Alt and Battery going bad?

How can you tell if your alt and batt is going?

06 tacoma base. 1 diehard platinum up front a group 34 prob. a 2 battery out back next to the amp a optima group 31 battery 3-4 year old 4-5 year old rf sub amp 100 bdcp sundown 100.4 amp. my voltage read low 12.8 when im driving 12.4 when idle gonna have a friend bench test the alt and the battery soon any help thanks

 

voltage should be at least 14v at idle and I believe a little higher while driving.

 

Right before my alternator went out, about 7 years on stock 06 prerunner, it started whining while running for a couple days before it took a dump. As for as batteries go I have been replacing them every 3/4 years in SoCal weather. I just put in a Northstar 31m AGM a few weeks ago. Battery cranks over engine faster I noticed even when cold out.

 

Load test your batteries independently. If they pass then have your alternator load tested (remove from vehicle and place on test bench).

If your alternator passes and your batteries pass try recharging/conditioning/balancing your batteries. This is a long process so be prepared.

It took me two weeks once (just because I had the time) to have my batteries balance to each other. Most car audio people charge their banks as a single unit (3+ at the same time) and that's fine if they are balanced but very rarely do batteries have the same age and float off the shelf or without processing.

You basically charge/condition one battery at a time until full or it holds a float voltage close to 12.7. Then connect all charged batteries together in parallel without connecting to any devices.

Basically create a battery bank without powering anything or connected to anything else. For mine I gave it two weeks and once balanced my alternators are able to keep them charged without stress.

This is easier to do with agms and if all batteries have the same capacities and even easier if you just replace your alternator without battery processing. Either way I personally think your alternator is shot and your batteries are fighting themselves and the alternator.

I was never able to get more than 13.4 volts charging out of my stock alternator...I had to go dual aftermarket alternators with a voltage regulator/adjuster. 14.4 charge voltage all day now.

 

While I don't know anything about dual battery setups I have to agree with what Purpleman said

 

Today read voltage and was at 11.4 both front and back engine off.. The check engine light came on could that be related to the alt dying? Also the airbag light came on could it be cause low voltage ? How I check the cel?

 

How long you say I have? How much voltage to turn it over?

 

Your batteries are drained. Remove your alternator and have it load tested. The batteries may need to be recharged/conditioned or replaced. At that low of a voltage the battery is probably no longer holding a charge or taking any charge.