2003 ST 4 Stroke 9.9/15HP Johnson outboards Service Manual, Page 209Get this manual
Handling LeadAcid Batteries
Strong acid solution (sulfuric acid) If solution gets into eyes, wash and doctor immediatelyIf spilled or splashed on any part of body
Charging lead acid batteries
1) Attach and remove these cables with charger UNPLUGGED from 110 wall socket(Prevents shocks if charger is defective 2) Observe correct polarity when connecting these larger leads3) Always charge in well ventilated areaCharging causes acid solution to give off hydrogen gas through the vents in the capsMake sure vents are openIf clogged, pressure inside may buildBattery may EXPLODE