Replace the battery and the alternator and the battery light is still on. We have an 2006 SS - we have now had to replace 4 batteries in less than 3.5 years - everytime we take it to the dealer - can't find any reason the battery is being drained!!! Something MUST be eating it!!!! Any other suggestions??

Using a volt meter check the voltage at the battery with the vehicle running. There should be more than 13.5 volts. If it is more like 12 volts or less that the alternator is not working or there is an open between the alt and the battery. The most common cause of the battery light is a bad alternator.

Go through the basics.
1 Make sure the alternator is charging (operating properly) by measuring the voltage between earth (battery negative) and the live terminal. It should be greater than 13 volts, less than 15V.
2 Make sure the regulator is being excited. Most modern alternators need to have the dash light working in order to send a minute current to the alternator regulator to kick it into life. If the dash light is out, (fused) the alternator won't work.
3 If the voltage from the alternator live terminal is good, check the voltage at the battery. It should be between 13.5 and 14.8 Volts. If this reading differs from the reading at the alternator output, you have a wiring issue.
4 Wiring issues may include but is not limited to:
a) bad connection at the battery terminal
b) bad connection at the main fuse,
c) bad connection at the starter motor. Some vehicles pick up the main power from the battery cable at the starter solenoid.
d) broken wire, bad connections on the main power lead connection terminals.
Happy hunting!

connect a volt meter across the battery with the engine running must be around 14 volts, if not alternator or regulator at fault or drive belt loose . connect a amp in series with the battery with every thing switched off the draw must be less then 1/4 amp ,if not check for lights in trunk a glove box staying on.

Your going to need to replace the alternator, it may be charging the battery slightly and enough to get through a few days of starting the vehicle, but you will need to get the alternator replaced.

the battery light comes on to indicate there is a problem with the charging system, not necessarily the battery. With the engine running check for voltage at the battery. you should have more than 13.5 volts. Anything less than that and the alternator is not charging.

It is likley you need a new alternator.

check belt tension, check power alternator field and armature windings according to specs (smaller wires going to alternator harness) check voltage drop on main alternator cable going to battery, less than 0.4 volts ok, check alternator ground (less than 0.1v ok) check if alternator is regulating voltage in alternator or by pcm computer, also check if vehicle has a load detector to increas rpm while under electrical load....

Assuming a healthy car battery - most probable cause is dying alternator not supplying enough current to recharge the battery. The battery light lights up when it doesn't supply enough power.

Most plausible fix either a replacement of the alternator or the voltage regulator. Refurbished alternators can be supplied by Toyota as well.

Less common is a broken belt driving the alternator.

With a multimeter check the voltage at battery terminals. Should be more than 12.6Volts when engine off, and at least 14 volts with engine on. If the battery voltage is less than 12.6 volts then you need a new battery. If the voltage is less than 14 Volts with running engine, then the alternator is not working properly. A stop at AutoZone might help. They scan codes, and check alternators for free.

It can still be the alternator, the alternator can produce enough to keep the car running, but it may not have enough to keep the battery charged 13-14 volts. It don't take long for a new battery to drain if the alternator isn't putting out the right voltage. Find a voltmeter and check the output from the alternator to see what voltage is being produced. If it's not at least 12.5 -14 then have the alternator checked out.

That means it is not alternator, it was battery discharged. Have both checked for free at autozone store. Maybe you need to clean/replace heavy power cables coming out/to battery and out /to alternator, as the deposition reduces the current
Also check the alternator belt for tension by twisting (less when 90 deg) or pressing (1/4 inch freedom)
Hope that helps
Good Luck