volts is too much . 13.1 to 15 volts max. Battery light lit on the instrument cluster , check engine light on ?

Bad battery and/or alternator. Once it is running, test voltage coming from alternator with a multimeter. Should be over 13 volts. If not, alternator may be bad. If alternator is sending over 13 volts, battery may be bad. Possible bad connection between alternator and battery, but not as common.

Maybe it wasn't running long enough to charge up the battery?
Stop trying to jump it, when you are having problems with it dying, take the battery off and have it charged. And/or buy a $20 voltmeter to see what the battery shows. If it says less than 12 volts, it is discharged, and needs charging. A good battery that is not run down should have at least 12.5 volts. A new battery that is charged should have very close to 13 volts, maybe 12.8 volts.
When you have the truck running, check the alternator for charging. You put the voltmeter right on the battery and check it again. This time, with truck running, battery should show at least 13.5 volts. That shows the alternator is working. A good alternator will put out about 13 or 14 volts-the regulator will keep it from going any higher to prevent problems, like a melting battery or electrical parts burning out.
I would say get the charging system working right, then address the problem of dying at stop lights. That could be a carburetor problem, if the alternator is working.

The Battery CCA Rating is to turn over the starter

CCA--Cold Cranking Amps
That is at zero degrees
Total cranking amps is at 32 degrees

The car runs off the alternator
It puts out over 14 volts

The 12 volt battery is 6 cells @1.2 v each = 13.2 v

You need 12.6 volts to test a car or you charge it first

You need over 13 volts to charge a battery,thus the alternator

The only other job the battery has is a stabilizer in the electrical
circuit

So you need to have the charging system diagnosed as the
various light you have on should be off when engine first starts

Battery light usually means the alternator is not charging, get yourself a multimeter, select Volts, put red probe on positive and black probe on negative while the car is running, it should read above 13 Volts, not higher than high 14s, if it reads 12 volts or lower while engine is running than your charging system, alternator mainly, needs attention, any decent mechanic can check this in 2 minutes.

change fuel filter

Two things, 1- check and make sure the drive belt did not come off. 2- need to do an elec. check, if belt is ok, then more then likely the alternator has gone bad, if you can jump start your engine, check voltage at battery with engine running and jumper cable's remove, with volt meter check voltage at battery, that will be alternator out out voltage, should not have less then 13 volt's, if less, bad alternator, replace it.

altenators have to be running a 13.5 volt charge to be good

If the battery is bad it will not hold a charge. Have your battery checked a the local parts house or your mechanic. Also have the charging and starting systems checked to insure they are working properly. Battery should have a reading of about 12.8 volts, alternator should be putting out between 13 and 14 volts. Starter should not draw to many amps.

well if the alternator is not putting out at least 13 volts when it running most likly its bad i would take it some place and have it checked again