I'm on a 8000-mile road trip to Alaska. I noticed that my voltage is not great. I would only get to 12.5 and when driving sometime it would drop as low as 10.5. So in Seattle, I put in a new Altinator and I got 13.5, but after an hour on the road it would still drop down as low as 11.9. I have a dual battery set with a Luna National battery controller. When the second battery kicks on after the 5-minute delay I can only get 12.5 the second battery. I'm running a Dometic Icebox and I have to run it off the main battery while driving and when camping the second battery will keep the frig cold overnight but the car won't charge it up all the way on the road. I was hoping to do more camping but I need to charge the batteries overnight at the hotel so the Frig will stay cold for one night camping. I'm wondering if I got a bad replacement alternator or, is there something I'm missing, like a relay. Sorry for the rambeling here. Any help would be great.

 

How are you measuring voltages? I would use a multimeter at the two batteries while the engine is running. Should be above 13v. If the batteries are actually fully charged it might cut back to a lower voltage. Can't drive my car right now or I would check. Maybe someone else can check voltage with car running after a longish drive.

 

Could be your battery, could be voltage drop (bad cables, connections, and/or grounding issues), could be a bad new alternator. Could be 2 or 3 of those things. Could also be an issue with the pwm signal from the charging control circuit that's less likely.
Dropping to 10.5 while running should never happen. 10.5V not running means your battery is dead, at least. How and where are you measuring voltages?

 

On a cold start, you should be close to 14 if not a little over. 13.5 is a touch low on startup, even running that is a little lower than I would like but acceptable. I recommend pulling the starting battery from the circuit first. A bad battery can cause issues. Of course, as mentioned, it could be a bad replacement. Really need a more in depth diagnostic.

Does the aux battery have a separate ground to chases? (it should).
What size are both batteries? Are they both the same type of battery?

 

I thought I replied to this from my phone but I'm not seeing it. I found the culprit. There are many large wires that go to the auxiliary batter and the aftermarket amps. there was a fuse link that goes from the main battery to the 2nd battery that only had 12.5 volts at the 2nd battery side and 13.5 that goes to the main battery. I pulled it apart and gave it a good clean and that was it. Now I have 13.5 on the second battery. Reading all the comments helped me get to it. Thanks to all that replied.

 

That is really scary wiring work right there. Damn...

 

I bought it that way, it had a $7500 stereo installed in it. The big fat wires go to the amps that are under the seats and the smaller ones go to the second battery. It actually is a very well-done system and this is the only problem I have had with it.

 

FYI, I made it to Alaska almost 4000 miles getting here and two weeks of a journey. I did have to replace the driveline on day one and the alternator on day four. 14.2 MPG and zero oil consumed. Now it's got to make it home.

 

Awesome, enjoy it up there! I made 2 long trips there from GA over 2 consecutive years about 25 years ago. Amazing places. Camped the entire way, doing 'vanlife' overlanding before it was cool, in a 1986 Nissan Stanza wagon
It did about 15,000 miles over those 2 trips and just needed one alternator and 1 fuel pump ground wire along the way.
My favorite areas were the Kenai peninsula and the Valdez area.