Engine not charging batteries

[Pieter Wybro]

My batteries are not getting charged from the engine.  After starting the engine and running for a while, my engine start battery registered 12.4 V (and trended lower) and the house bank about 12.8 at the VDO panel meter.  My alternator seemed to be putting out okay (I measured 13.1 V at cruising rpm) at the connection to the "battery isolator" which is a small electrical box located behind the house battery box that is under the sitting area in the aft cabin.  This battery isolator also has connections to the house bank and the engine battery.  The voltages of the engine battery terminal at the isolator showed a drop of about 0.6V relative to the alternator terminal at the isolator, which indicated the engine battery was not connected to the alternator.

 

Once I shut down the engine and switch on the shore power charger, the batteries all charge up nicely to 13.3 V, so I don't think the problem is with the batteries themselves.

 

My question is, has anyone experienced this problem?  Is 13V output of the alternator normal, or should it be higher?  Sould I see the same voltage at the engine battery as the alternator output (13.1 V), if not does that indicate a problem with the battery isolator?

 

Any advice from the esteemed group is welcomed.

 

Thanks,

 

Pieter Wybro

B40 hull #144

[sam]

The isolators are known to fail. Sounds like that is the issue here. Is the alternator stock ? here is a file on the isolator .

[Chuck (C_Note)]

You have to be carefull charging dead or severely depleated batteries by running the engine/alternator. With the new alternators, with the regulator built in, they are not real big and will blow out quickly. I've found that you can install an older, external regulator that's built much stronger and will take the current better. I installed a new Delco marinized alternator with the small internal regulator and tried to charge low batteries. Didn't work. The repair tech explained what happened. That's why you need to charge up your batteries first or have something like a shore powered charger or even a solar charger hooked up to keep them topped off. 

[Mike Brown]

The standard 80amp alternator will only put out about 35 to 45 amps at 1200-1400 RPMs, about what you might expect while trying to recharge at anchor. Since these boats can easily consume 120+ ahrs a day it only takes about 3 or 4 days and you'll need to plug in to shore power to fully recharge the house batteries. That's based on a reluctance to not run the engine for 3+ hrs a day to recharge back to 80%. It's also based on the 450 ahrs we have on our boat. Smaller battery banks will only make the need to plug in happen sooner. I believe the battery charger also passes the current through the isolator so if you charge up while plugged in then it may be you're just using more than you can replace while at anchor.