Question for those using hellroaring...

[Razor J]

I didn't realize the one way block either. When you first start the engine, the primary battery will require high voltage to recharge, the module will engage, and it should then top off both batteries before the voltage falls and the module cuts out. Now, if you were driving a long time and pulling power off the secondary battery for some accessory, you could discharge the battery and the module may not kick in. I bet this is why HR recomends pulling all power off the primary battery and not the backup battery (I wondered why they didn't recomend running a secondary fuse block off the back up battery like I did)
The thing about our FJ's is the small fuel tank. You will be restarting the engine every few hours when you fill up. I would think that both batteries would then get "topped off" with charge.
I'm not sure I am correct in this analysis, but I still suspect your intermittent system will keep both batteries charged. Put a charger on your backup battery once a month to see how many amps it will take--I'll bet it will be fully charged.

[Mr R2FJ]

I think you're right as well. I'll throw a charger on there from time to time if nothing else to double check the condition of the battery.

When you first start it in the morning it always seems to come on with a nice high voltage. IT's only after you've been running all day do I see this "issue". It does make sense.

[DEWFPO]

You can just throw a voltmeter on each battery and check voltages. The standard is to wait ~8 hrs after shutting off the vehicle and then put a voltmeter on each battery. A healthy, fully charged battery should indicate ~ 12.7 volts.

DEWFPPO