Why battery switch between starter and starter battery?

[Kassiopeia]

Hello guys!



I am rewireing my electrical System and this question came up to me...

In cars etc. the starter battery is always connected directly to the starter!



Why there is a switch in every boat I know?


The generator can/will be damaged if someone switches off the battery switch while engine is running!

[s/v Jedi]

Originally Posted by Kassiopeia

Hello guys!

I am rewireing my electrical System and this question came up to me...

In cars etc. the starter battery is always connected directly to the starter!

Why there is a switch in every boat I know?

The generator can/will be damaged if someone switches off the battery switch while engine is running!

This is called a service switch. It’s only used when servicing the engine so that no accidental short circuit can happen with a tool etc.

The switch should never be used while the is engine running.

[smac999]

Because when your car catches on fire you pull over and get out and walk away. When your boat catches fire you’re dead.


I’ve seen a starter solenoid fail. Starter keep cranking. No battery switch to shut off power to starter. 2/0 Battery cables caught on on fire. And the starter stoped after they Burt in half

That is also why abyc says the battery switches Must be accessible outside the engine room. So if their is an engine fire you don’t need to enter engine room to shut it off.

[hjohnson]

The other reason for the cutoff switch is theft prevention. To be blunt, the key switch for my engine control panel is hilariously insecure. I’ve often wound up using the wrong key, or the key upside down.

The battery switch is down below, where it’s both out of sight, and at least protected by a padlock.

[sailingunity]

The battery switch on my boat also allows me to switch between start batt and house batt as the energy source for the starter.

[SailingHarmonie]

Originally Posted by s/v Jedi

This is called a service switch. It’s only used when servicing the engine so that no accidental short circuit can happen with a tool etc.

The switch should never be used while the is engine running.

Actually, no. This function could be easily implemented by just disconnecting the battery, like one does on a car.

As others have pointed out, the primary reason for having a switch on the battery is safety. It allows you to cut ALL the power to the boat everywhere in the event of an electrical fire. Now you can use water to put out the fire.

The design of my boat has one, DPST high amperage switch that cuts both the 12V starting battery and the 24V house battery out of the circuit. We think of it as the "Ah S--T, there's smoke!" switch.

This is not to say you can not use the safety switch as a service switch, or that it in not handy to have for working on the power system. But it is REALLY important for other reasons.

If you ONLY think of it as a service switch, you might just forget about using it in an emergency. On our boat it is always called the "Emergency Switch" for just that reason. Even though we have only ever used it (and hope to only ever use it) as a service switch.

[boatpoker]

Originally Posted by SailingHarmonie

It allows you to cut ALL the power to the boat everywhere in the event of an electrical fire. Now you can use water to put out the fire.
.

This has to be a slip or a joke, I know you know better than to use water on an electrical fire but someone may take you seriously.

[alctel]

I definitely use mine as a service switch, way easier than disconnecting everything. I generally leave it turned off when not using the engine

As my alt goes to my house bank, not my starter bank, I also have another switch on the alt + as well to disconnect the engine fully from the bank.

[s/v Jedi]

Of course this thread is about the switch between main engine and start battery, not the main switch on the house bank like some suggest.

It is a service switch, this is not a car and cables are not removed from the start battery during engine maintenance… the switch is out to the OFF position.

Yes, it’s wise to keep it switched off during periods that the engine isn’t used.

It’s better to have the switch in the negative cable so that all metal from engine, propshaft and prop is isolated from DC negative. You can use a neat dual pole switch for both positive and negative like this: https://www.bluesea.com/products/551...Battery_Switch

[NorthCoastJoe]

As a Firefighter, I see no problem using water to put out an electrical fire if it's all in the 12 volt system. I can't count how many car fires we put out with just water.

To me the greater threat is the smoke coming off a plastic fire. How long can you hold your breath.

My boats have battery switches on both positive and negative cables.

[Cheyne]

On several of my cars, mainly ones that don’t get used that much, we have switches. Same for our semi’s that we use to haul grain and all our tractors.

On our boat I have cut off switches between the battery and the starter. After I turn off the engines I turn off the switches. Not for security. Shoot I have instructions at the helm on how to start the boat. There are no keys involved. Just push buttons. The only connection to the starter batteries are Victron trickle chargers when the switches are off.

[CharlieJ]

To expand this concept a bit: Many engines arrive from the engine manufacturer with the following wiring: Start battery B+ > engine start solenoid and, to save money, Alt. B+ > engine start solenoid. Most owners that install a higher capacity house bank split these two “systems/functions” into start battery B+ > engine start solenoid (unfused) and Alternator B+ > house bank B+ (fused at battery). In both arrangements, the alternator B+ terminal is always at B+. If the alternator needs to be worked on (belt tension, removal, etc.) the wrench used to perform the maintenance can easily come into contact with the engine block, causing a short circuit.

The simple fix for this is to put a Maintenance Switch (SPST) in the alternator output cable. I use a key operated switch (Blue Sea 6004), lock the switch closed and remove the key and attach it to the cable immediately next to the switch.

This is not original with me. RC on Marine How To wrote an article on his site years ago.

[hjohnson]

Originally Posted by alctel

I definitely use mine as a service switch, way easier than disconnecting everything. I generally leave it turned off when not using the engine

As my alt goes to my house bank, not my starter bank, I also have another switch on the alt + as well to disconnect the engine fully from the bank.

On my boat, the starter battery is at the bottom of my Lazaret where I usually forget about it (in a suitable battery enclosure, don't worry). The battery disconnect is down below where I can disconnect the engine primarily for security, and secondarily for safety.

My alternator also goes to my house lithium bank. For that, if I need to kill it, I have to also flip (and lock out) the BlueSea RBS that controls the high/start side of my house system.

The nice thing about my system is I can keep the rest of the 12v system running (for things like lighting and such) because it's controlled by a separate contactor (well, actually, a Victron BatteryProtect).