Chassis Batteries and Fuses

[AXett]

Ok, Chassis battery/fuse question...

First, in the photo you can see several fuse holders sitting between positive lines run off the battery posts, then off to the bus. They are labeled things like "Transmission ECM". They look like factory labels - is it safe to assume these are just that: fuses for some of the large chassis items? I'd think these would be up in the engine fuse bay? They all wind out into a cable bundle that runs into the bus (see blue/grey line on schematic).

Second, in the attached piece of fine paint art, I've got the expected three parallel batteries. They daisy chain together with what looks like 2/0 and then have core leads off the final in 4/0 out to the bus. Makes sense.
The thing I can't sort is why the first battery in the bunch ALSO has dedicated 4/0 lines heading out into the bus?

Possibly bringing me to my Third question... I've yet to find anything in the bus actually running on 24V, at least in the body, yet many (not all) of the fuses are labeled for 24V (despite only having 12V across them). Any insights here? Are they just over-spec'ing the fuses for safety? And more broadly, are there big engine bay items running 24V (hence the need for the series leads out of the battery bay)?

[rossvtaylor]

Your bus is 12V, as you noted, and always was (unless it was a transit or coach and later converted to 12V). Fuses and other overcurrent protection devices will have a current limit and a voltage rating...but the voltage rating doesn't change anything you need to worry about, as long as you're not exceeding that. So having 24V-rated fuses is just fine.

I'm afraid I can't offer any insight into the other bus-specific questions. Sorry!

[AXett]

I don't believe it was converted - all has seemed pretty "vanilla", came from a school auction. Is it safe them to assume if I blow one of the OEM "24v" fuses, I can swap in a standard fit 12v automotive fuse?

[rossvtaylor]

Originally Posted by AXett

I don't believe it was converted - all has seemed pretty "vanilla", came from a school auction. Is it safe them to assume if I blow one of the OEM "24v" fuses, I can swap in a standard fit 12v automotive fuse?

Yes, so long as you retain the same current (amp) rating.

[ewo1]

Originally Posted by AXett

I don't believe it was converted - all has seemed pretty "vanilla", came from a school auction. Is it safe them to assume if I blow one of the OEM "24v" fuses, I can swap in a standard fit 12v automotive fuse?

fuses are designed for current protection not voltage protection.
There are other devices that provide voltage protection such as diodes or voltage regulators.

Essentially you can have a very small gauge wire, say 26 gauge, and run quite a high voltage on it. Lets use 110 volts....what is critical to know/realize is that it is the amount of current on that wire that is important !

A 26ga wire at 80c can handle up to 4 amps through it. it then starts to get warmer and eventually hotter , burning the wires as you increase the current flow on that wire.

fuses are just like that little wire...the thicker the wire the more current it is capable of handling.
18ga wire at 80c can handle upto 15 amps.

So if you look at your fuses you will see that the little wire in the fuse gets thicker as the current rating increases and that is so it can handle the heat generated from the current flow.

To go back to your concerns, on a bus it really doesn't matter if it's rates for 12v, 24v or 110v. Your bus wiring is based on a 12volt system.

There's more to know but this is the basics....

[AXett]

Ohm was a good dude! Fuses care about wattage, not current because they're designed to work in a fixed-voltage system. So yes, in a system we all assume is 12V, a given gauge of wire offers a fixed resistance proportional to the voltage - thus burning up at a specific current.

I didn't know if these busses were half 24V or some other madness, which would impact the current flowing through the fuse at a given wattage.

The saddest part here is a good portion of my formal education was in electrical engineering, but it's not what I do day to day anymore, so I find myself second guessing things because I assume things are more complex than they are!

Do you think each pair of those big 4/0 leads off the battery are serving the chassis vs body fuse panel?

[AXett]

Talking battery madness, any reason I can't feed a trickle charger through an "always on" circuit with an adequate fuse like the cig lighter? Tapping that would be way cleaner than trying to route into the battery compartment...

[jimmythomas]

A trickle charger / battery maintainer should always be wired directly to the battery terminals with the shortest wires possible. Fewer chances for bad connections, and if your bus has a battery disconnect switch you can still charge the batteries.
A parked bus should always have a battery maintainer hooked up and running, it really helps battery life.