Battery disconnect for a generator > How to prevent alternator damage?

[oak_box]

My boat has an Onan 8MDKDB generator, that I **FINALLY** have working again (possibly 4-5 years since a previous owner had it running).

I'd like to put a contactor between the battery and the generator, so that the battery is fully disconnected when the generator isn't being used. The contactor (big relay) will allow me to remotely connect the battery to the generator and start the generator without going into the engine room.

QUESTION: Is there a simple way to put a dummy load on the alternator to prevent the alternator diodes from blowing if the battery relay accidentally disconnects the battery from the generator while the generator is running?

 

[DavidM]

Balmar and others make an alternator protection module that will keep the diodes from blowing when a load is suddenly removed.

David

 

[luna]

Good on you for getting the generator running again. They seem to be good machines. I hope it serves you for many years.

I also have a 8MDKDB (from1995) on board that continues to run well, although it looks like I need to replace the rear main seal soon. I will do the isolators as well.
I have never experienced an issue with the battery, it's charge level, the starter, the start circuit, or the small permanent magnet OEM alternator that slowly charges the battery. Everything, after 4000+/- hours and 29 years just seems to work.
Why is it that you want to isolate the battery while at rest? There isn't any parasitic loads as far as I know.

Is there something I should look out for?

 

[more]

My boat has an Onan 8MDKDB generator, that I **FINALLY** have working again (possibly 4-5 years since a previous owner had it running).

I'd like to put a contactor between the battery and the generator, so that the battery is fully disconnected when the generator isn't being used. The contactor (big relay) will allow me to remotely connect the battery to the generator and start the generator without going into the engine room.

QUESTION: Is there a simple way to put a dummy load on the alternator to prevent the alternator diodes from blowing if the battery relay accidentally disconnects the battery from the generator while the generator is running?

you don't need a contactor

 

[Iggy]

I am wondering too, why you want to add a switch?

Just an FYI, I just installed this on my bow thruster. I found it much easier to install because the terminals are facing out. Egis 8710-1500B XD Single - Remote Battery Switch with Knob - Tinned Wires

 

[oak_box]

I'm probably over thinking this, but there were several reasons behind wanting to isolate the battery from the generator.

First - it seems that ABYC wants to see a disconnect (battery switch of some sort) between a main battery and any loads. Generally - there also has to be a fuse or breaker, though I know most people don't do that for start batteries. I'm assuming that's because the huge inrush current would trip any breaker, and blow fuses that weren't rated far in excess of the "average" current. I really wouldn't mind having a breaker immediately next to the battery - but not sure if this would work for a start battery. I'm at least assuming that the start current for a generator would be less than for my main engines - but might not be by much.

Second, I just like the secure feeling of knowing the battery is fully disconnected when I'm not going to use the generator for a while, or when I leave the boat.

 

[luna]

I would not characterize your wants as over thinking this. It is your boat, you need to be comfortable with the way it is set up.
If you don’t have a battery disconnect switch now, a remotely operable one will likely be a welcome addition.

The notion of arriving at the boat after a few days absence, and attempting to use any engine without first checking it over and then watching it start is something I would not do. The first start of the day is always done at the engine.

Turning a manually operated switch to On would just become part of the normal start up activities, although not for me as the last time I turned the genset’s battery disconnect switch to Off was over a year ago.

Using this kind of procedure, the need for a remotely operated switch is largely diminished, as it only becomes useful in the case of some kind of catastrophic event, like fire.

Is the seacock that provides the genset with access to raw water also remotely operated?

 

[oak_box]

Yes, all engines should be checked in the engine room after an absence. But once that's done, it could still be nice to isolate the battery.

I had also thought it would be nice to have a remotely operated valve after the manual seacock - but I'm not familiar with such a thing... And didn't want to spiral this thread in that direction... But it would be cool in both boats and RV's to have reliable, large, solenoid actuated valves.

 

[Greg QS]

Another vote for the Balmar alternator protector, simple and effective.

 

[oak_box]

At the first mention of the Balmar device in this thread, I googled it... $80 on Amazon, but not much information. It connects to B+ and B- of the alternator - no mention if this is equivalent to just connecting it across the battery leads (as the generator obviously doesn't have a Balmar alternator).

I had two lines of thought.

The first is that yes, this is exactly what I'm looking for, and (assuming I just connect it across the battery leads) is easy to deal with.

The curious part is, "what's in the box"? Is it a reverse diode, a MOV (Metal Oxide Varistor), a resistor? They mention that the device is somewhat "sacrificial". If the LED goes out or goes to the wrong color, the box needs to be replaced. Yes, the $80 box is cheaper than an alternator - but a lot more expensive than a couple of diodes (if one knew how, and was inclined to dig into the alternator, find, and replace the diodes...)

The statement of the box being sacrificial, and possibly going out after enough spikes suggests that the key component may be a MOV.

I was wishing I could find a really good electrical technician / alternator guy that could tell me what the device is, so that I could order a component or two and put together my own device.

I balk a bit at the $80 for the black box, but I do get that by the time you count development cost, packaging, and that these are probably sold in relatively very low volume, and that the business realistically needs to probably make a 40-50% profit to make sense

 

[Jerry formally Epoxy Lady]

Post #6 is correct that ABYC requires a disconnect sw between the start batt and the starter, I installed them on both M.Engines and the Generator. Good insurance against a shorted/run away starter motor.

 

[twistedtree]

Post #6 is correct that ABYC requires a disconnect sw between the start batt and the starter, I installed them on both M.Engines and the Generator. Good insurance against a shorted/run away starter motor.

The required battery switch is at the battery and is to disconnect the battery from all loads. It will of course disconnect power to a started if wired to it, but the switch isn't for the starter. There isn't a requirement for a switch dedicated to the starter.

 

[oak_box]

Is it reasonable to expect that the "battery switch" must be a mechanical switch, and not a remotely activated electromechanical device (i.e. contactor or relay)?

 

[twistedtree]

Is it reasonable to expect that the "battery switch" must be a mechanical switch, and not a remotely activated electromechanical device (i.e. contactor or relay)?

It can be remotely controlled. That was clarified in the upcoming E11. But it also needs to be controllable at the battery. A lot of the available devices are remotely controllable, and also have a mechanical over-ride switch which accomplishes the goals. There are also location and accessibility requirements.

 

[Tom Daly]

Post #6 is correct that ABYC requires a disconnect sw between the start batt and the starter, I installed them on both M.Engines and the Generator. Good insurance against a shorted/run away starter motor.

I'm skeptical that a battery switched can be rotated off without welding if a big engine starter were running, never mind shorted. And those magnetic latched remote switches - I'm very skeptical. No experience, but hard to imagine.
Perhaps the only reliable emergency cutoff is a giant cutter - but even that might weld during the cut.
Maybe an explosive disconnect ?

 

[SteveK]

Maybe an explosive disconnect ?

How about a standard battery on/off switch. one wire to the positve cable and one to the negative cable. Close that switch to stop the run away starter.

Caution: do not try this unless you are a trained professional.