98 Grand Cherokee 4.0 Alternator Recommendation

My Jeep is dead. I put a trickle charger/battery condition on it and it wouldn't even notice there was a battery there.
The battery is maybe 16 months old. I jumped it and it fired right
up. I ran the car for about 15 minutes and shut it down. Waited 5 minutes or so, went to restart and it barely turned over. Tried a couple more times and then the battery cut out entirely.
I presume, 10 years old and 160,000 miles that it needs an alternator.
Rock Auto has a rebuilt AC Delco 90 amp, and a brand new NSA. They also have other options including a 120 amp, but I have nothing in the Jeep in the way of accessories to need the extra power.
Anyone recommend one brand versus the other here? They all seem to be $125-$150, so there isn't much of a price difference to make one more compelling than another.
Planning on doing this in my driveway Saturday, never changed an alternator before, but I did all the ignition wires, distributor, plugs, oil/filter changes, radiator replacement, hoses and coolant, serpentine belt over the last 12 months or so, I presume this won't be more complicated.
Thanks in advance for any opinions, Phil
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PhilB wrote:

One way to know for sure if the alternator is the problem is to put a voltmeter on the battery terminals with the engine running a little above idle speed (1000-1200 rpm). If it measures above 13.6 volts, your alternator is not the problem. Also - if when you ran it for 15 minutes after jumping it off at idle, the alternator may or may not put out enough current to charge it much at idle.
To answer your question, if the price on the 120 amp alt. isn't that much different and it's a direct swap and not an inferior brand, no reason not to get that one.
You might also compare what's available and pricing with a local auto parts store - I find that after shipping, Rock Auto is not necessarily cheaper - often is not. Just don't get some off-brand rebuilt. The quality and reliability on things like rebuilt starters and alternators carried by places like Advance can be arocious. You might check NAPA out - they tend to use better quality sources for things like that, though they may cost more. Anwyay - that's my suggestion.
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Bill Putney
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Thanks. I plan on calling the local parts guy today (Preferred Auto or Pep Boys in my neighborhood).
I presume it is the alternator because why would an essentially new battery fail - in my head it was because it wasn't being charged. However, I plan on checking the voltage to see what's up. You mention the volt meter should read 13.6 volts with the car running at 1000+ rpm. The battery is 12 volts. Is the extra 1.6 volts attributable to the charge coming from the alternator?
Thanks, Phil
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IIRC, Lead Acid battery chemistry gives 2.105 volts at full charge. There are 6 cells in a car battery, so nominal voltage of a charged battery should be 12.63 volts. To properly charge a battery, the current must flow in the correct direction (into the battery), so the charging voltage must be above the battery's full charge voltage. This is why you should measure somewhere between 13.5 to 14.x volts at the battery when the alternator is running. I believe the nominal design for charging voltage is 13.8V and this is why Bill suggested you should see above 13.6V.
Bob
wrote: Thanks. I plan on calling the local parts guy today (Preferred Auto or Pep Boys in my neighborhood).
I presume it is the alternator because why would an essentially new battery fail - in my head it was because it wasn't being charged. However, I plan on checking the voltage to see what's up. You mention the volt meter should read 13.6 volts with the car running at 1000+ rpm. The battery is 12 volts. Is the extra 1.6 volts attributable to the charge coming from the alternator?
Thanks, Phil
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Bob Shuman wrote:

Yep. :)
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PhilB wrote:

If you're going to go with a parts-store rebuild, my advice would be to find a NAPA or CarQeust. My experience with Pep Boys and Autozone rebuilt parts is absolutely awful. My first choice would be to take the alternator to a local starter/alternator rebuilding shop. There's very little to actually go wrong with an alternator, usually just a new pair of slip-ring brushes and new bearings is all they need. Sometimes a diode may be blown, but not too often these days.

Because they just do. Fairly often.
- in my head it was because it wasn't being charged.

A "12 volt" lead-acid battery doesn't really start accepting a charge until 13.5 volts, so automotive "12 volt" systems are regulated to about 13.6 volts to keep the battery charged.
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PhilB wrote:

Sounds like a bad battery to me, since niether your charger nor alternator would get it to hold a charge.

That doesn't make any sense. Did you put a voltmeter on the system while the engine was running? If it showed normal voltage, then your alternator is fine. You're drawing a conclusion based on ZERO factual information. The alternator may indeed be bad, but nothing you've mentioned so far implicates it at all.
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