Charging Voltage over 13.8VDC
Depending upon your battery type (and ambient temperatures), typical Charging Voltages are always considerably higher than 12Volts.
Bulk (Absorption) Rate:
Flooded Lead Acid: 14.4VDC (@ normal temp. 77 deg. F - Lower V for Hot temperatures, and Higher V for Cold temps.)
Be aware that your small outboard may not have a “Regulated Output” alternator, suitable for Battery Charging. Many small O/Boards only have an “Unregulated” power supply, suitable for operating lights.