I did learn this the hard way - my Merc mechanic learned me up on it when I took it in to him after being helped back in. This was a 1999 200, so maybe something has changed in 11 years. I have different boat and a 150 ETEC now with two batteries with a Perco switch. I don't know if it has the same cranking amp requirements as the Opti, but I am cautious anyway. I start on battery one and head out. When I get to my fishing grounds, I switch to battery 2 to anchor, troll or whatever. Runs radio, GPS, bilge pumps, baitwell, etc. Then I have battery one ready to crank at full charge again because it charged all the way running out - sometimes a two hour run. If battery two won't crank it to head back in, I can crank on battery one, then switch to "both" and run back in, charging both batteries. I also have an on-board charger system and I charge both of them the night before I head out.

But - to be honest, I have never had a situation where either battery would not crank the ETEC, even after a day of fishing. Maybe I am over-cautious with what I do, but I sure don't want to be stranded again. Maybe someone else knows the ETEC requirements? I can't find anything in my owners manual that covers it.