Alternator/battery

I have a 2000 NIssan Frontier Pick up. The alternator went out. Got one from a salvage yard and installed it. Found out that the truck was still running off the battery after leaving me stranded on the side of the road. Purchased a new alternator and installed it only to find that it is not sending enough voltage to the battery to recharge the battery. I’ve tested everything that I can think of that could cause a the problem. I’ve even checked the fuses and everything seems to be good. The only thing that I found is that the alternator belt is not tight enough and I can turn the spindle on the alternator by hand without moving the belt. My question is: Is it the loose belt that is causing my problem or is it something else that I’m overlooking. Needing help. ihatecars

You might look at the voltage regulator and seriously inspect cables and connections.

The only thing that I found is that the alternator belt is
not tight enough and I can turn the spindle on the alternator
by hand without moving the belt.

This condition is the cause for many a dead battery. Fix it first before wasting time on anything else.

Maybe it would be a good time to test the battery? Even if it was good when all this started, it may not be good now.

Fix/tighten/replace the alternator belt. You should NOT be able to rotate the alternator pulley by hand with the belt on if it is on correctly. The proper tension should be no more than 1/2" of flex on the belt between pulleys. Belts are cheap, compared to the alternator. Replace if it is at all suspect, like glazed, chipped, or cracked excessively.