Mercury Optimax Models 135, 150, Direct Fuel Injection., Page 77Get this manual
CHARGINGSTARTING SYSTEM
1DO NOT smoke near batteries being charged or which have been charged very recently2DO NOT break live circuits at terminals of batteries because spark usually occurs at the point where live circuit is brokenAlways be careful when connecting or disconnecting cable clamps on chargersPoor connections are common cause of electrical arcs which cause explosions3DO NOT reverse polarity of battery terminal to cable connections