Dying battery/No start

My daughter's 96 Blazer keeps needing jumpstarts. I've checked the battery terminals and cleaned them thoroughly, made sure the battery had enough electrolyte in it, and checked all the wires at the alternator.
With the engine running, I was only getting 12.9 V at the battery, and at the alternator terminal - I would have expected something in the 13-14 V range.
The idler pulley is chirping loudly, so I know that'll need replacing, but can't really tell if there's any noise coming from the alternator itself (without losing an ear to the fan).
Is there something else I should check before possibly investing in a trip to the shop?
Vito
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Have you load tested the battery, or checked alt. output under load, basic diagnostics for this problem.

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I don't have a manual for this vehicle, so do not have the procedures down and am not familiar with the terminals on the alternator.

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The alternator should put out 14.25 to 14.5 voltes at the battery. If you are getting only 12.9 V when running, test as follows, Test 1. Turn off engine, battery should read about 12.6, if not charge battery for a while and see if voltage comes up. If battery will not come up, then you have a bad battery (open cell). Test 1a. Turn off engine, note battery voltage, turn on headlights, battery voltage will drop a little, 1 volt or less. If more than that, try test #1. Test 2. Find output terminal of alternator, should be a heavy terminal . Run engine and measure voltage at alternator output. If voltage at alternator is about the same that you see at the battery or a little higher (+0.5 V) then I'll bet your alternator is bad. If it reads 13.5 V +, battery bad. If your battery old (4+ years) change it too, She your daughter after all.
Mike

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Thanks for the info and test procedures Mike. I'm gonna go thru it now. I'll let y'all know what the outcome is.
Vito

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Just got done with testing and here are the numbers
Test 1: Battery read 10.89V so I ran engine for 20 mins to charge up. Test 1a: Battery read 11.51V after 20 min run, but steadily dropped. Once headlights turned on got 10.81V and another steady drop. Test 2: Alternator output terminal reading 12.91 with engine running Battery reading 12.89-12.90 with engine running
I'm guessing both a bad battery AND a bad alternator
Vito

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I know that Auto Zone will test your Battery and Alternator for free, so that would be an easy way to figure out what is going on before you start replacing things.
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Yep. A good battery will read 12.6 to 12.8 after sitting awhile. If less than this, you might have a weak of dying cell. When running the alternator will try to bring her up to the 14.25 to 14.5 V range and will put out max current. This is very hard on the diodes (rectifiers) and will cause them to overheat and burnout. This is for all who are reading this, If you are having problems starting your truck, ie slow cranking speeds ect... I would test your battery and your charging system first before looking at the starter IMHO. I dont sell batteries for a living. Just been working on my Chevy cars and trucks for 30 years. Good luck
Mike

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I have a 93 S-10 4.3 Vortec..that I just got and I just charged the battery and it seems to stay up good and cranks the engine good....I checked at the battery at engine idle with voltmeter and it's showing...14.25...which seems kinda high.The alternator may already have destoyed the Diodes?? Bobby
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