who knows what these are?

[topaz surfer]

Ok I'm starting my dual battery set up and trying to clean up some wiring. Anybody know what these are? I have three of them that are all wired into a single positive connector to the pos. terminal.

[IDBoating]

Looks like a 24VDC 40A auto-reset circuit breaker. With corroded terminals.

[chpthril]

^^^^^ Yep, 40A Auto-reset breaker.

[hicrawler]

Is your boat on a 24 volt system? If not, this circuit breaker needs to be replaced with a 12 volt breaker

[IDBoating]

Is your boat on a 24 volt system? If not, this circuit breaker needs to be replaced with a 12 volt breaker

Not necessarily. A breaker reacts to current flow, not voltage. You must be careful to use a DC breaker for DC systems (and not just a repurposed AC breaker intended for home use!), but otherwise a 24VDC breaker should be fine.

Also, consider that the "12 volt" systems in boats and cars actually see voltages in the 13.8-14.5 volt range when the alternator is running (i.e. when the engine is running). To charge a battery you must run current into it, and to do that you must charge with a voltage that exceeds the battery's present voltage. Hence electrical components on boats, cars, etc. must actually be designed to accommodate voltages that approach 15VDC. A breaker rated for 12VDC would be outside its specs in that environment.

[hicrawler]

Not necessarily. A breaker reacts to current flow, not voltage. You must be careful to use a DC breaker for DC systems (and not just a repurposed AC breaker intended for home use!), but otherwise a 24VDC breaker should be fine.

Also, consider that the "12 volt" systems in boats and cars actually see voltages in the 13.8-14.5 volt range when the alternator is running (i.e. when the engine is running). To charge a battery you must run current into it, and to do that you must charge with a voltage that exceeds the battery's present voltage. Hence electrical components on boats, cars, etc. must actually be designed to accommodate voltages that approach 15VDC. A breaker rated for 12VDC would be outside its specs in that environment.

Yes the charging system will exceed 12 volts when charging but it is still on a 12 volt system. The amperage is what makes the breaker blow. If the two batteries are wired in series as a 12 volt system it will still be a 12 volt system. But if they are wired with one side power to ground on the battery terminals it will now be a 24 volt system. I'm not an electrical genius but it doesn't make sense to me to run a 24 volt breaker on a 12 volt system.

[topaz surfer]

I thought they were some kind of breaker. My boat is 12v with one battery and im setting it up with a dual battery set up now and trying to clean up the previous owners electrical. I don't know what they go to yet so I thought I would ask. Thanks for the help guys.

[topaz surfer]

And yes I will be cleaning them up. Lol.

[IDBoating]

If the two batteries are wired in series as a 12 volt system it will still be a 12 volt system. But if they are wired with one side power to ground on the battery terminals it will now be a 24 volt system.

Actually, that's exactly backward. Batteries in series add their voltage, and two 12VDC batteries in series yield 24VDC across their terminals. Batteries in parallel add their current, so two 12VDC batteries in parallel will still yield 12VDC but have double the current capacity (presuming they are identical batteries with identical charge).

I'm not an electrical genius but it doesn't make sense to me to run a 24 volt breaker on a 12 volt system.

I am an Electronic Engineer and I promise it's just fine. The breaker will trip on current flow. It would be a far worse problem to run a breaker that is underrated as to voltage, and in a 13.8VDC nominal auto/boat system a 12VDC device is already more than 10% beyond its spec.