Interesting. I hadn't come across these 'advanced' alternator regulators. Intelligent, multistage charging in cars seems a good idea.
(Car manufacturers are very conservative when it comes to changing tried-and-tested systems. One consequence of that conservatism is the vastly increased reliability of most modern cars compared to older cars, but another consequence is that cars still use very out-dated technology. One example I'm struggling with right now is the blower motor speed control mechanism. My car uses an array of resistors to slow down the motor, totally wasting about 50w of energy at all speeds except the maximum speed. I've just bought a Pulse Width Modulator speed controller which should save me at least 40 of those 50w and gain me one or two mpg in the process.)
But have you seen these...?
Using a capacitor array instead of a massive lead acid battery for starting would mean there is absolutely no problem with running a slightly higher voltage as there's no battery to overcharge. Your DC/DC converter could be set at 15v, effectively disabling the alternator. When the auxilliary battery is no long able (or no longer allowed) to power the DC/DC converter, the voltage will drop and the alternator will kick in and take over automatically. No change to the basic electrical system necessary: just plug in the auxilliary battery and DC/DC converter to a cigarette lighter socket or similar and away you go. Any brief current peaks will be easily handled by the capacitors so a steady 30A from the DC/DC converter should be adequate.
However, I've changed tack on my auxilliary battery idea and I'm going to go with a single, large AGM battery for the time being. I got the largest AGM battery I could cram into my existing, stock battery tray without cutting or extending anything. It's 90Ahr so not enormous, and it cost £90. Apparently AGM batteries are well suited to daily discharges of up to 50% as long as they are then recharged daily. The manufacturers claim this one will be good for 1000 50% discharge cycles and it comes with a 4 year warranty.
The reason I changed my mind and decided to see how I get on with just one battery is that I measured my car's actual current draw under a variety of conditions (various different equipment switched on) and was pleasantly surprised at how little current it takes just to run the engine. Other equipment, like the blower motor and windscreen wipers, used more than I expected, but for the ignition (well, it's a diesel) the CPU and the engine - the minimum necessary to drive the car in daylight - I only need 6.4A. I reckon I can mostly get away with using less than 20A, so that's a good 2 hour range on a 90Ahr battery without going below 50% SOC. For long journies I'm going to need the alternator anyway, and since there is no point running the battery down just to run it back up again with the alternator on the second half of a long run, the savings can only really be made on shorter trips. (Did that sentence make sense? It's been a long day!) But, the vast majority of my car use is for daily short journies from home, and is generally well under two hours, so I think I should get away with a single battery. At least I'm going to try that first. The 10A waterproof, temperature compensated, all singing automatic AGM battery charger arrived today and I'm testing it out before installing it in the car.
[Edit: With a second, auxilliary battery of course on long journies the same saving is possible as with shourter journies since the auxilliary batery can be used to save all the alternator induced fuel use up to the point where it becomes depleted. Still, most of my journies are short so that's where most of my fuel savings are to be found.]
I managed to identify and disconnect the field wire to the alternator (the 'ignition wire' that goes from the dashboard battery warning light) and that successfully prevents the alternator from starting up. An alternator ON/OFF switch on the dash will be easy to implement.
So my attention is turning at the moment to making the car electrical system less wasteful so as to maximise my alternator-free range and maximise mpg on longer trips. Hence looking at the blower speed controller. At the lowest speed the blower uses 25w but WASTES 50w in the resistor! At Speed 2/4, which I most often use, the blower uses 53w and the resistor wastes 53w. In winter especially I can't drive without the blower motor on and it makes me sick to think I'm wasting all those amps through those prehistoric resistors.
I also replaced all the light bulbs I could with LED's and got more efficient, low energy headlight bulbs. The lighting upgrades save me 52w in total when I'm using headlights, which is definitely worthwhile despite the cost of all the LED's and all the hassle of sourcing and fitting them, but the blower motor is on ALL the time and is wasting about the same amount (53w). The PWM to replace the resistor array is costing just £10.
(These wattages are at 12.4v without the alternator charging. At 14.4v they're all increased by 30%)