I have a 2 bank alternator charger from minn kota that basically piggybacks off of the starting battery.  It says "In order to achieve full output, the minimum input to the Alternator Charger from the engine alternator should be 25 amps" for charging 2 trolling batteries.  I have a 50 horsepower mercury efi.  My google search says it comes with an 18 amp alternator.

 

From Minn Kota's information resources, it basically charges the trolling motor once the starting battery is over a certain volt reading (13.6V I believe) and stops when it falls to a certain point which is why it doesn't allow a lithium battery to be the starting battery (it also does not charge lithium trolling batteries).  The reason being that lithium batteries never drop below 13.6V so it can potentially just drain the starting battery completely.

 

Would anything negative happen by using this?  I'm not the most knowledgeable when it comes to electronics, but I believe it will work just fine.  My thinking is that it will not charge as often (shorter cycles).

Give it a try.

 

"In order to achieve full output,"  may not be necessary depending on how hard you are on trolling motor batts.

 

It says you need minimum of 25a for full output, but if full output is not necessary then maybe your 18a will be fine, but is this starting batt used for anything else like bilge pump and maybe live well pump too and lights and stereo? Just make sure all 18a are going to trolling motor charging and not bled off elsewhere further reducing charge ability.

 

Give it a try. Got nothing to lose but time fishin...

Seems like a solution looking for a problem.

 

Or a problem looking for a place to happen.