# WARNING..shore power ON, engine ON is a NO-NO



## Giulietta (Nov 14, 2006)

Guys,

Just got back from some sailing, as we landed a few friends were discussing in a friend's boat.

The boat almost caught fire...here is the deal.

He had several problems with his Alternator, and had to change them twice before.

This time it almost set fire to the engine...here is why..

He has his boat with the shore power allways connected, every single day.

3 times a week, a care taker starts his engine (and mine) and lets it run in neutral at higher RPM...as everyone here does.

Turns out that his alternators are being damaged because the shore power is connected while the engine's alternator is also charging the batteries...

I always have disconnected (for more than 20 years) the shore power when we start the engine, but here its not a comon practice, annd most run the engine whilst still on shore power.

Normally the alternator's are protected with a bridge or something, but...never knwing..

So be warned, if you run your engine, make sure you disconnect the shore power, just in case.


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## sailingdog (Mar 19, 2006)

Hmm.... interesting... 

I'd be curious to see how he had things wired on his boat. Of course, if you don't have an AC-based charger, this isn't going to be an issue.


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## TSOJOURNER (Dec 16, 1999)

That's a coincidence. Last weekend I disconnected the shore power to start the engine for the first time ever (I suddenly recalled having heard that is was better, without knowing why).

Knowing why and the possible consequences will definately help to remember to do it in the future. So thanks a lot for the warning.


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## NOLAsailing (Sep 10, 2006)

As an aside, why are you running your motor without load that frequently? Small diesels don't really respond well to that over time.


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## Giulietta (Nov 14, 2006)

NOLAsailing said:


> As an aside, why are you running your motor without load that frequently? Small diesels don't really respond well to that over time.


Over here is comon practice to start the engine every week for 10 to 15 minutes, leave it idle for 2 minutes, then rev it up to 1500 or more, in neutral for a few minutes.

We all do it, and we never had any problems. Mind you most boats get used a lot so this is not needed. Only for boats that are long time stopped.

We know about cylinder glassing, but its a minimal price to pay for not runing the engines. Our diesel is also very good and has algae deterrents from the refinery, and is sold as marine diesel.


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## Idiens (Jan 9, 2007)

I guess there must be an odd way to wire a boat so that the shore powered battery charger fights the alternator charger. 

Not a problem on my boat though, DC is DC and diodes normally are diodes. Sounds like he had a diode failure somewhere.


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## TSOJOURNER (Dec 16, 1999)

I always disconnect first but I've been doing it just because starting the engine is usually the last thing I do. Did not realize it could be a problem if you don't. Is that a normal problem or is something wired wrong on that boat?


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## JohnRPollard (Mar 26, 2007)

I have started our boat many times while still attached to shore power without any incidents. But I have always shut down the shore power/charger almost immediately thereafter. So I can't say whether I would have a problem or not if I left it connected during prolonged periods of engine operation. Edit: We are running a Xantrex TrueCharge20 charger.

It would be interesting to know more about the wiring details.


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## swadiver (Jan 17, 2007)

You are the man. I often start my engine if I will not be sailing for more than a week. I try to remember to disconnect the shore power but sometimes I forget. I have not experienced a problem, but this make sense to me. I will be sure to incorporate shore power disconnect prior to engine start in the furture. Thanks so much for your post.


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## artbyjody (Jan 4, 2008)

*Shore power*

Older chargers that do not have blocking diodes (such as some of the cheap automobile variety) - yes, probably not a good idea to have shore power attached. However, if you have a newer system in place - you will never have a problem.

Here is why:

When the internal engine is running and the alternator is running correctly - the alternator has Voltage sensor just as the AC chargers do. There will be an initial switch on and switch off between the units but the Alternator will usually win the battle for charging the missing cranking amps, as the AC Charger is more sensitive and cuts off more quickly when sensing the voltage is higher than the float / max charge level (which the alternator puts on the line). The alternator will be a bit slower as it is sensing the pull it using to crank the engine but the AC charger will immediately sense it and shut off for protection. However, if the battery is severely drained - it is possible for both to charge at the same time - but the AC charger will go into trickle mode.

However, older systems - older Charge Units - are not necessarily as sensitive / smart with the charging but they too usually should have no problem being on when the engine is running as they are all designed to shut off when max Voltage is detected.

Causes of fires etc, are usually improper grounding or not using the appropriate gauge of wiring on either of the systems causing overheating and eventually fire. Or placement of fuel tanks too close to the engine without the space being properly ventilated before being engine is started. Most accelerated corrosions or quickly dying alternators are usually caused by fuel vapor or oil coming in contact with with the windings in the Alternator. These deposits in the end cause the alternator to arc and spark internally (and if not fire - will kill the alternator rather quickly).

Considering that both RV and Marine utilize similar techniques (actually the same - except the chargers for boats are 'marinized', for example the same Xantrax charger I have on my boat is the same as the on on my Airstream) . I would of blown my truck's alternators a long time ago by having AC power hook-up to the RV at the same time the truck is running and connected to the RV (as the RV's battery banks can be charged from the trucks trailer connection).

While it probably is best to disconnect - as it puts one in the habit of not pulling out without dragging the shore power cable from the dock... it is very unlikely with the proper system in place - that electrical issues would result with having shore power and the engine running less a failed component, poor ventilation if fuel vapor is present in the area of the engine (alternator), or improper wiring.

Just my 2 pesos...


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## Valiente (Jun 16, 2006)

Good post. Thank you.


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## djodenda (Mar 4, 2006)

ArtByJody is correct


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## denby (Feb 21, 2007)

I believe ArtByJody is correct, but I always disconnect shore power or ac charger first. Just a habit.


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## Idiens (Jan 9, 2007)

What do you guys do when you have solar, wind, shore and alternator power available? - Disconnect all but one source?

The boat should be wired to accept any combination of input without damage, which includes the inverter being automatically switched out when shore power comes on.


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## donradclife (May 19, 2007)

There should be no problems with running both shore power and the engine if things are working correctly. The boat in the original post probably had a defective regulator in the engine charging circuit.


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## chucklesR (Sep 17, 2007)

Correctly wired, it shouldn't matter. Blocking diodes are supposed to prevent any problems - automobile chargers used by people trying to save a buck might blow their systems, but not a properly installed marine grade system. - ArtbyJody writes it up much better than I can - what I can add is hands on.

Summertimes when working on the boat pierside I run the A/C on shore power(after all, why not be comfy) - and the engine at the same time. 
I'd hate to have to disconnect my solar panel to run the engine - don't you think that would be a little odd?
I plug a small generator right into my shore power outlet to fire up my 110 only a/c, while underway on the motor, it's two separate systems made separate by folks with electrical engineering creds


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## chuck711 (Dec 25, 2002)

*I must be missing something here*

I must be missing something here. Just because you are connected to shore
power it doesn't mean your AC battery charger is on. Isn't the charger on a breaker switch. You only turn it on periodical.

I find its bad practice to leave your AC charger. It would hide a possible weak battery.


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## artbyjody (Jan 4, 2008)

chuck711 said:


> I must be missing something here. Just because you are connected to shore
> power it doesn't mean your AC battery charger is on. Isn't the charger on a breaker switch. You only turn it on periodical.
> 
> Your right - it doesn't mean that is on - but it probably should be. As you will most likely weaken your battery by not doing so as they have self-discharge rates (plus what happens if you leave a light on or the bilge pumps cycles through a few times and you are gone for a few weeks). Having a trickle charger on them will keep them at optimum charged capacity.


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## Valiente (Jun 16, 2006)

I tend to unplug the "yard shore power" when I'm off the boat, because these days I'm down there every two or three days...or more if work allows.

My other boat is loaned to a woman skipper who lives about 60 miles north of here, so I encouraged her to take that boat's batteries to her house and to keep them trickle-charged on occasion.


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## TSOJOURNER (Dec 16, 1999)

Agree there should not be any problem running alternator a battery charger at the same time. If something bad happens, probably a result of a fault that already existed and they should be happy it happened at the dock! 
About chargers: Anyone using a cheap auto charger should stop immediately. Those things are not ignition protected, and they may contribute significantly to galvanic corrosion, and worst case may cause stray voltage to enter the water via DC ground and hurt possibly kill someone. This is due to the fact that they may not use an isolation type transformer. So there are 3 reasons why using a cheap automotive charger may screw up someone else's boat, and I know there are a bunch of you doing it!!! arrrggg. 

So.. trickle charging good or bad?? I have always left my 10amp double bank guest charger on at the dock, never noticed any significant loss of water from the batts. Going 2 years now and I haven't noticed any loss of capacity.


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## haffiman37 (Jun 4, 2004)

General rule when charging is to DISCONNECT the battery from alternator, normally turn off main. The reason is that all diodes have a certain leakage. Charging from shorepower with main on, this leakage might lead to overheating of the diodes in the regulator and short it. If it 'burns off' (open) nothing will happend except the alternator will not work, but if shorted, unregulated AC goes out, and that might cause fire in cable harness due to overcharging. Another reason for the incident might have been that the batteries have been shorted, and the alternator has been charging at max amp output, more than the harness have been able to handle. Shore power chargers normally goes to 20-30 Amps at max, if an alternator of 75Amps ore more starts it might be a problem. Have see a lot of people changing alternators from std 55-60 Amps to 100+, but they never emodify cable harness accordingly.


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