Charging system failed. Why is new alternator output ranging between 11 1/2 and 12 1/2 volts?

13.7 volts is within the correct charging range. While charging it should read between 13.7 and 14.7.
How old is the battery and the alternator? If the vehicle is difficult to start and you are worried, get a good mechanic to 'Load Test' the battery for voltage output. You could also have the alternator output tested to see if it is charging at the correct rate. A good Auto electrician can do both of these jobs for you.

Check that your alternator is actually charging the battery. With volt meter and car running check for at least 13.5 volts at battery. If not check fuses, big red wire from alternator to battery and for actual output of alternator at bolt where big red wire attaches to alternator. Should have battery voltage with car not running and at least 13.5 when engine is idling(motor running)

Dear Joyce,
The "battery not charging" light actually will tell you many things, and that one light is affected by several parts of the car.
If your battery is new, then the light is telling you some other part of the charging system is failing or has failed.
The most common part is the alternator.
The alternator, or the voltage regulator which is built inside the alternator, can both fail, and the light is telling you.
Those need to be checked by a repair man. (mechanic)
If you have no corrosion on the battery terminal, or the wires, or inside the insulation of the wires, the alternator should be your problem.
God bless your efforts.

Yes and no.
It sounds like you have a wiring problem.
The alternator output could be 14 volts and only one or two amps, but that would be enough to kill the warning light. Its more likely the alternator is charging but the current is not reaching the battery.

Hi William
to start with, check if there is output voltage from the alternator it self ( at the back of the alternator ) using a voltmetre
Chould give 14.0 Volt, if is OK, that means you have a broken wire between the alternator and the battery, this is common.
good luck

The voltage for the alternator appears to be in the range expected of a good alternator. Have the fault codes read and look for an ignition sensor that is not working correctly as it sounds like a loose connection or failing coil.

Vehicles: any failing to keep its battery charged.

A vehicle unable to charge its own battery has one of 4 problems:
(a) alternator failure
(b) voltage regulator failure
(c) battery failure
(d) wiring problem between battery and alternator/voltage regulator.

One most modern vehicles (including 2002 Lexus RX300 - 2WD and AWD), the voltage regulator is an integral component of the alternator and is not separately serviceable.

In the US, one can get a free "charging system diagnosis" from the popular auto parts chains: AutoZone, Advance Auto Parts & Pep Boys. You needn't remove any parts from the car to get this diagnostic, since they can attach a diagnostic meter to the charging system in the parking lot. This diagnostic aid will tell you exactly which component has failed - battery, alternator or voltage regulator.

In case the vehicle is immobile, one can DIY (do it yourself) the diagnosis.
(a) inspect the wiring for corrosion/loose connections/loose connectors/etc.
(b) check alternator belt/pulley - if drive belt is properly turning the alternator pulley (no slippage/misrouting/etc.), then the mechanical tests are complete, and you'll need to continue testing the electrical performance of the charging system components.
(c) first component to test: battery
DIY test 1: remove battery from car and put battery on a 120VAC automotive battery charger and charge it fully (or just check it in the car with motor off, since the car's charging system is a type of automotive battery charger).
test parameter: a fully charged lead-acid automotive battery should read 12.45 volts on a VOM/DMM
DIY test 2: disassemble battery out of car after driving it to one of the auto parts chain stores (Advance/AutoZone/Pep Boys) for a free battery test. These testers will test the battery under load, which is not possible with just a DMM.
(d) if wiring is good, and battery tests good under load, then
the failed component is the alternator/voltage regulator - by process of elimination.
(e) DIY test 3: direct alternator/voltage regulator test (car must start and idle successfully to perform this test)
Start the car, and put a VOM/DMM across the terminals of the battery. Since the car is running, you'll be reading the output voltage of the alternator and not the output voltage of the battery. The acceptable ranges for alternator/voltage regulator output are:

ALTERNATOR CHARGING VOLTAGE

Most alternators that are charging properly should produce a voltage of about 13.8 to 14.2 volts at idle with the lights and accessories off. Always refer to the vehicle manufacturer's specifications. Many Asian vehicles, for example, have higher charging voltages of around 15 volts.

When the engine is first started, the charging voltage should rise quickly to about two volts above base battery voltage, then taper off, leveling out at the specified voltage.

The exact charging voltage will vary according to the battery's state of charge, the load on the vehicle's electrical system, and temperature. The lower the temperature the higher the charging voltage, and the higher the temperature the lower the charging voltage. The "normal" charging voltage on a typical application might be 13.9 to 15.1 volts at 77 degrees F. But at 20 degrees F. below zero, the charging voltage might be 14.9 to 15.8 volts. On a hot engine on a hot day, the normal charging voltage might drop to 13.5 to 14.3 volts.

Here are the full specs for installation of the 2002 Lexus RX300 alternator - you may be able to check these specs yourself (with a torque wrench), or pass them along to your mechanic.

Note: the VIN 8th digit should be "F" for the 2002 Lexus RX300 (2WD & AWD)

2002 Lexus RX300 (2WD and AWD) - 3.0L Engine, VIN "F" SFI DOHC

Alternator

Drive belt. Tension the belt to 170-180 lbs. for a new belt or 95-135 lbs. for a used belt.
Adjusting alternator lockbolt. Tighten the bolt to 13 ft.-lbs. (18 Nm).
Alternator pivot bolt. Tighten the bolt to 41 ft.-lbs. (56 Nm).

Glossary of acronyms
--------------------------------
DIY = do it yourself
DMM = Digital Multimeter
DOHC = Dual Overhead Cam
SFI = Sequential Fuel Injection
VOM = Volt Ohmmeter

References
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How to test a Car Alternator - todayifoundout.com

Alternator & Charging System Checks - aa1car.com

Have an OBII scan done and reset dash lights with same tool/scanner. Autozone, Advance or any good parts store or shop can do it for you and most will do it FREE

take the battery in to have it tested it may be defective. if the battery tests ok your alternator isn't charging. if you have a volt meter , your battery voltage should be no lower than 12.5 volts dc and with the motor running you should see no lower than 13.5 volts dc with everything turned on (lights, ac on fan on high) if the voltage drops below that your alternator is bad.

STARTING YOUR DIAGNOSIS
What happens when you attempt to start the engine? If nothing happens when you turn the key,"http://www.aa1car.com/library/2003/us20310.htm"to determine its state of charge. Many starters won't do a thing unless there is at least 10 volts available from the battery. A low battery does not necessarily mean the battery is the problem, though. The battery may have been run down by prolonged cranking while trying to start the engine. Or, the battery's low state of charge may be the result of a charging system problem. Either way, the battery needs to be recharged and tested.
If the battery is low, the next logical step might be to try starting the engine with another battery or a charger. If the engine cranks normally and roars to life, you can assume the problem was a dead battery, or a charging problem that allowed the battery to run down. If the battery accepts a charge and tests okay, checking the output of the charging system should help you identify any problems there.
A "http://www.aa1car.com/library/2002/cm10220.htm" that is working properly should produce a charging voltage of somewhere around 14 volts at idle with the lights and accessories off. When the engine is first started, the charging voltage should rise quickly to about two volts above base battery voltage, then taper off, leveling out at the specified voltage. The exact charging voltage will vary according to the battery's state of charge, the load on the electrical system, and temperature. The lower the temperature, the higher the charging voltage. The higher the temperature, the lower the charging voltage. The charging range for a typical alternator might be 13.9 to 14.4 volts at 80 degrees F, but increase to 14.9 to 15.8 volts at subzero temperatures.
If the charging system is not putting out the required voltage, is it the alternator or the regulator? Full fielding the alternator to bypass the regulator should tell you if it is working correctly. Or, take the alternator to a parts store and have it bench tested. If the charging voltage goes up when the regulator is bypassed, the problem is the regulator (or the engine computer in the case of computer-regulated systems). If there is no change in output voltage, the alternator is the culprit.
Many times one or more diodes in the alternator rectifier assembly will have failed, causing a drop in the unit's output. The alternator will still produce current, but not enough to keep the battery fully charged. This type of failure will show up on an oscilloscope as one or more missing humps in the alternator waveform. Most charging system analyzers can detect this type of problem.
thanks,please rate the solution positively.