I don’t understand why you would put a choke there.
I didn't either, especially since the choke gets installed between the onboard charger and the cranking battery. Since the Humminbirds are connected to the cranking battery, it would seem that they are isolated from the trolling motor. Since the onboard charger isn't being run while the boat is on the water, there shouldn't be any risk of anything from that either.
So, I called Humminbird to ask them about it. Here's what they told me:
Because the trolling motor is connected to the 3-bank onboard charger, there
is a connection between the trolling motor and the cranking battery, via the wiring and circuitry in the onboard charger. So this makes it possible for electrical signals and possible interference to make it's way through the trolling motor, into the onboard charger, and then to the cranking battery, to which the Birds are connected.
Once they explained it to me, it made sense.
Now, if I had only a two-bank charger connected to the TM batteries, and the cranking battery was charged separately, either by a single bank onboard charger, or a single external charger, there would be no need to install the choke. But since I do have a 3-bank charger that has all three batteries connected to it, they strongly recommend installing the choke.
And since they strongly recommend it, I will go ahead and do it, just to make sure I eliminate the possibility of any problems.