Battery not holding charge

[BillyGoat16]

I have a 91 Sea Ray 200 with a Mercruiser 5.7 V8

The battery is not holding a consistent charge. I have put in a new battery, new alternator and it continues to not charge the battery. I have had it out on the water 3 times since changing these items. The first day it held a charge in the mid 12v. The second day it dropped to the low 12v. The third day it started where it left off the second day and it dropped to the mid 11v. All three days it ran well and we were on the water for 3-5 hours each day. So it seemed the alternator is not charging the battery to full capacity. Is there something like a relay, solenoid, or fuse that may be keeping the alternator from charging the battery to full capacity.

 

[alldodge]

Measure the battery voltage before starting.
Start the motor and while running, measure voltage reading at the battery

 

[isaacs]

You're going to ruin your new battery if you keep letting it run down. Be sure to charge it with a plug in charger 'till you find out what's wrong.

 

[Chris1956]

An ammeter between the battery pos post and the pos battery cable will check for a short.

Your alternator is wired correctly, right? They sometimes have 3 connections, where two are light wire and the third cable is a heavy charging wire.