Found a stat that shook me about alternator output

I was reading a Bosch technical guide and learned that a standard alternator only puts out about 60% of its rated amps at idle. That seems low. Has anyone else tested this in their own shop and seen similar results?

3 comments

3 Comments

jenny_lee16d ago

The output can drop even more just from being hot." Oh great, so we're supposed to install an air conditioner for the part that makes the air conditioner work. That's just perfect. Might as well mount the alternator on the roof with a little fan blowing on it while we're at it. But hey, at least the heat gives us a good excuse when the battery dies at a drive-thru. "Sorry boss, the alternator just needed to cool off.

the_holly16d ago

That 60% number sounds about right from what I've seen testing customer cars over the years. Had a guy come in complaining his lights dimmed at stoplights and his battery kept dying. Pulled out the meter and sure enough, his stock alternator was only pushing about 50 amps at idle when it was rated for 90. Matches up pretty close. For daily drivers, you might want to check the pulley ratio if you can. Smaller pulley on the alternator will spin it faster at idle and bump up that low speed output. Had a buddy do that on his truck and it made a noticeable difference. Also keep in mind that heat kills output. If the alternator is sitting in engine bay heat after a long drive, that 60% can drop even more. I've seen them lose 10-15% just from being hot. Good ventilation around it helps a lot with that.

hannahcraig16d ago

Curious about the pulley swap, did your buddy have to change the belt length too, or was there enough slack in the tensioner to handle a smaller pulley? I've been thinking about trying that on my Civic since the headlights dim pretty bad at idle with the AC on, but I don't want to mess with the belt routing. Also, any idea if a smaller pulley puts more strain on the alternator bearings over time, or is it usually fine for daily use?