Mercruiser 5.7L charging question

[Phlorida]

I went to start the boat yesterday and the start battery was completely dead. I used the combiner feature of the battery switch and it started with the house battery. I have a BEP battery switch cluster with VSR connected to two Optima D31M batteries that are 18 months old. The batteries pass a load test and appear to be fine. The battery switches appears to be working fine as the house battery isn't drawing down and the VSR light comes on as soon as the engine starts. There are no direct connections to either battery other than the battery cables (not even the bilge pump as the boat is dry stored with the drain plug out). Both battery switches are turned to the off position when the boat is put away.

This would seem to me to indicate that the starter or the alternator is the cause of drawing down the battery but I thought that I would ask for an opinion on the forum. The starter operates consistently. I do notice that at idle RPM's that the voltage on the volt meter reads less than 12v but it comes up to normal ranges with even a little more RPM's. Thoughts?

 

[steelheder]

Is your boat moored or on a trailer? If it's moored, is there any possibility the bilge pump is kicking on and drainig the battery?

 

[Phlorida]

I keep the boat in dry storage. The bilge pump isn't connected directly to the battery.

Is your boat moored or on a trailer? If it's moored, is there any possibility the bilge pump is kicking on and drainig the battery?

 

[CliffA]

I do notice that at idle RPM's that the voltage on the volt meter reads less than 12v but it comes up to normal ranges with even a little more RPM's. Thoughts?

if the voltage gauge is not reading around 14 - 14.5 Volts while charging at higher engine rpm's i would suspect a problem with the alternator or the belt slipping on the alternator and not spinning it fast enough....

cliff

 

[Phlorida]

if the voltage gauge is not reading around 14 - 14.5 Volts while charging at higher engine rpm's i would suspect a problem with the alternator or the belt slipping on the alternator and not spinning it fast enough....

cliff

I am suspicious of the alternator as well. I didn't realize that the gauge should be reading that high when charging - mine runs very consistently at 13.5v when running at cruising speed.