40 amp fuse

[KYAvion]

Trying to understand a few things...

The batteries have a 40 amp fuse. Obviously a fuse protects the wire, so I assume the 40 amp fuse is appropriate for the gauge wire that is run between the battery and converter. Therefore, if a larger converter is installed, then a larger wire is needed, and thus also a larger fuse appropriate to the wire size? Correct?

Also, the fuse is placed closest to source of the current draw to protect the wire from overheating. In this case the source of the current draw is the battery, so I assume the fuse is to protect the battery from drawing too many amps from the converter when charging? If the batteries drew too many amps, the fuse would blow before the wiring gets too hot. Correct? If this is the case, then theoretically if a significantly larger converter is installed (let’s say 90 amp) without increasing the fuse size (and wire size), then the batteries would attempt to draw close to 90 amps from the converter when charging, and would therefore blow the fuse? Am I understanding this correctly?

[Salty]

There's a few variables here, but I'll address your protection questions first. The fuse being placed close to the battery protects the wire And the load. Mostly the load. Not so much the battery, but it all works toward the end of preventing excessive current from damaging something.
Relating to wire size, wire ampacity is rated for continuous current carrying capacity. That isn't to say that for short periods of time that the wire couldn't carry a larger load current.
As a side note, your 40 AMP fuse would likely not blow until the load reached 45 amps.
Battery charging rates:
Batteries charge at a lower current rate, and the rate of charge varies with the amp hour size of the battery and its construction and the number of batteries installed. . LiFePo4s charge differently than lead acid than gel cell. But in none of these cases would you push 90amps at a single or pair of batteries.you would boil a lead acid battery and likely damage a gel cell. More like 10 amps per battery. LiFePo4s are a bit different I believe, hence the use of a separate charge controller. I could be wrong on this.
The advent of charge wizards are boon to RVers, as are gell cell maintenance free batteries. LiFePo4s are an excellent battery, in theory.. Their use is relatively new in Rvs and life cycle claims seem outrageous.
I know wire gauge size is important to you, it is not the first time you ask this question, just in a different way.
EDIT :
Wire gauge from battery to load is based on the anticipated max load current. You may also wish to add a voltage drop calculation when selecting the wire gauge size, as, when voltage drops, current draw tends to increase.

[KYAvion]

Thanks for the reply. Let me clarify a couple things so I’m sure I’m understanding this correctly. I thought the purpose of a fuse (or breaker) was to protect the wire from being overloaded to minimize the risk of fire, or as you said excess current from damaging something. You mentioned the primary purpose though is to protect the load. Would you elaborate on that?

My trailer appears to have come with two batteries and a 40 amp fuse from the factory. Given each battery will charge at about 10 amps, why a 40 amp fuse and not a 25 or 30?

And last question, at least for now. With the approximate 10 amps each battery is charging at, is the battery pulling the current, or is the charger pushing it? Or both? What keeps the charger from pushing out more than the battery can handle?

[KYAvion]

Ok. One more question. Why is it in a scenario like our trailers we have main fuses at the box AND individual fuses at the components (e.g., water pump, batteries, fridge), but we don’t do this in a 110v AC environment?

[Salty]

Interesting question!
What follows is a certain amount of speculation. Largely, the 120VAC outlets are there to run consumer electronics/electrical devices . Blow dryers, coffee makers, TVs, computers etc... . The responsibility for fusing those devices is on the manufacturer.
Load balancing at the power center is one way to protect the system. Sizing the breakers of individual circuits appropriately is one step, segregating known steady loads (AC, refrigerator,) from convenience outlets is another.
GFI outlets protect the system and humans from a dangerous ground leak (think shock hazard) but are not an overcurrent protection per se.

[Salty]

Thanks for the reply. Let me clarify a couple things so I’m sure I’m understanding this correctly. I thought the purpose of a fuse (or breaker) was to protect the wire from being overloaded to minimize the risk of fire, or as you said excess current from damaging something. You mentioned the primary purpose though is to protect the load. Would you elaborate on that?

My trailer appears to have come with two batteries and a 40 amp fuse from the factory. Given each battery will charge at about 10 amps, why a 40 amp fuse and not a 25 or 30?

And last question, at least for now. With the approximate 10 amps each battery is charging at, is the battery pulling the current, or is the charger pushing it? Or both? What keeps the charger from pushing out more than the battery can handle?

OK, saw this after the other post.
The load is anything that the battery is supplying power to. In this case, an engineer (I would hope) determined that under max normal load the current draw from the battery would be less than 40 amps. The wire is sized in accordance with the max current draw and the acceptable voltage drop at the load end. It's a balancing act. The wire from the battery to the power center is likely 6 or 4 gauge (I don't know) which is rated for more than 40 amps, but is that large due to the distance, so it doesn't drop the voltage. Remember, DC, unlike AC current, is more subject to voltage drop over distance related to the resistance of the wire.

The charge circuit pushes current to the battery. You cannot be in a discharge condition (drawing power from the battery to run lights etc...) and a charging condition at the same time.
The current being pushed to the battery /batteries is regulated.
Something of note: You may have seen auto battery chargers that are of the quick charge type. These are usually rather large devices due to the large transformer installed. These chargers push a high current charge to the battery in a short period of time. These can damage a battery if left on too long.

[KYAvion]

Ok. So the load is no different from line vs load in an AC circuit. I guess the 40 amp fuse at the batteries would be analogous to a 200 amp main breaker in a household AC box?

But then we’ve got the 12v kill switch, which also seems to serve as a main breaker for the 12v system? Simply redundancy in the system?

I follow on the DC and voltage drop vs AC drop. When I installed my 12v tank heaters I attached the common to the frame because otherwise the wire size necessary would have been pretty large.

Now here’s where I am getting slightly hung up. I get why the fuse is at the batteries—to protect the load from the batteries. Conversely, is the fuse also there to provide protection from the batteries pulling too much current from the converter, the same as a fuse at the water pump provides protection from overdraw at the pump?

Thanks for taking the time to walk me though this.

[Salty]

Protection from the converters pushing too much current to the batteries is or should be at the converter (the source).
Kill switch :
This is a disconnect to stop anything you have (accidentally?) left on from draining the batteries.

[KYAvion]

Yes, I understand the kill switch is to prevent battery drain. However, I have tripped it before by running too many 12v fixtures/appliances at once.

[Salty]

Interesting. Since my control center isn't live at this time, I haven't experienced this phenomenon. I need to check into this further. Every other kill switch I've looked into recently is simply a switch.