Charging woes. Help

94 4.0 auto full gauges.

Today while driving it was hot and cloudy. So I flipped on the lights and AC. And watched the guage drop from 13ish to 10- just outside the red. Yikes.

Pulled into the local auto parts place for a batt check. Registered at 5 volts and 0 CCA. It was a cheep Walmart battery the po put it there years ago. Replaced batt.
Rechecked battery still falling when loaded. My initial thought was alt. So I put a new one in. Still problematic.

The biggest drop is when the AC is turned on. But a noticeable drop when lights are turned on.

It use to run a solid 14 all lights on AC max radio in. Now it will barely maintain 10 with just headlights and AC.

All stock components, 1 year old ac comp. Stock head lights with relay mod. New alt is a 90 it's all they had in stock. I'm not at home. I'm out on the road.
No starting problems. Hit the key and go.
Help. I'm beginning to think the 23 year old cables are failing or I've got a major ground problem somewhere.

The voltage regulator that controls the alternator is in the ECU in those year models.

Clean the contacts on the ECU as a first step, then clean all the grounds.

Great. I'll see what I can do. Nee cables might help the connecting at the batt is looking a bit crusty and green.
Thanks chris

My wires were very corroded about an inch up from the connectors. I replaced them, as well as sanded all the ground points and ran new wires were they were pretty bad, and a lot of electrical problems went away.

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If the battery gauge is low, suspect the alternator. Those Chinese rebuilt alternators have a really high failure rate. As suggested, all the wires need be free of corrosion, and all the connectors and connections need to be clean and tight.