when at idle the volt meter drops to about 15, but when increased rpm it shows a on 2001 Toyota Sequoia

on my 2001 toyota sequoia when at idle the volt meter drops to about 15, but when rpms increase it goes back to almost 18, if u turn off engine and if lights are left on( or radio for that matter) ignition in acc position the batt. will not start the engine only click. the car does not have to be in this position long. So do i have a bad batt. or is the alternator going out. HELP

Asked by for the 2001 Toyota Sequoia
Sounds like the Alternator is over charging your battery, the voltage regulator inside is not keeping the voltage at the correct voltage. It may have already damaged your battery as well, take it to your local parts store like Advance, AutoZone, O'Reilly, they will test both parts for you free of charge.
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2 more answers , 2 more comments
Just a comment, but in many cases the alternator will act that way when it's trying to charge a bad battery, still needs to be tested, but who am I.
you're ProfessorG........ and the alt & bat should both be tested has been stated.
Agreed as stated ????
Don't trust the voltmeter on the car alone. Get a real meter on it and see what is going on. Your truck should be charging between 13.9v and 14.7v at idle and at 2000 rpm.