Shore power and starting engine issues?

[MalcolmP]

Over on our FB group there was an issue I had not previously contemplated. When at the dock and on shore power apparently there is a risk of damaging the alternator if the engine is started.

I don't recollect any warnings about this previously, and touch wood I don't think I have had a problem. EG when changing filters etc when I just start the engine even though will still be on the mains charger.

Any thoughts from our knowelable sparkies?

Thanks

[Trevor]

Hi Malcolm,

No risk at all in starting the engine with the yacht plugged into shore power.

It is simply not an issue.

Regards,

Trevor

[hoppy]

A sparky warned my uncle against it with his 36i. I think it was related to the battery charger being on. Not sure of the risk.

Never had any issues when I did it.

[MickeyB]

Very interested in this.  I was told when I bought the boat to never be on mains when engine on.

They told me it was the battery charger and over charging risk.  To be safe I always disconnect but it would be so much better if I didn't have to.

We sure this is safe?

Mike

[Trevor]

We usually have a few charging methods for our batteries on a yacht. Ultimately these charging methods are generally in parallel to charge the batteries. Sometimes we may have isolation devices like diodes or mosfet isolators but the principal still holds.

The charging mechanisms are demand driven. If an alternator "sees" a flat battery it drives hard to charge it. It is actually trying to maintain a certain voltage and the only way to achieve that is to provide current to charge the battery.

Same with the solar regulator. If it sees a flat battery it uses all of the capacity available to it to charge the battery by trying to raise the battery voltage by supplying current to the battery. The same can be said of a mains powered battery charger. It supplies current until a certain battery terminal voltage is achieved.

If any of these devices sees a battery that is fully charged as denoted by its terminal voltage, they stop charging. They do that by settling on a voltage that simply holds the battery to a full state. I have sold the process a little short because some of these devices are very sophisticated and deliberately change the voltage to allow various stages of charging like bulk, absorption, float etc but one of their main jobs is to not overcharge the batteries. Each device is designed to take that responsibility seriously and individually they achieve that.

If they see a voltage that is saying the battery is full they simply maintain that voltage. They don't increase the voltage to overcharge the battery. The exact voltage that deems the battery to be full is usually selected by selecting the battery chemistry, eg AGM, GEL, etc. When the battery if full they simply hold at a certain value and sit in a balanced state where they may simply provide a few milliamps to keep the battery at a state of full charge.

On that basis, provided the battery charging systems we are using have a reasonable degree of sophistication, we can put as many of those in parallel as we like and no harm should occur as each charging device is presented with the same voltage and each device will work in the same way until the voltage reaches the status of "full". At that point each device will simply present a voltage to maintain the battery "full" state.

I liken it to having multiple water storage tanks all joined by a connecting pipe at the bottom of the tank. They all sit in equilibrium with the same water level in each.  Attaching another tank with the same level of water in it does not make the existing tanks water level rise.   It simply sits in a state of equilibrium.   it is a crude analogy but that is the mental picture to have when considering multiple charging sources to a battery bank.

Regards,

Trevor

[MickeyB]

Now you have put that clear picture in my head, I feel foolish in never thinking it through myself. The solar panels give out large voltage at peak times and I often run the engine at the same time - never any damage there!

I suppose the mains is 'just' another charge method.

Many thanks - no more unplugging for me!

Mike

[MalcolmP]

Many thanks Trevor for such a comprehensive answer as always

Empirically after many years doing this I had never found an issue, so will carry on remaining confidently pluged in to shore power when starting the engine when in the dock, so long as I remember to unplug when leaving....I did do that once...😭😭

[mikebz]

To put Trevor's description another way:

The alternator regulator will do all it can to maintain a constant voltage at its output.  This is often 13.8V (which is poor in terms of how fast it will recharge your batteries and therefore potentially bad for battery life, but that's another subject) so I will use that as the example.

If the battery is at 13.8V without the engine running, and then you start the engine, there will be no voltage difference between the regulator and the battery so no current will flow.  If the battery is < 13.8V then an amount of current will flow - this will depend on the battery voltage, the battery's internal resistance, and the alternator's capacity (in Amps).  The point is that the regulator's function is to maintain a constant voltage - in doing so the amount of current which flows will depend on the state of the battery.  A simple regulator has no notion of the battery being 'full', it simply does its best to regulate its output and the voltage difference does the rest.


The alternator has a finite capacity (rated in Amps), so if the if the current required to pull the battery terminals up to 13.8V would exceed the alternator's capacity then regulator's output voltage will be lower, but will gradually rise as the battery charges and its demand for current decreases.  At the point where the demand for current has reduced to the maximum capacity of the alternator the terminal voltage will thereafter be held at 13.8V by the regulator.

If you switch on another charging device which e.g. regulates at 14.2V (and is capable of supplying enough current to pull the battery up to over 13.8V) then the alternator will cease to supply any current.

I once destroyed the regulator in a car alternator by disconnecting it from the battery whilst the engine was running - this causes a voltage spike which can blow diodes in the regulator.  I wonder if this possibility has been twisted into the belief that switching on the battery charger whilst the engine is running could cause the same effect.

[ForGrinsToo]

Mike and Trevor are spot-on. And a common cause of alternator damage with diesel engines is if the key is turned off while the engine is running - the voltage spike blows diodes.

I'll note that I occasionally start the engine with shorepower attached and battery charger operating, as the shorepower is about the last task before leaving a slip.

Geoff