Missing from the formula señor is the amperage rating of your alternator and the ampere hours of your battery bank. You COULD if you wish to get into this is to SHUNT part of the field negative from your alternator to negative. Yep the check engine light may come on. notice I said PART not all. Newer system use type A field control (called N by marine electrical types) this means the ECU throttles the NEGATIVE. The positive to the alternator field is total uncontrolled (except by the ignition switch) 12 volt positive.
Wanna get exotic? Dig up a THIRTY OHM EIGHTY WATT RHEOSTAT, and wire that into the negative field shunt that you made to chassis ground. Now you are the boss, not a 29 cent chip. And NO it will not "hurt" the ECU unless you forget what you're doing and pass out in a hammock. This is FULL MANUAL CONTROL of your alternator. If the engine bucks and farts when and if it goes into open loop because you are bypassing all those carefully manufactured insane-asylum grade software setpoints in the ECU so-be-it. It'll get over it. If a mechanic screams NO NO NO, ask him "What happens WHEN the alternator brushes wear out and the field goes to ground?" Oh I get it the alternator catches fire, the rig torches and this happens all the time, right. Look at all those flamed hulks pushed off the side of the road.
