Class T vs ANL fuse

The specs on my Overkill BMS says it takes 10 seconds for this over current protection to kick in. For a 100 amp system at operating at 120 amps, that's quicker than my fuse is likely to blow, but with 200 amps being pulled from the batteries, the fuse would blow near instantly.

BMS tend to claim short circuit protection. But I would discount the ability of anything based on a few FETs to deal with 20,000A. Only if current rise time was delayed (e.g. due to large inductance) so they could disconnect as current crossed 200A. But then it would have to survive the inductive kick (which is reverse polarity, so a diode snubber works.)

At 5x rating, fuses tend to have specs of about 10 milliseconds to interrupt current.

I see now in my Overkill manual I have short circuit protection. Some things I'll just have to finish installing to see how it works.

Seems like its configurable, but as look at the XiangXong Battery app, I don't see where to configure it.

chrisski said:

The specs on my Overkill BMS says it takes 10 seconds for this over current protection to kick in. For a 100 amp system at operating at 120 amps, that's quicker than my fuse is likely to blow, but with 200 amps being pulled from the batteries, the fuse would blow near instantly.

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I changed mine to 5 seconds and I believe it is at 130 amps. Any fuse should be fine at 200 amps.

Page 22 of the Overkill manual 3.1.10 describes short circuit protection of a few milliseconds. Not visible in the Apple app.

Reading up on the subject, there are several sources saying that FETs shouldn't be relied on as overcurrent protection like a fuse would because during a high overcurrent event, the doped semiconductor junction inside can overheat so severely that it can kind of melt and the device becomes unable to turn off. In other words that it can become no longer a semiconductor, and can fail in the closed circuit state.

The overkill has both overcurrent protection, and short circuit protection. They are different. The short circuit protection is nearly instant, and it works. I have directly shorted to test. Big spark, and it turns of for a few minutes. Reportedly, if done enough times it will damage the BMS. You still need a fuse.

Ok so I think I am comfortable with my simple mega fuse and the BMS combined without worry of going class-t.

ohthetrees said:

I got the (I guess wrong) impression that 400a was max from reading the Blue Sea fuse selection chart and seeing that the maximum class T size was 400 amp. Also, The marine stores I frequent seem to only carry up to 400 amp. Here is a link to the chart:
http://assets.bluesea.com/files/resources/newsletter/images/Choose_the_Fuse_Amperage.jpgView attachment 61620

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Class T fuses increase in size (in groups) with increasing current rating. The Blue Sea class T fuse holders (two sizes) only accommodate 110-400A fuses. Below and beyond that (and the fuses go much higher in capacity), you need to provide your own mounting system.

Just to correct an earlier post about the ABYC standard for Lithium Ion Batteries (TE-13), it does NOT mandate class T fuses. This "Technical Information Report" simply requires fuses of sufficient AIC (Ampere Interrupting Capacity). Unfortunately, as most battery manufacturers I'm aware of don't specify available battery short-circuit currents, the user is left with guesstimating instead of proper engineering to decide which fuses to employ.

ABYC do mention Class T fuses in a note, but there is no requirement to use Class T.

That said, I would not use anything else in a Li-Ion system. The cost of the right fuses and fuseholders pales in comparison to the cost of batteries and inverter-charger, etc. costs. Safety and peace of mind are much more important to me. I still can't get over a major manufacturer employing Mega fuses in this situation, when they are only rated by the inventor/manufacturer (Littlefuse) to interrupt 2000A at 32VDC and 1000A for the 58V version! (See https://www.victronenergy.com/upload/documents/Lynx_Distributor/Lynx_Distributor-en.pdf.)

I would add that for 48V systems, the Class T fuse is really the only proper option. ACK fuses could be borderline for a bolted fault, and are slower interrupting (plus you need to be careful you don't end up with the time delay versions often used in forklifts). Class T fuses also have relatively low voltage drop.

alanapar said:

I still can't get over a major manufacturer employing Mega fuses in this situation, when they are only rated by the inventor/manufacturer (Littlefuse) to interrupt 2000A at 32VDC!

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Which manufacturer?

alanapar said:

That said, I would not use anything else in a Li-Ion system.

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Just want to be sure we're talking about Lithium Iron Phosphate, and not Lithium Ion Batteries.

chrisski said:

Just want to be sure we're talking about Lithium Iron Phosphate, and not Lithium Ion Batteries.

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To be pedantic, LiFePO4 is a type of Lithium Ion. They all are.

chrisski said:

Just want to be sure we're talking about Lithium Iron Phosphate, and not Lithium Ion Batteries.

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That's a common misunderstanding. Lithium Iron Phosphate ARE Lithium Ion Batteries, just one of several common chemistries employed in Lithium Ion Batteries.

I think the reason people do this is as an (incorrectly formulated) attempt to distinguish the relatively "safe" LiFePO4 chemistry from the other most commonly employed chemistries.

alanapar said:

That's a common misunderstanding. Lithium Iron Phosphate ARE Lithium Ion Batteries, just one of several common chemistries employed in Lithium Ion Batteries.

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I don't think I'll go down that road.

LCO is what a lot of people maybe imply with

“Lithium Ion”

LiFePO4 = Lithium Ion Subtype

LiCoO2 = LCO = Lithium Ion Subtype

this isn’t something that should be painful.

LMO LTO LFP LYP NCA NMC LCO are all Lithium Ion chemistries. There is no more to it than this if it uses lithium ions then it’s a lithium ion cell.

saying a dog is not a mammal is strictly technically incorrect and worth pointing out

Your all wimps! I just got me a Manly Sized Class T fuse for my system.




JUST KIDDING GUYS I thought that size counted in everything

robby said:

Your all wimps! I just got me a Manly Sized Class T fuse for my system.

View attachment 74115


JUST KIDDING GUYS I thought that sized counted in everything

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Oversized protection can be worse than no protection (in all things) ;-)

gonna need a lot of wire! XD (what was the temperature rating of this insulation again….)

alanapar said:

Oversized protection can be worse than no protection (in all things) ;-)

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Well not in "All things" A nice 50 cal is better protection than a .223 and thick wires with thin Insulation pass more electrons than thin wire with thick insulation

alanapar said:

Just to correct an earlier post about the ABYC standard for Lithium Ion Batteries (TE-13), it does NOT mandate class T fuses.

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Good point. What is the TE-13 reference?

alanapar said:

The standard simply requires fuses of sufficient AIC (Ampere Interrupting Capacity). Unfortunately, as most battery manufacturers I'm aware of don't specify available battery short-circuit currents, the user is left with guesstimating instead of proper engineering to decide which fuses to employ.

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This is true. And the ABYC E-11, barely considers LFP as it is. Considering the predicament you outlined above (unknown short circuit current) this is the relevant excerpt:

11.10.1.2.3 For batteries or battery banks with a CCA rating greater than 2200 CCA, or 500 amp hours, battery
overcurrent protection shall have a minimum ampere interrupting capacity (AIC) rating at least as great as the battery
manufacturer’s short circuit rating OR be rated at a minimum of 20kA at 125 VDC or higher.

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Since short circuit current is unknown, the latter should apply. It is unclear if the above pertains to LFP or not, there is no indication that it does not.
The above is probably why Class T has come to be the defacto standard recommendation (probably not a coincidence that both Blue Sea (Eaton-Bussmann) and Littelfuse have class T fuses rated for 20kA AIC @ 125VDC). So while there is no specific requirement for Class T, it is a sensible choice, and the most readily available fuse type that satisfies this standard.

alanapar said:

That said, I would not use anything else in a Li-Ion system. The cost of the right fuses and fuseholders pales in comparison to the cost of batteries and inverter-charger, etc. costs. Safety and peace of mind are much more important to me.

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Dzl said:

Good point. What is the TE-13 reference?

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TE-13 13.7.8.1 If necessary, a battery bank should be subdivided into units such that the ampere interrupting capacity (AIC) of the overcurrent protection device is not exceeded.

NOTE: Generally, fast acting current limiting fuses such as Class T fuses in an approved Class T fuse holder have an AIC of 20,000 amps at 12VDC and will be adequate for a subdivided bank.

[My comment: why only 12VDC?]

Dzl said:

This is true. And the ABYC E-11, barely considers LFP as it is. Considering the predicament you outlined above (unknown short circuit current) this is the relevant excerpt:

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A lot of ABYC standards (and standards in general) fail to keep pace with technology. I implemented a second-generation LFP system on my former boat over 5 years before TE-13 was released. And although I don't fully understand ABYC's "TE" designation, it is labelled a "Technical Information Report" and AFAIK, is not a required standard.

Frankly, I am disappointed in TE-13. There is a lot it could cover and doesn't. For instance, how can you require adequate AIC if you don't require the short circuit current to be specified? Having the 20kA fall-back in E-11 seems to be ducking the issue.

And subdividing a bank to accommodate protection devices that can't handle the full bank short circuit current begs the question of how you protect downstream devices and branch circuits.

And what about segregation of parallel strings (or groups of parallel strings) with contactors to limit fault effects and total loss of power with cell failures and BMS shutdowns. I know people who have lost all 24V power on their boats because the Li-Ion bank / BMS employed a single master contactor to "protect" the bank (while leaving all parallel strings still connected to the faulted string).

Dzl said:

Since short circuit current is unknown, the latter should apply. It is unclear if the above pertains to LFP or not, there is no indication that it does not.
The above is probably why Class T has come to be the defacto standard recommendation (probably not a coincidence that both Blue Sea (Eaton-Bussmann) and Littelfuse have class T fuses rated for 20kA AIC @ 125VDC). So while there is no specific requirement for Class T, it is a sensible choice, and the most readily available fuse type that satisfies this standard.

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Interestingly Mersen (Ferraz Shawmut) A3T (160V DC rated class T) fuses are rated at 50kA interrupting capacity. Presumably this means they were qualified to this level with UL or whomever. I bought Mersen fuses for the extra "reserve" whether it's real or not.

Dzl said:

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alanapar said:

Frankly, I am disappointed in TE-13. There is a lot it could cover and doesn't. For instance, how can you require adequate AIC if you don't require the short circuit current to be specified? Having the 20kA fall-back in E-11 seems to be ducking the issue.

And subdividing a bank to accommodate protection devices that can't handle the full bank short circuit current begs the question of how you protect downstream devices and branch circuits.

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good point

Short Circuit Ampere rating seems good for lithium cells to carry and post visibly?

alanapar said:

people who have lost all 24V power on their boats because the Li-Ion bank / BMS employed a single master contactor to "protect" the bank (while leaving all parallel strings still connected to the faulted string)

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?

alanapar said:

Frankly, I am disappointed in TE-13. There is a lot it could cover and doesn't. For instance, how can you require adequate AIC if you don't require the short circuit current to be specified? Having the 20kA fall-back in E-11 seems to be ducking the issue.

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That is one way to look at it. Another way is that its just a statement made in recognition of the same problem that you stated--that AIC is often not listed by LFP manufacturers or assemblers--and guidance that can be followed without knowing that info would be useful to builders.

alanapar said:

And subdividing a bank to accommodate protection devices that can't handle the full bank short circuit current begs the question of how you protect downstream devices and branch circuits.

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If each subdivision was properly fused, with properly sized AIC and current ratings, downstream would be protected wouldn't it? Only the first fuse for each subdivision would need to handle the full AIC of the pack I think. Beyond that, lower AIC fuses are okay I think. I may be misunderstanding something.

alanapar said:

Interestingly Mersen (Ferraz Shawmut) A3T (160V DC rated class T) fuses are rated at 50kA interrupting capacity. Presumably this means they were qualified to this level with UL or whomever. I bought Mersen fuses for the extra "reserve" whether it's real or not.

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Nice to have a little extra breathing room, particularly considering the unknowns. The 20kA stated in E-11 is referenced as the minimum if short circuit current is unknown, so exceeding it is reasonable. Was the fuse more pricey than other class T fuses you looked at?

alanapar said:

NOTE: Generally, fast acting current limiting fuses such as Class T fuses in an approved Class T fuse holder have an AIC of 20,000 amps at 12VDC and will be adequate for a subdivided bank.

[My comment: why only 12VDC?]

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Not sure without more context. My guess is they are just making a fairly safe generalized statement for the most popular system voltage. The Class T fuses I have looked at are all rated at 20kA for voltages above the common 12/24/48 system voltages (most commonly 125V or 160V DC)