This baffles me. You are trying to not have the echo circuit connected to ground but a separate neg buss because your fear corrosion
from stray electrons?
Or you want to charge the start directly from the alternator?
What is the goal of this exercise?
I believe the idea of the echo charge is to isolate two somewhat dissimilar batteries of different sizes and charge requirements. The start bank needs lots of cranking power and is used only for brief spurts. The house banks is used "continuously" and requires more capacity. So the alternator is connected to the house bank which under normal use will require more charge to top them up. The echo charger then siphons off the excess charge and send it to the smaller start bank WHILE the alternator is on and WHILE other charging sources are on and even WHILE the house bank is on shore power
of WHEN it simply is full up and can send some little charge over to the smaller start bank.
I nhave on echo installed and I have also altered the wiring of my key switch. What I discovered is that the power coming to the key switch is then used for the sarter solenoid AND the engine instruments, including lighting
AND the ventilation for the engine compartment. So this circuit is under load and draining the battery WHILE motoring, NOT just for starting! I moved all loads EXCEPT the solenoid from the key switch and so the only drain is the starter motor
and closing the solenoid.
I have found that my start bank is always 100% topped up. It an Optima blue top 55AH and the engine is a Volvo
36HP. Starts are no problem!
The house bank are 2 AGM
8Ds in parallel with 100 watts of solar
trickle charge with a PV14 regulator. The alternator is a 120 amp output with a MaxCharge612. Either the house bank does not become deeply discharged or the regulator alternator system is off, but I rarely if ever see more than 40 amps output and only for a few minutes as it ramps down and settles float in the 5-10 amp range. I suppose I never get deeply discharged to demand oddles of charging for the house bank. And this may be because we use our engine at least an hour or more a day when on board for refrigeration
(engine drive), making hot water
(heat exchanger via engine) and shower
and vacuum (AC loads) during this "charging period". When all is being done, we are showered, the boat is topped up, the hot water
topped up, the batteries topped up, the refer cooled down. Now we use the windlass
(engine on) to weigh anchor
out of the harbor to raise sail and revert to use of the house bank for electricity. When we are going to make landfall we use our topped up start bank to get the motor going and the alternator cranks out as many amps as required to replenish the used power of the sail.
This is simple and seems to work. Try it!