Supra SL Owner's Manual page 71

This situation could occur if you run your accessories such as ballast or stereo
for a long time without running the boat. The dual battery setup is designed
to create a redundant system allowing the user to have a backup battery. The
switch must be in the "1" or "2" position to create the redundancy.
The 4 positions available on the battery switch are: Off / 1 / 2 / 1&2. These
choices indicate which battery is being used to power the boat being charged
by the engine's alternator.
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We recommend using only position 1 or 2 while boating.
Battery switch position "1&2" is available but not recommended as power will be
drawn from both batteries and the alternator will have difficulties keeping both
batteries charged simultaneously. It also creates a ground differentiation which
in turn will create issues with sensitive electrical components such as the Vision
screen. The only time you may need Position 1 & 2 would be in an emergency
situation where both batteries are low and need power from both to help start
the engine.
Do not turn the selector switch to the "Off" position when the motor is running. The
alternator will be permanently damaged and is not covered under Skier's Choice
or Indmar warranty.
It is recommended that Position "1" be used in most situations so the secondary battery will be available to start the engine from Battery 2 (secondary)
if necessary.
Be aware that if position "1&2" is selected, both batteries will drain power at the same time and could leave you stranded due to weak batteries.
NOTE: It is recommended that you fully recharge your batteries using a battery charger periodically or after a period of high discharge (running many
accessories for an extended period of time).
NOTE: Skier's Choice highly recommends using quality "Dual Purpose" batteries.
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©2017 Skier's Choice, Inc.