Thanks for the suggestions. Here is a better description of what I'm trying to do:
This is for a Ford E350 V10 Triton-powered Motorhome I recently bought. It seems that the parasitic current drain to keep the ECU alive with engine off is ~31mA, which will kill the starting battery while parked for about 1 month. If you disconnect the battery while parked, the ECU looses its engine constants, and it reverts to the "limp-home" mode, where the engine runs like crap. If you need to get the state-mandated emissions inspection, you have to drive it 100mi just so the engine begins running well enough for it to pass the inspection... (What a @!#$$%^%%$ STUPID DESIGN)!!!
The solution will be a charger which will be plugged-in to the AC for the several months at a time while the motorhome is parked. I am planning to operate from a 24VAC 2A transformer, which will be in my utility shed, and the wire running to the motorhome will only have 24VAC on it for safety. That way, if I accidentally hit it with a snow shovel, I wont electrocute myself.
The charger will rectify the 24VAC and make the FLOAT voltage for 23.75hours, and will bump up to the CHARGE voltage for 15min per day, simulating driving the motorhome once per day. The CHARGE voltage is similar to what the alternator voltage regulator puts out, however, since the engine compartment wont come up to normal operating temperatures, ao the temperature compensation will follow the electro-chemistry based temperature correction.
The data for temperature dependence of the FLOAT and CHARGE voltages came from
Dr Mark's web site, here . The attached sim shows the required temperature correction coefficients for a 6cell battery in the behavioral voltage sources, and the plots show the temperature dependence.