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CATL 302Ah Battery Fuse

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packrat1969

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Hello

I‘m planning on building a single 4S battery pack with (4) CATL 302Ah cells soon, and have been here gathering info here on all the things needed. One very important component for this battery, which will be installed in my travel trailer, is the battery’s master current disconnect device. I’ve been looking at Class T fuses, and, based on the theoretical capacity of the 200A BMS I’m using, I expected to need one in the 250A range. Isn’t the rule of thumb max amps X 1.25?

The problem I’m having, is that 250A fuses seem very rare. I mean, I can find them, but every fuse holder/fuse combo package I’ve seen seems to straddle that value and uses either 200A or 300A fuses. 250A fuses also seem to be more expensive. What am I missing here? Is this just a seldom-used value? Is it safe to go up to the next even-hundred amperage?

packrat
 
The fuse size is based on the wire size which is based on the load size.
The load size is typically the max continuous rating of your inverter.
 
The Class-T fuse is primarily there to deal with the “Oh Sh*t, I just dropped my wrench across my bus bars” situation. They’re fast blow, with large (20kA) interrupt ratings. What you need to be aware of is the inrush current to things like your inverter. If you’re not running a precharge circuit, the inrush to your inverter can pop a class T.

In my system, I have a 400A class T, and the largest current I ever expect to see is around 200A. I do have a precharge for my victron, so I’ll be fine.
 
I understand that the fuse is there to protect the wiring. When I built my system 3 years ago, I used some surplus 00 AWG (I think that translates now to 2/0). At the time, I had (2) 6V T105 cells in series and was using a 150A circuit breaker as the interrupt. Well, thinking back on it, that probably wasn't a good idea with large-capacity FLA cells and, moving forward, it certainly isn't acceptable with LiFePO cells. Time for some upgrades. Batteries, fusing, charger, service disconnects, and whatever else I have time for...

Now, according to one of the Blue Sea fuse selection charts I found on this site, the largest value fuse for single 2/0 wire isn't even listed. It stops at 0AWG with a 300A fuse. If the BMS has a "capacity" of 200A, what happens to it when an unexpected short dumps 300A of current through it? Even for fractions of a second. Will a 300A fuse blow before any damage occurs? Would a 250A be better? My current inverter maxes out at 2000W, which could put me in the 170A range (depending on voltage), but that may not be the only current draw. I don't intend on loading up the inverter while moving the trailer's slide or tongue jack, but who knows.

As for inverter precharge, I don't do this, but then the inverter is connected to the battery all the time. I've never noticed any issues when I turn off or disconnect the batteries. Is there an inverter size at which this becomes necessary?
 
packrat1969 said:
I understand that the fuse is there to protect the wiring. When I built my system 3 years ago, I used some surplus 00 AWG (I think that translates now to 2/0). At the time, I had (2) 6V T105 cells in series and was using a 150A circuit breaker as the interrupt. Well, thinking back on it, that probably wasn't a good idea with large-capacity FLA cells and, moving forward, it certainly isn't acceptable with LiFePO cells. Time for some upgrades. Batteries, fusing, charger, service disconnects, and whatever else I have time for...

Now, according to one of the Blue Sea fuse selection charts I found on this site, the largest value fuse for single 2/0 wire isn't even listed. It stops at 0AWG with a 300A fuse. If the BMS has a "capacity" of 200A, what happens to it when an unexpected short dumps 300A of current through it? Even for fractions of a second. Will a 300A fuse blow before any damage occurs? Would a 250A be better? My current inverter maxes out at 2000W, which could put me in the 170A range (depending on voltage), but that may not be the only current draw. I don't intend on loading up the inverter while moving the trailer's slide or tongue jack, but who knows.

As for inverter precharge, I don't do this, but then the inverter is connected to the battery all the time. I've never noticed any issues when I turn off or disconnect the batteries. Is there an inverter size at which this becomes necessary?
Click to expand...
00 awg = 2/0 awg ~= 70mm2

2/0 awg pure copper wire with 105c inslutaion is rated 330 amps by the AYBC.

Interactive Wire Size Calculator

Battery Chargers, Inverters, Solar Components, and Wiring Supplies for Boats, RVs, and Off-Grid Applications.
baymarinesupply.com baymarinesupply.com
The fuse rating must not exceed the wire rating.

The fuse has 3 important criteria which are all related.
Ampacity this is the current that it is rated to carry without tripping.
In reality the trip curve will allow a bit more for a short amount of time.
Voltage the maximum voltage that a fuse is rated for.
AIC=Arc interrupt capacity this is an ampacity at a given voltage where the fuse is rated to quench the arc.
This one is often overlooked and very important for LFP batteries because they can dump huge amounts of current into a dead short.
Fuses that would be fine for a lead acid battery bank can blow and sustain an arc leading to a very bad outcome including metal turning to plasma and the highly toxic contents of the cells venting.

The largest commonly available fuse under 330 amps is 300 amps.
300 fault amps * .8 fuse headroom = 240 service amps.

That means your total loads should be 240 amps or less.
for a 12 volt inverter that means...
2400 ac watts / .85 conversion factor / 12 volts low cutoff = 235.29 amps
 
Last edited:
Yeah, I think I should be able to keep the system under 200A of total operational draw. The BMS will make sure of that. So, in total, you're saying that as long as I keep the battery fusing below the ~12V wire rating (with some padding) AND select a fuse that has an appropriate AIC for lithium, then I should be all right. I feel like I'm back where I started with the 250A Class T fuse idea, but by a different path. If I understand you properly, I do have the option of moving up to an easier to find 300A (Class T) fuse.

I recently came across a discussion you had with another forum member regarding the benefits of using a Class T fuse vs an MRBF fuse for the main system fuse and that was why I was leaning in that direction. As much as I hate the idea of having to replace a $40 fuse if I do something stupid like svsagres' wrench incident, I think it's better than replacing $25K worth of camper and gear if something happens when I'm not able to do something about it. This battery is going in a moving vehicle after all...

Any idea why Blue Sea/Bus doesn't have the interrupt amperage spec for voltages other than 125VDC? A lot of us still work with 12v systems.
 
packrat1969 said:
Any idea why Blue Sea/Bus doesn't have the interrupt amperage spec for voltages other than 125VDC?
Click to expand...
You can count on the the fuse being at least as good as its AIC rating at any voltage below 125VDC.
 
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