A marine battery switch with an alternator field disconnect switch built into the assembly would typically be used with a field-excited alternator. The use of the alternator field disconnect switch is to add a circuit to the alternator wiring. The field current will be routed through the battery switch field disconnect switch. In this way, the battery switch can also control the alternator field current.
The purpose for doing this is to avoid a possible risk of damage to the alternator
if the battery switch were to be moved to the OFF position while the engine was still running. The risk is that without a load on the alternator to absorb the output current being created, the alternator voltage would rise, possibly rising so high as to cause damage to the internal diode rectifiers in the alternator. The arrangement of the alternator field disconnect switch is done so that it will open BEFORE the main battery switch contacts open, thus shutting off the alternator just before the battery will be disconnected by the other contacts in the battery switch. The field current controls the alternator output current, but the field current is much lower than the output current.
If the OEM battery switch in your boat did not have the alternator field disconnect switch, you have been operating the battery switch with this inherent risk all the time you have been using the boat.
I am curious:
Q2: did your mechanic explain why he has recommended replacing the battery switch?
Q3: what fault is there in the present switch?
Q4: did Sue Lodel explain her recommendation to replace the original switch with the field disconnect option?
I wonder if Boston Whaler or Mercury are suggesting this change.
I strongly recommend contacting Boston Whaler customer support, or speaking with a Mercury engine service technician before making a change to original battery switch wiring. Unless there has been a recommendation from either Boston Whaler or from Mercury to modify the battery switch model, I would hesitate to make the change.
On the other hand, Sue is usually VERY well informed. I tried to reach her by telephone today (Monday), but she was away until July 25 (tomorrow). I will ask her about this.
If you were to change to a field disconnect switch, you will have to install the necessary new circuit to run to the battery switch from the engine and then back to the engine. This means the alternator must make that circuit available for external wiring. An alternator of the “unitized” type may not even provide access to the field winding circuit.
For more discussion about marine battery switches with alternator field disconnect switches, see
https://shop.pkys.com/What-is-the-alternator-field-disconnect-terminal-on-a-battery-switch-for_b_26.html