Is a 345A switch enough for my engine?

[LifesBetterWhenYoureBeating]

My boat came with the engine starter wired to the battery via a Guest 2300A switch (rated to 600A momentary), but there's also a Guest 2111 switch (345A momentary) right next to the engine.

I'm now using the 2300A for my house bank, and plan to use the 2111 for the starter, but my engine's (Perkins 4.108) starter motor max current is 900A (according to the manual), so I'm wondering if it'd be safe to do so?

Thanks in advance

[Wotname]

I would caution against using the Guest 2111 switch in your starting application.

Every time you start the engine, you will overloading (i.e. heating) the switch contacts beyond their designed rating. The more times you cause damage to the contacts, the worse the problem becomes. At some stage in the future, the switch will fail and will either prevent the engine from starting or start a fire or both.

[smac999]

are you sure the manual doesn't say 900cca battery? 900a starter draw is massive.

I would bet 200-300 is more like it.

[LifesBetterWhenYoureBeating]

Originally Posted by Wotname

I would caution against using the Guest 2111 switch in your starting application.



Every time you start the engine, you will overloading (i.e. heating) the switch contacts beyond their designed rating. The more times you cause damage to the contacts, the worse the problem becomes. At some stage in the future, the switch will fail and will either prevent the engine from starting or start a fire or both.

Hmmm, ok, but out of curiosity, what are you assuming to be the in rush current when starting the engine? If we assume 900A, even the Guest 2300A wouldn't be enough, and I wonder which switch model would?

[Sailmonkey]

I’ll bet your starter is 900 watts, not 900A.

Starting current would then be below the max intermittent rating of the switch.

[LifesBetterWhenYoureBeating]

Originally Posted by smac999

are you sure the manual doesn't say 900cca battery? 900a starter draw is massive.

I would bet 200-300 is more like it.

It says "maximum current 900A" for the starter motor

[Jammer]

Originally Posted by gsalgado

My boat came with the engine starter wired to the battery via a Guest 2300A switch (rated to 600A momentary), but there's also a Guest 2111 switch (345A momentary) right next to the engine.

I'm now using the 2300A for my house bank, and plan to use the 2111 for the starter, but my engine's (Perkins 4.108) starter motor max current is 900A (according to the manual), so I'm wondering if it'd be safe to do so?

Thanks in advance

1) Regardless of what the manual may say, the starter will never draw 900 amps. It might perhaps draw as much as 300 amps on a really cold morning. I say this based on having owned several diesels and measured the starter current on them, with a meter.


2) The rotary battery switches, regardless of rating or manufacturer, are a fruitful source of electrical system failures and should not be installed gratuitously. If you really need a battery disconnect for the engine for reasons of safety then I would suggest using an Anderson Powerpole connector instead of a rotary switch. The powerpole connectors have better contact wiping action when connected and disconnected and can readily be inspected and cleaned as needed.

[Wotname]

According to your manual, the maximum current needed at the beginning of the start cycle will be 900A, this overcomes the inertia and first compression. The cranking current then varies as each cylinder passes through compression with the average cranking current will be about one third of the max cranking current.

Normally a cranking current rating of a switch is timed over 30 seconds and the intermittent rating of a switch is timed over five minutes.

The Guest switch specs do not list a cranking current rating so this is an unknown nor do they specify the time for the momentary rating so another unknown.

Compare this to the smallest Blue Seas switch which has a continuous rating of 300A, an intermittent rating of 500A (for five minutes) and a cranking rating of 900A (for 30 seconds). Using this as a guide, I would not use the Guest 2111 switch but I would use the the Guest 2300 switch - you might decide different!

As far as the switch ratings are concerned, the time periods as just as important as the current as it is the time that determines how hot the contacts before.

The maximum cranking current of 900A will only exist for a second or so and the average cranking current of say ~300A with exist for up to 30 seconds. In the absence of better data from Guest, I would err on the safe side!

BTW, the max stater current 900A also tells us the battery has to be able to supply at least an initial current of 900A.

[phorvati]

Wire the starter directly to dedicated starting battery with a slow blow fuse, like 500A ANL via 1/0 welding cable. Don't forget the black return wire. Don't run it via engine block. Then tricke charge that battery via another source.
I've been using an office UPS backup AGM battery like this to start my lehman 90hp. Its been since 2018 and works really well.
One trick, my 1/0 welding cable round trip is 5 feet and all the crimps are new.
Previous owner had a starting battery, giant mastervolt 4D 160AH, 25 feet away so 50 foot round trip. I repurposed that cable for better things.

[LifesBetterWhenYoureBeating]

Originally Posted by phorvati

Wire the starter directly to dedicated starting battery with a slow blow fuse, like 500A ANL via 1/0 welding cable. Don't forget the black return wire. Don't run it via engine block. Then tricke charge that battery via another source.
I've been using an office UPS backup AGM battery like this to start my lehman 90hp. Its been since 2018 and works really well.
One trick, my 1/0 welding cable round trip is 5 feet and all the crimps are new.
Previous owner had a starting battery, giant mastervolt 4D 160AH, 25 feet away so 50 foot round trip. I repurposed that cable for better things.

The ANL fuse sounds like a better solution, but I'm curious why you say not to run the negative via the engine block. My positive cable is 2/0 and around 4', but the negative is 4/0, with one 12' run to the negative buss and another 8' run (also 4/0) from there to the engine block.

Also, what do you mean by trickle charge the battery via another method? Currently my alternator is only connected to this battery as I have to rely on a dc2dc charger to charge my LFP house bank.

[LifesBetterWhenYoureBeating]

Originally Posted by Jammer

1) Regardless of what the manual may say, the starter will never draw 900 amps. It might perhaps draw as much as 300 amps on a really cold morning. I say this based on having owned several diesels and measured the starter current on them, with a meter.


2) The rotary battery switches, regardless of rating or manufacturer, are a fruitful source of electrical system failures and should not be installed gratuitously. If you really need a battery disconnect for the engine for reasons of safety then I would suggest using an Anderson Powerpole connector instead of a rotary switch. The powerpole connectors have better contact wiping action when connected and disconnected and can readily be inspected and cleaned as needed.

The main reason for the switch was so that I could disconnect the battery in case the starter motor gets stuck -- someone mentioned that in another thread and it scared the hell out of me. But in another answer on those thread someone suggested a 500A slow blow fuse, which I suppose would blow in case the starter motor gets stuck? I have a 300A MRBF fuse on the way that I was planning to use on this battery. Would that save me in such a case? FWIW, I don't plan to ever go far from the tropics and my engine always starts in less than 2s

[Jammer]

Originally Posted by gsalgado

The main reason for the switch was so that I could disconnect the battery in case the starter motor gets stuck -- someone mentioned that in another thread and it scared the hell out of me. But in another answer on those thread someone suggested a 500A slow blow fuse, which I suppose would blow in case the starter motor gets stuck? I have a 300A MRBF fuse on the way that I was planning to use on this battery. Would that save me in such a case? FWIW, I don't plan to ever go far from the tropics and my engine always starts in less than 2s

It is uncommon but not rare for starter motors to stick mechanically so that the pinion gear is driven by the engine after the engine starts.


On the other hand, I've never actually had a starter motor get "stuck on," electrically, on anything, gasoline or diesel, or heard of it happening to a friend's car/boat/skidloader/etc or from a mechanic. Recall that diesel starters are technologically no different from automotive starters except for size. It's a wide world out there and all kinds of weird stuff goes on so I suppose it does happen. If it does, the starter will take one or two minutes or more to fail, not seconds. I'm not sure I see the benefit of a second switch, but it's your boat.


If you want to fuse the starter so tight that it will blow after a minute or two of continuous operation you'll probably need a fuse that is more precise than an MRBF. MRBF fuses are good for what they are but they have a wider tolerance band than more traditional designs. Here's a curve from Blue Sea Systems. As you can see, there's about a 10x-15x spread in times between "guaranteed to blow" and "guaranteed not to blow" so there's no choice that will reliably blow after, say, 120 seconds with the same load that it will reliably carry for 15 seconds. The variation in starter current based on temperature will make this worse.

[NPCampbell]

Originally Posted by Jammer

On the other hand, I've never actually had a starter motor get "stuck on," electrically, on anything, gasoline or diesel, or heard of it happening to a friend's car/boat/skidloader/etc or from a mechanic.

A mechanical failure can cause electricity to be stuck on though. If a started solenoid is "tack welding" on, has a broken return spring or a faulty ignition switch then current will also be "stuck on". Obviously, it won't stay stuck on for long before something fails.

[Bowdrie]

I suppose you can get something like this that's going on ~100 years old.
Could break both the legs if the starter went wild.
You get extra points if the switch is labeled "General Electric" or "Westinghouse".

[wingssail]

Originally Posted by Jammer

1) Regardless of what the manual may say, the starter will never draw 900 amps. It might perhaps draw as much as 300 amps on a really cold morning. I say this based on having owned several diesels and measured the starter current on them, with a meter.


2) The rotary battery switches, regardless of rating or manufacturer, are a fruitful source of electrical system failures and should not be installed gratuitously. If you really need a battery disconnect for the engine for reasons of safety then I would suggest using an Anderson Powerpole connector instead of a rotary switch. The powerpole connectors have better contact wiping action when connected and disconnected and can readily be inspected and cleaned as needed.

This makes me wonder. My battery control switch is a single Marinetics rotary switch on the Master Power Control Panel (see photo). It switches all power to and from my batteries including to the engine starter which is essentially on the house side of the switch. I have additional distribution panels and other switches and meters but this is the only battery switch I have.

This switch and this panel have been on the boat for 40 years. As far as I know the rotary switch has never been changed or serviced in any way. (there are no log entries for Battery Switch or marinetics, etc).

I wonder if this switch is worn out or causing a voltage drop when starting amps for my yanmar are going through it. Can these switches last forever?