Battery light at startup, goes out after 30 seconds

First post, googled this question as well as searched here and ttora.

Little back story, 99 Tacoma 3.4 l, 270,000 miles, had it for ten years, bought it with 100,000 miles, daily driven, still has original alternator I believe and an Odyssey pc1200 that's at least 8 years old, has always worked awesome, only drained it once leaving the stereo on too long. No real loads, no big stereo, lights, winch etc.

The predicament: My battery trouble light has just recently started staying on after startup, it starts just fine, then goes out and stays out. I don't know electrical well, but I assume the alternator at idle charges it enough to get it to go out, but the battery is dying so it's not holding a charge when it sits overnight.

Plan to buy new Pc1200 as primary, then eventually going to add a second pc1200, upgrade to 1 gauge welding wire and a dual battery system and upgrade the alternator with the popular Cadillac swap: I can do the alternator now or the battery, not both, so I would love it if some electrical experts could chime in and say what they believe the problem is.

Recently had bad ground at the frame but fixed with new copper lug, can tell when that's the culprit because headlights/dash lights start flickering and dash battery indicator light flickers as well.

Thanks, appreciate any help

 

do battery first, you know the alternator still works or it wouldn't turn the light off

 

If the battery is losing that much charge overnight then it would probably not crank and start. I'd check the voltage at the battery in the morning. Should be 12.6 volts. Then check voltage right after it starts, then once warm at normal idle and also with the engine revved up a little. Ideally you have around 14 volts with the engine running. If it's dipping down below 13 volts at idle, then it's hardly charging at all. That could be caused by some bad diodes in the alternator, so you don't get the full power out.

 

Start simple. Any chance the belt is slipping/loose? Wet? Oil leaks on it, slips till dried up a bit, no squeal?

Then I'd just pull the battery and have it load tested. But at 8 yrs, I'd expect to replace it soon anyway. Generally the indicator light is not measuring the battery, but is warning you about the charging system.

Then check the running output voltages of the alternator. I don't know if the regulator is built into the alternator or into the ECU on this vehicle. I've seen both types on other marquees.

Anyway, rather than a reman alternator if that winds up being what's needed, I'd have my OE unit rebuilt at a local specialty shop. I've always found that to be the same or less$ than an OTC parts house bit, and having much better QC.

 

simple way to check is disconnect battery overnight then see if you still get the battery light in the morning, if you do then battery is bad, if you don't get the battery light in the morning after disconnecting the battery over night, then something is shorted and draining the battery over night and you need to find out where the short is at.

 

Yup, thank you for the replies, I'm just starting to get into the electrical, so I agree, battery is going to need replacement asap. I called a local auto electric place, the mechanic told me it was my alternator or charging system, no doubt about it. He said the battery indicator comes on only if the issue is its the charging system, so when that light comes on in many cases you're running on battery power alone or the charging system is weak enough that you'll drain the battery in short order.

Truck sat for last three nights and started up no problem, so I can rule out a parasitic drain or a bad battery as far as I can tell.

I ordered a 95 Cadillac Deville alternator from Autozone with a new harness and plan to redo my ground to the engine by cleaning the wire, recrimping onto a copper terminal lug, soldering if possible and heat-shrinking it.

My plans are to do a dual setup with 1/0 welding cable between a Blue Sea ML-ACR and two Odyssey PC1200's after I read the entire dual battery setup thread which was incredibly informing. Thanks again for the replies, great forum

 

Nothing is certain unless you do some diagnostic checking to determine real issue.
Follow what dr oz and bill said to do first.