Here's one for ya. I have 2005 Ranger 4.OL just recently when I accelerate the battery light comes on. No light when driving normal. Stopped by O'reillys had them check the battery, alternator, starter. They said battery bad. So I dropped 130.00 on new battery. Light still comes on when accelerate. Any thoughts. THX David

 

Make sure the nut holding the pulley on the Alt is tight. Belt might be slipping. Make sure tensioner is good. Lastly if the truck has a lot of miles on it it can be the brushes are short.

 

THX wtroger I will check all of that this weekend. David

 

There is a single wire coming out of the top of the alternator. It's purpose is to put the light on, bears checking.

 

THX Dan I will check. David

 

I think Idiot lights are for idiots. Tells you what type of engineers Ford hires.
My advise is to spend a few bucks on a volt meter and throw it across the battery to see what's going on.

 

Check the belt tensioner casting belt tension range marks, to see if belt tension is within range. If ok, check the under side of the belt for missing rubber, or coolant, oil, power steering fluid, grease, ect, contaminates, that could cause it to slip under load.

Did the store rev the engine when they checked the electrical system output????