I have a 40 hp 4 stroke which was difficult to start yesterday early morning . Eventually the motor fired and there were no further problems during the morning with dozens of stops & immediate restarts. The boat is garaged and the battery is always maintained at full charge by recharging every few days , although the boat motor had not been run for 6 weeks. I have also have a 100 amphr deep cycle battery which is used solely for powering a Minna Kota. If I connect the 2 batteries in parallel (both fully charged) will the deep cycle battery give any substantial boost to the starting battery on cold mornings or is there a danger of damaging the deep cycle battery?
Also if the batteries remain in parallel during 7 hours use, will the motor alternator charging damage the deep cycle battery?