The single engine on a Boston Whaler 235 CONQUEST is probably a VERADO. The principal purpose of the two batteries on your boat will be to start the engine, from either battery in the event of a failure of one of them. The requirements of the VERADO for its starting battery will constrain your choices for battery selection. Both batteries must meet the VERADO requirements, which specify a massive battery capacity and insist on absorbed glass mat construction. Based on
information from Mercury themselves, the battery requirements for the VERADO are:
--must be AGM
--must have 800-MCA and 135 minute Reserve Capacity 25-Ampere discharge rating for USA (SAE-rated) battery, or
--must have 975-CCA and minimum 65-Ampere-hour for international (EN-rated) rated battery
This same specification appears in the most recent editions of the
VERADO owner's guide.
Only a limited number of batteries can meet those strict requirements. You'll be buying batteries in the US-$300-or-more price range. I recommend you contact Brunswick's Mercury Marine division customer service, and ask them for a list of AGM battery brands and models that satisfy the requirements for your VERADO engine. They should be able to provide you with the exact manufacturer name and exact model name of batteries that are recommended for this service.
By my observations, the ODYSSEY EXTREME SERIES battery model PC1500 34T appears to meet those requirements. Cf.:
http://www.odysseybattery.com/extreme_battery_specs.aspxI presume the present battery wiring on your Boston Whaler 235 CONQUEST provides an OFF-1-BOTH-2 battery switch. If better management of battery charging is desired, change the battery wiring to a configuration using an automatic combiner relay (ACR) or voltage sensitive relay (VSR).
If the Boston Whaler 235 CONQUEST boat does not already have a battery charger that runs from 120-VAC, then I recommend getting the BLUE SEA SYSTEMS battery charger and ACR combination model 7605 product. Please visit
SMALL BOAT ELECTRICAL for more information on the Blue Sea System charger and ACR model 7605:
http://continuouswave.com/forum/viewtopic.php?f=9&t=1545If you want to isolate the chart plotters from the engine cranking battery, you could designate one of the two AGM engine cranking batteries as the HOUSE load battery, and change the primary power distribution switch to an isolating dual-pole battery switch. If you did that, the Blue Sea Systems model 1155e switch would be a good choice. Follow up in
SMALL BOAT ELECTRICAL if you want more details about the electrical wiring.
If you want to maintain isolation during engine starting, employ the optional wiring for engine cranking isolation. The details of that wiring are provided in the Blue Sea Systems literature for their products.
Topics related to the electrical systems of the electronics used on small boats are best discussed in
SMALL BOAT ELECTRICAL.