While on holiday the engine battery failed
Fortunately I have one of the £20 Quick Starts
A gel battery in a plastic box
It was then two years old and has worked for the last 3 years
It fitts neatly in the pedistal
In fact I fitted 2 just incase
I have a 15 HP engine I use the boat for 3 months in the summer and start it a few times a day
It turns the engine over with out difficulity and I guess two would have no problem with a 50HP
£20 for a gel battery has to be cheap and you get a battery isolator, socket, meter and compressor as well
If you engine does not have a regulator diode split charge the battery with the main battery
Outboards without regulators current charge and the gell battery will not like that