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


Old 01-04-16, 07:38 PM
Thread Starter
Join Date: Nov 2008
Location: USA
Posts: 1,338
Received 6 Votes on 6 Posts
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.

Sponsored Links
Old 01-04-16, 07:43 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 57,733
Received 924 Votes on 863 Posts
It is very likely that the battery voltage was too low and the alternator was protecting itself from charging what appeared to be a shorted/dead battery.

Many home battery chargers work similarly.
Old 01-05-16, 07:30 PM
Thread Starter
Join Date: Nov 2008
Location: USA
Posts: 1,338
Received 6 Votes on 6 Posts
Thanks for quick reply PJ. Just seems strange that there was enough voltage to start the engine yet not enough to tell the regulator or alternator "hey I'm not dead yet, just low!" Maybe the alternator cuts out if it sees a resistance so low that the charging amperage would exceed X Amps. Like I said previously, my home charger that's rated 10A was indicating 13A when first plugged in.
Old 01-05-16, 08:43 PM
iceman681's Avatar
Join Date: Jul 2012
Location: United States
Posts: 465
Received 3 Votes on 3 Posts
my bet would go with a bad connection at the battery, "fixed" when you put the charger on, and really fixed when the battery was replaced. but i have also seen where temp will play a part with an alternator.
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Ask a Question
Question Title: