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If the alternator is fine, and it uses an internal regulator, but the batteries are not charging?
Either the alternator is not being spun properly (very loose belt or glazed belt so it is slipping badly), OR you have a "one wire" installation which shorts the alternator sense lead back to the output post. In order to get the alternator to kick in, you have to blip the engine, typically to 2-2500rpm to get the initial output from it before it will turn on and regulate itself.
OR you have a wiring problem. Could be your battery terminals or cables or switches, or the harness leads from the alternator, are not making a good contact. Could be there's a blown battery main fuse that's out of sight.
If the belt is nice and clean and tight, and you can't turn the alternator without the belt and flywheel turning, the belt probably is OK.
Check your manual to see if you have a "one wire" setup, or check the wiring to see if the sense wire has been joined back to the output.
Then start checking the rest of the wiring. Note that if the alternator is SPINNING at normal speed, but the sense wire is disconnected, you can blow the alternator out in a minute flat. So check the wiring out before you restart, and then when you restart, use a voltmeter connected directly to the alternator output terminal to see if it really IS putting out.
Odds are something is just not connected.