Battery not keeping charge

[Bsolomon6946]

I have a 2005 Mercury 150XL SW 2 stroke motor. While out on the water, it is only good for 5 or 6 cranks. I have a brand new marine cranking battery so the battery is not the issue. It’s like the battery is not recharging like it should. After charging it at home overnight it fires right up. Please help!

 

[whitegreg56]

With the engine not running, put a voltmeter on the battery terminals (don't short anything).
Have someone watch the meter. Then, start the engine and race it a bit. If the battery
voltage doesn't rise slightly, your alternator/rectifier is not working.

 

[fastjeff]

And if it's charging properly, try rebuilding the starter motor.

Easy enough to do: Drop the end cap, shine up the commutator with fine sandpaper, oil the bearings, and reassemble.

Jeff