Batteries won't stay charged

[Rinkerguy]

I have a 2003 Rinker, Mercury 5.0 MPI motor, and dual batteries

While connected to shore power, batteries fully charge

While under way, volt meter registers maybe 12V

After anchored for a couple of hours, battery that I'm running off of is dead

Batteries have both been tested, and are o.k

Anyone have any suggestions as to where to start looking?:confused:

[ITD]

Underway, volt meter should register about 14 volts, I would start simple, alternator belt, battery cables and connections, then work up from there.

[jrc]

An accurate volt meter on a running motor should read a lot more than 12 volts. Your dashboard voltmeter may not be that accurate so you should check with something accurate, a cheap Radio Shack digital meter is plenty good. I can't remember the exact number but charging voltages should be around 14 volts. Any less and your alternator is not working. Did you accidently set your battery switch to OFF when the engine was running. Even if just for a second, that can burn out the alternator diodes.

A good alternator shop can rebuild it for you, just make sure you don't put an automobile alternator in there. I'm pretty sure the alternator is one of the electrical parts that must be marine, to prevent explosions from gas fumes.

[Kevin C]

Have the batteries been load tested? This is the true test of battery strength.

[Rinkerguy]

Batteries checked out both O.K according to one of those computrozed testers. Cables look O.K

Alternator was our thought, but didn't want to replace until we were sure we exausted all options ( +/- $175 to replace)

Don't own a volt meter, but going to probably invest in one of those cheap Radio Shack types.

Thaanks all for the input

[ITD]

If you get one of those volt meters, I would check the output of the alternator and all the connections between the alternator and the battery. Would be mighty embarrassing and expensive to bolt that new alternator in place and still have the problem. Good luck.

[NightWing]

Places like AutoZone will test your alternator for free. Just don't buy one there if it is bad because you need a marine (sealed) alternator.

[NightWing]

Just another thought, you should have a starting battery for operation (#1) and a deep cycle battery (#2) for anchoring. When you are at anchor, switch to battery #2. Even if you run that one down, the starting battery (#1) should be fine. Exception is if you are only charging 12 volts, which will draw the battery down.

[Dave R]

I hope it's simply a charging system problem. Look into repalcing just the brushes and regulator on the alternator, rather than the whole alternator, if possible. On some, the whole job takes 5 minutes without removing the alternator.

Something tells me it might be more than just a charging system problem. A couple of hours at anchor should not discharge a battery. Perhaps something else is drawing lots of current and the alternator is struggling to keep up, hence the low voltage.

[ApS]

"...I have a 2003 Rinker, Mercury 5.0 MPI motor, and dual batteries...While connected to shore power, batteries fully charge...While under way, volt meter registers maybe 12V..."
Are the batteries as old as the boat? Most factory batteries begin to fail after 3 years.

Did the "electronic testing" include a load tester? That type puts a "short" into the circuit, heats up, and gives you a real idea of its potential.

A load-testing meter is available today for $10. I got mine (the same one) a few years back, for $30! Lucky to be you. :)

(And don't use one of those those computrozed testers—they're no good :D ).

There's a chance that you have a leak while at anchor. Remove one cable and put a 12V light bulb between the battery post and the cable. With all equipment off, the light bulb shouldn't glow—even dimly.

Your voltmeter should show 14.7 volts at full charge.

[jeffk]

I am wondering along the same lines as Dave R. After you are on shore power and the batteries are fully charged (I am assuming an on board battery charger keeps the charge at peak), disconnect the shore power and run whatever you normally would at anchor. I assume when you are "at anchor" you mean the engine is off and you are on batteries only. If you try this "at dock" test you eliminate the alternator charging system as culprit because you are not using it. If batteries die at the dock then either your batteries aren't really in as good a condition as you think OR you have a heavy current drain that is tapping them out. You could set up to test the amperage draw on the battery but I'm not sure what "normal" would be.
One thing to look for might be lights left on in enclosed spaces (engine compartment, glove box, etc.). I left my trunk light on in my car and it drained a brand new battery in a day. If it is a current drain it could be coming from just about anywhere in the system. Checking amperage through various wires could help narrow it down.
Good luck.