Battery charger only charging Genet battery?

[Dark Horse]

Hello all, I have a 2001 Mariner 350. It has 2 main batteries and 1 battery for the generator.
Recently the engines would not turn over so I replaced both the main batteries. When I went back to the boat 2 weeks later, those batteries were almost dead. The boat is in a wet slip with dual 30 amp power.
So i checked the battery charger and it appears it's only charging the Genset battery. The charger is a Mastervolt 12/3-40. I can check each bank and it shows that the main battery is low. (Shows a series of light bars and while the genset battery shows 3 yellow bars which is good, the main batteries show 1 red bar not so good)
Is it possible that the charger has failed?
I checked all the wiring and nothing loose or corroded. Checked with a volt meter and it seems like its only putting out a reduced voltage (7 volts or less). Maybe swap wires from genset to main and see what it does?
I am thinking I will have to replace the charger as I am not sure how to further test and / or fix this issue.
Any ideas?

[km1125]

There should be fuses or breakers between the charger and each battery. Are there, and have you checked those?

[Dark Horse]

I have not as I didn't know there would be any inline fuses. Not sure where to look for those but will check tomorrow when I go to the boat.

[km1125]

There definitely should be and they should be closer to the battery end of that link. The idea is to protect that wire if somehow it got compromised and shorted to ground closer to the charger end.

As a TEST, you can also swap the connections on the charger end and see if the problem (low volts) changes when you move the generator connection over to where one of the starting batteries is hooked up. CAUTION - realize that those cables are connected to the battery (via previously mentioned fuse or breaker) so those ends will be "live" and need to be protected from shorting as you removed and replace them.

[Dark Horse]

I do know that the positive wires from the charger end at the battery switch that is on a panel in the cabin not at the batteries themselves. Hopefully those fuses are not behind a wall panel.

[bud37]

QR_BBPOST I have not as I didn't know there would be any inline fuses. Not sure where to look for those but will check tomorrow when I go to the boat.

If your boat is still wired as factory then the charger 12v side goes thru breakers in the panel where your battery switch is.......aft corner wall of salon above the couch....they are labeled. Become familiar with that panel and the relationships with the battery 1, 2 ALL positions as that will portion out the battery reserve to the boat when you are away from the boat.....even with that switch turned to off the charger will charge whatever batteries you have told it to with the breaker switches.

That 7 volts is probably the dead battery reading. Check out what you have before you decide the charger is no good.

The 120v breaker for the charger is in the panel below the entertainment center......hopefully no previous owners have messed with this......

[Dark Horse]

No I am lucky my boat is stock and as it came from the factory. I did pull that panel off the other day- thats how I know the charger wires end there.
To test the voltage i took both ends of one of the drained batterys and put the voltage meter to it and thats where I ready 7 v. I am guessing the 7v was coming from the charger as I had battery switch off so it was isolated from other battery.

[bud37]

The charger should not be controlled by the switch......just the 12v from either bat#1/Bat#2/ or all or off. That is the way our boat was anyway.

I have an idea that charger has been changed as I believe based on our boat and a few friends the factory charger was a Charles 5000 series, but my old memory could be wrong....

Disconnect the leads at the batteries themselves and take your readings there.....FYI...be careful with that wiring bundle behind that panel as I am sure you found out, it is real tight and it is easy to break switch/breaker terminals.

[Dark Horse]

Yes the boat orginally had a Charles battery charger. I have all the orginal paperwork.
And to measure the charger voltage I do just that. I disconnect both positive and negative cables from one battery, turned master switch to off so I got no back feed voltage from other battery and measured. It read 7.3 volts. I belive it should be 14 v according to owners manual.
Both battery bank 1 and 3 (main batterys) are showing low battery while the generator battery is just fine.

[km1125]

If the batteries were down to ~7 volts, I doubt they would take a charge. Not sure on that specific charger, but some need to sense a "reasonable" voltage before they'll start charging. Sometimes you can do that with just paralleling a good battery to the bad one for a few seconds (or minutes) to get the charger to "kick-in", then it would do the rest of the charging itself. A lead-acid battery is DEAD when it's resting voltage is below 11 volts. (some would say more like 11.8VDC). Below that it's pretty much damaged goods and no measurements are really meaningful.