Worried about overcharging new AGM

Hey everyone!

Making a post to see if anyone has had similar experience. So recently, I had symptoms of alternator dying. I decided to swap everything out and do some key upgrades; new 180amp alternator from BrandX, Deka 9A34R AGM Intimidator Battery and the 'big 7' wiring upgrade from JeepCables.


I've noticed that while running, my voltage is quite a bit higher then my stock alternator. I have been running typically 15.2-3 volts in the first 10 minutes, and then it seems to settle after to 15 to 15.1-2 thereafter. I am getting these readings from my scanguage in the obd2 port, I don't currently have a multimeter to test with, but was going to use my test driver to sew what the voltage at the battery when off and when running looked like.

I am new to AGM batteries, but does this seem high or enough to where I am overcharging/hurting the new battery? A quick google.search comes up with a variety of answers, some stating that they are sensitive and anything over 15v is going to harm it, others stating that it is temperature derivative and 14 to 15 volts is normal in colder Temps, with it dropping in the summer months...


My old.battery was a maintenence free smaller Deka, non agm, and was typically charging and operating right around 14volts before it went south (11 volts one night driving home, when I decided it was time to upgrade).

I had the new battery on a trickle charge for a week, so it should have been close to a full charge before install. I will put my old battery back in tomorrow and see if there is any difference.

 

Update;

Went ahead and installed my old battery to test. It was interesting, on first startup I let the truck run at idle for a half hour, and in that time it seemed to be charging at an acceptable 14.2 to 14.8v range. I turned the truck off for a few minutes, and upon restart it was then charging at 15-15.3v consistently. Drove to work, and home back night and it was 15+ on both trips.

I guess the next step is to contact BrandX and Deka to see what their thoughts are...

 

On their website, they offer a 7-14 Tacoma and a 16+ Tacoma option but nothing that fits your Tacoma.

 

I run Odyssey AGM batteries in another truck, when I contacted them I was told 14.0-14.7v is what I should see for charge voltage, and the higher end of that is preferred. That truck consistently shows 14.7-14.8v using an OBD scan tool (measurement should be taken at the battery though). My inquiry didn't involve over charging but based on the above v spread, over 15v does sound high.

 

I bought this alternator thru the JeepCables website, it was listed for a 3rd generation 4runner. A quick rockauto comparison showed that they used the same stock alternator for the 3.4, so I grabbed it. Everything bolted up fine, but I did have to go to a new plug style as this alternator used I think the GM style. It had a yellow and blue wire, which per the brandx instructions I wired up as yellow to yellow and blue to white (that was the sense wire). My red ignition wire was left unused and off the new plug.

 

Most oem spec say 12.8v-15v at 2k rpm under electrical load. I would say 14.4v is the sweet spot. Anything over 15v will kill the battery. Jeep cables doesn't sell alternators but it sounds like you upgraded the battery cables and swapped to a gm alternator. You probably messed up the wiring for the gm alternator I would double-check that. How many pins does the alternator have?

 

The alternator was purchased thru jeepcables, it was a BrandX 180a marketed for 3rd gen 4 runners. Yes, I believe they use a GM style plug on it, I would post a picture but I don't know how. The new alternator had 2 pins, a yellow and a blue wire. My old harness was a 3 point, having yellow, red and white. The brandx instructions said to connect yellow to yellow, blue to the 12v sense wire (white) and really didn't mention anything about red (ignition). I ended up getting an adapter connector from mechman, it was wired ton put yellow to yellow, and blue to white. The red remained untouched.

 

You are correcct to be concerned. +15VDC is too much for your AGM (or any lead/acid type) battery.

Also, I would not totally rely on the scangauge readings thru the OBDII port. You really should be checking/verifying at the battery with a multi-meter.

 

Quick update:

Still driving it, old battery hasn't given up the ghost yet.

I've noticed it's going 15.3v on startup and for 5 or so minutes, then back down to a static 15-15.1v for the remainder of the drive.

Still haven't found a conclusion if this is bad for a standard 12v battery and/or an AGM. Some have said it's fine, others tell me I'm cooking the battery.

 

It is odd that it is charging a full volt higher that I'd expect it to. 14.3 is typical at start up, tapering down to 14-14.1 as the battery tops off.

If the alt is actually malfunctioning with an out of spec voltage, in the long run you'll fry both batteries.