Alternator won’t charge below certain voltage threshold???

2005 Scion xB. 4-year-old battery. Has been starting fine up until this morning. Cranked noticeably slower but fired right off. Immediately noticed many "idiot" lights illuminated on dash. Idling fine, not missing a beat. But won’t accelerate much above idle without cutting out. Turning on headlights immediately causes engine to want to die, but I turn them off and engine recovers.

Back in ’07 I installed a ScanGuage II (so I could have real-time engine info to compensate for the useless idiot lights). One of the 4 parameters it indicates is battery voltage. Right after noticing the idiot lights, I see that the battery has only 7.6 volts. I let the engine idle to warm up past open loop temp (138 degrees, another parameter indicated on the ScanGuage) to see if anything changes. No change. By now, the voltage is down to 7 or 7.1. Five minutes later at 6.8V the engine starts to miss at idle. I shut it down.

I get the battery charger hooked up. Initial amperage showing 13A. Ten minutes later 7A. 15 min. later, 4A. I disconnect charger, try to start car, it cranks at normal speed, fires up and is now charging normally, showing 13.9 to 14.0 system voltage.

I drive to Toyota dealer, they replace bad battery, and CHECK CHARGING SYSTEM. No problem with alternator/charging system. The senior service adviser at this top-rated dealer did not have a clue as to why the system voltage was indicating so low this morning and refusing to charge the battery. He said maybe the battery was so bad the alternator wouldn’t even try.

It’s been my experience that a low battery will indicate 13-14V while the engine is running, but won’t hold 12V after shutting off the engine.

A buddy of mine said maybe modern alternators have to have a certain threshold voltage before they will put out a charge. Another friend said the voltage regulator won’t work right if it doesn’t have a certain threshold voltage as well.

And before you ask, no the belt was not slipping/loose. There were no lights or other voltage drain left on overnight. And it’s been about 45 degrees ambient temperature in my garage overnight for the last week or so.

Needles to say, everythings fine now with new battery, but I’m still curious as to why the alternator refused to try to charge while the voltage was below 7.6V.

Ideas??

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