Alternator not strong enough for dual batteries?

[aawolf]

I've been running my dual optimas with National Luna Intelligent solenoid setup for a couple of weeks. The solenoid comes with a monitor and I'm able to see what the voltage on each battery is in real time.

I've noticed that once the solenoid is active (i.e. both batteries are receiving charge from the alternator) the voltage on the batteries is between 12.6 and 13.0 when idling at 700 RPMs. Is this too low?

While driving I'm seeing the charge on both batteries get up to around 13.5, but I'm worried that prolonged idling where, no real charging is actually happening, is going to have an adverse affect on the longevity of my batteries. Are these concerns unfounded?

EDIT: Whether it's bad for the batteries or not, isn't this a bit weak on my alternator's part? The Life Remotely guys, who did a similar 4Runner Dual Battery install, apparently were able to charge their setup 100% while idling in under an hour. At my rate, it may not charge at all.

 

[IBallEngineer]

First check voltage with a real voltmeter. Yes that is low, not terminally, but it is low. What is Battery Voltage at rest? It will charge as long as you are above battery voltage, and the real issue is what happens when you have the lights, fan, 6000W stereo running.

 

[aawolf]

First check voltage with a real voltmeter. Yes that is low, not terminally, but it is low. What is Battery Voltage at rest? It will charge as long as you are above battery voltage, and the real issue is what happens when you have the lights, fan, 6000W stereo running.

Yup, I have already verified the meter's values with a voltmeter. Every time I've spot checked it it has been right on. I believe the data can be trusted.

What do you mean by at rest? These are the idling values in normal usage, though I did have my stereo and small amplifier powered on when taking this picture.

 

[IBallEngineer]

Yup, I have already verified the meter's values with a voltmeter. Every time I've spot checked it it has been right on. I believe the data can be trusted.

What do you mean by at rest? These are the idling values in normal usage, though I did have my stereo and small amplifier powered on when taking this picture.

I mean engine off, nothing on. It should be 12.6 or 12.7. Check each battery on it's own, in case one is low. I realize you have them isolated, but I don't trust those kits, although if it using a solenoid they are generally OK.

So if the batteries are at 12.7 at rest, and 12.7 at idle, that would mean you aren't adding charge, only maintaining. Again, what is more important is what happens when you turn things on.

 

[pluton]

I have the same controller, except that the 2nd batt is in the National Luna "Portable Power Pack" so it can switch between vehicles.
Mine reads at 12.6(100%) with the batteries separated, engine off after sitting overnight.
With the engine warmed up and idling at 700/800 RPM, and the batts connected, it reads at either 13.7 or 14.2(the two highest indicator dots).

 

[Brveagle]

I wouldn't say it's too low, but sounds like your alternator is tired.

Basically at 12.6 volts, you aren't charging the battery at all.

 

[pluton]

I wouldn't say it's too low, but sounds like your alternator is tired.

Basically at 12.6 volts, you aren't charging the battery at all.

This is my assumption as well. Have the alternator output checked.

 

[fourwd1]

You need a minimum of 13.8V to charge a battery.
And if your isolator uses diodes, you loose .7V through each one.

 

[jeremyc74]

What happens to the voltage when you add electrical loads like headlights and A/C fan? Does it hold where it's at, or drop even further?

You may have a voltage regulator that isn't doing its job, or a bad diode(s) in the alternator.