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I am not familiar with the 1996 40hp Johnson.
Can it be emergency rope started?
Do you have the emergency rope that came with it if it had one?

No need for an extra battery on a small boat if you can rope start the motor unless you also have an electric trolling motor.

However, on some of your trips, you might consider taking along a jumper battery system. Even has an air compressor, electrical outlet, etc. Even a worklight if it gets dark.
Not bad to have onboard for longer trips.
Here is one of many.
http://www.walmart.com/ip/Wagan-900-A...r/14521520