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Cruisin Cat · 01-12-2011, 07:58

Hi all,

I need some advice on disconnecting an alternator from the batteries when it is runing.

On occassions when I have to run my engines for long periods (eg no wind on a passage) my standard volvo alternator with internal regulators are overcharging my batteries. I have disconnect switches on the boat that will disconnect the alternators from the batteries (effectively isolate the alternator electrically from everything)...but not sure what will happen if I open it while the engine/alternator is running.

I would appreciate advice on this....ie engine/alternator running and batteries already charged so charge current from the alternator is only about 5A...is it safe to open a switch that will isolate the alternator from batteries and all boat electrics? My question is really about whether it will damage the alternator /regulator?

Looking forward to receiving some comments on this.

Target9000 · 01-12-2011, 08:01

Do not isolate the alternators. It will generally blow the diodes and render the alternator unusable without repair or replacement.

MarkSF · 01-12-2011, 08:05

Quote:

Originally Posted by Target9000

Do not isolate the alternators. It will generally blow the diodes and render the alternator unusable without repair or replacement.

Like he said. Don't do it.

Target9000 · 01-12-2011, 08:09

I often see threads like this with titles asking a very specific question about a possible solution to a problem, but it might be more fruitful for you to ask about the problem, instead of a possible solution.

What I mean is... Instead of "Can I disconnect my alternator from my batteries?" You may consider asking, "How do I prevent over charging my batteries with long running internally regulated alternators?"

You'll get a lot of answers and if you're lucky, you'll start an argument between some people as they try to answer you.

**colemj** · 01-12-2011, 10:08

The switch would have to go in the field wire to stop the alternator from charging without damaging it.

The standard volvo alternator should not overcharge the batteries under any condition or any length of motoring. If it really is doing this, you have a problem with the alternator.

You may have a problem with the batteries. What are the symptoms that you describe as overcharging?

Mark

Maine Sail · 01-12-2011, 10:16

Quote:

Originally Posted by colemj

The switch would have to go in the field wire to stop the alternator from charging without damaging it.

Bingo! But this may be hard to do on the stock Volvo alts. Some are self excite and some are "key on" excite. If it is key on excite it is fairly easy to turn it off via the excite wire. If not you'll need to dig in and find the field wire..

Disconnecting the "load" or battery banks can cause the voltage to spike, the regulator really can't react fast enough to stop it, and you may blow some or all of the diodes depending upon how hard the alt was working at the time.

Quote:

Originally Posted by colemj

The standard volvo alternator should not overcharge the batteries under any condition or any length of motoring. If it really is doing this, you have a problem with the alternator.

You may have a problem with the batteries. What are the symptoms that you describe as overcharging?

Mark

Spot on.. Unless your alt is feeding your banks voltages in excess of 14.6V for looooong periods of time your batteries should do just fine, though they might take a little more water.. Most of the Volvo alts I've measured are 14.2V to 14.4V and I can't recall if I have seen one with absorption set to 14.6V or not....

Cruisin Cat · 09-12-2011, 12:34

Thanks everyone for your comments. I apologise for not replying sooner...I've been sailing in the western Venezuelan islands and had no internet.

Yep, I had a feeling that disconnecting the load (batteries) from an alternator would not be good for it.

The problem I have with my standard Volvo alternators is they just sit at about 14.2 V for ever. I have AGM batteries (so can't top up water) and the manual says charge voltage should be reduced to around 13.3V when charge currend drops below 0.5% of bank capacity.

My alternators have internal regulators so not easy to put a switch in the field wire. I'm reluctant to start stripping them down as right now I'm headed to Panama and the Pacific.

There may be an external key on power supply to the alternator...I'll check that.

Any other thought are very welcome.

nial · 09-12-2011, 12:45

If your alternator's field wire is switched on/off by your ignition key you could use this to stop charging but then all your sensors; oil, temperature, etc. will not function. Big no/no.
nial

Cruisin Cat · 09-12-2011, 13:04

Quote:

Originally Posted by nial

If your alternator's field wire is switched on/off by your ignition key you could use this to stop charging but then all your sensors; oil, temperature, etc. will not function. Big no/no.
nial

If I found the wire from the key to the alternator (assuming there is one which I need to check) and put a switch in it...wouldn't that just shut off the alternator without effecting other enging systems?

Richard Harper · 09-12-2011, 13:18

I had that problem on a long hot night motoring. Smelled sulfur and found the batteries boiling away. After everything cooled down I pulled the plug from the stock alternator which has the excitation for the field so it would not put out current. This got us home. The batteries may have been near the end of life but I felt they were still good enough for a few weeks cruise. I put a switch in the line from the key to the alternator after that. When I switched to an aftermarket alternator with external regulator I kept the switch just in case.