Battery light on

I have an 08 Tacoma that I’ve owned its entire life. It’s been incredibly reliable. I’m religious about the maintenance and never skimp on parts.
I’ve replaced the battery a number of times given the typical 5 year life and the tell tale symptom is hard starting and then no start.
Today the battery light came on and stays on solid. I have not seen that before The battery is due to be replaced but I’m thinking the alternator is causing the light. Wondering if anyone can advise me on this before I replace the alternator. Thank you!

 

Test the alternator. It's likely the problem, but you can prove before replacing.

If there is a long term electrical shop in your area, having them rebuild yours could give you a better product at lower cost than a parts house reman

 

When you say due to be replaced, I think you might be thinking car batteries expire like milk. Replacing every 5 years isn’t a bad plan. What I do is at around 4 years I begin to think it might be getting weak and do a starter voltage drop test. From there I’ll decide to be comfortable knowing it’s good, retest in a couple months, or start shopping for a new one. I’ve had them not make it 3 years, to test good up to 14 years.

Since you have a battery light, I’d check to make sure the terminals and posts are clean and acid free, then test the battery and alternator.

If you remove the terminals from the posts to clean with a battery brush, stick on some corrosion prevention felt pads. I never believed in them til I saw some at Wally World for super cheap. They worked so good I bought some for fleet maintenance, now the terminals are all always acid free.

$1 NOCO NCP2 MC303 Oil-Based Battery Terminal Protectors, Anti-Corrosion Washers, and Battery Corrosion Pads (Pack of 2)

https://www.amazon.com/gp/product/B001DKUTR0?ie=UTF8

 

Battery light is controlled by the alternator, it should only illuminate when there's a problem with the alternator. The only other thing that would turn the light on is if the L wire was shorted to ground.
It's easy to test the alternator, battery voltage should be 13.2-14.8 with the engine running and all loads (blower on high, headlights on high, etc) on. If it's not first check the ALT-S Fuse (7.5A) in the underhood fuse box, if it's good you need to check for power on the big wire at the alternator as well as pins 1 and 2 of the alternator connector with the key on, if that all checks out but it's not charging then the alternator is bad.