Mercury 225 Optimax will not start and barely turns over real slow

[Khrisomey]

Ok guys I replaced my low pressure fuel pump and cranked it here at home and let run for about 5 minutes. Went to lake and launched and boat cranked right up. Got up on plane and about 15 seconds later boat just stalled and shut off. No alarms or anything. It happened right after I was trying to trim motor up a bit after I got up on plane. I have replaced plugs, fuel water separator, new cranking battery, and it still will not fire up. When I turn key over it makes initial beep because its in the run position, then when I go to crank it, it just stalls and finally turns super slow. What do I need to check. 2001 Mercury 225 optimax motor.

[Krazymaan]

Everyone hates to hear this but check the battery connections even if you absolutely know they are good and perfect. Check connections at starter and under shroud ground. If you have a voltmeter, even better. While its in the dead mode. Touch grounds and positives under the shroud from battery and see where the weak link is (low voltage). I've been stubborn enough to stomp my feet and yell, its not that, many times. To find out it really was that, I didn't look at it deep enough. Or you didn't plug something in tight enough when you had things apart and your motor ran off of the battery until it died.