Battery current interrupt fuses

[tdf-texas]

I feel really embarassed.

I assumed that since my trailer came with a Mega fuse installed for the inverter cables, all I had to do is add a fuse holder and all would be good.

I should have known better to trust anything Escape did as correct. The Mega fuse only has an current interrupt rating of 2000 amps and is marginal for a lead acid battery. An ANL fuse at 6000 amps could be used for lead acid and with lithum, class T must be used. (Thanks, Jon Vermilye for the info)

I wish I could go back and delete all my posts using the Mega fuse so no one else goes down that path but alas, that is not possible without moderator privileges.

Here is a link to my thread where I installed the Mega fuse holder. It is currently being replaced with a ANL or class T fuse holder as the Mega fuse does not supply the current interrupt rating required by lead acid batteries for short circuit protection.
https://www.escapeforum.org/forums/f...les-19164.html

Sorry for the mistake, Tom Fowler
________________

[Centex]

Your correction and explanation of the underlying reason is greatly appreciated as are all of your very useful posts and guidance on these electrical matters.

Thank You for continuing to share your expertise Mr. Fowler (and Mr. Vermilye)!

[davidmurphy02]

Tom, I have learned - and continue to learn - a tremendous amount from your posts on all things electrical. Being a woodworker at heart, electricity is not at all intuitive for me and my knowledge of it is fairly shallow so I really value the expertise of you and others on the forum.

[tdf-texas]

Here is the technical data for why, depending on the application, specific fuses are to be used. It's all about the interrupt rating.

Interrupt rating refers to the level of current that a fuse can safely interrupt. In severe overcurrent situations a fuse will blow but vaporize and the metal will condense right back on the base it has just vaporized from. The end result is that it doesn't interrupt the circuit and the fault continues despite the fact that the fuse has blown.

ABYC standards require all positive battery cables to be fused within 7" of the battery terminal for short circuit protection. A short circuit on a unfused battery wire creates a fire hazard due to the wire melting and flaming when subjected to the high short current.

ANL fuses have an interrupt rating of 6000 Amps, which by the ABYC standards, allows them to be used as the primary circuit protection for lead acid batteries.

MRBF fuses have an interrupt rating of 10000 Amps and meet ABYC standards but require a mouning area of over 2" above the battery terminals. This height requirement makes it hard to install these in battery boxes.

Both these types of fuses are acceptable for current interrupt service for use in a single lead acid battery installation per ABYC standards.

Class T fuses have an interrupt rating of 20000 Amps and should be used in lithium or paralleled lead acid battery installations. Both of these types of battery installs could produce current interrupt values that would exceed what an ANL / MRBF fuse could interrupt safely.

Maxi/Mega fuses have only an interrupt rating of 2000 Amps and should not be used as primary protection on lead acid or lithium battery banks.

Hope this helps.

[oldwave]

Originally Posted by tdf-texas

Here is the technical data for why, depending on the application, specific fuses are to be used. It's all about the interrupt rating.

Interrupt rating refers to the level of current that a fuse can safely interrupt. In severe overcurrent situations a fuse will blow but vaporize and the metal will condense right back on the base it has just vaporized from. The end result is that it doesn't interrupt the circuit and the fault continues despite the fact that the fuse has blown.

ABYC standards require all positive battery cables to be fused within 7" of the battery terminal for short circuit protection. A short circuit on a unfused battery wire creates a fire hazard due to the wire melting and flaming when subjected to the high short current.

ANL fuses have an interrupt rating of 6000 Amps, which by the ABYC standards, allows them to be used as the primary circuit protection for lead acid batteries.

MRBF fuses have an interrupt rating of 10000 Amps and meet ABYC standards but require a mouning area of over 2" above the battery terminals. This height requirement makes it hard to install these in battery boxes.

Both these types of fuses are acceptable for current interrupt service for use in a single lead acid battery installation per ABYC standards.

Class T fuses have an interrupt rating of 20000 Amps and should be used in lithium or paralleled lead acid battery installations. Both of these types of battery installs could produce current interrupt values that would exceed what an ANL / MRBF fuse could interrupt safely.

Maxi/Mega fuses have only an interrupt rating of 2000 Amps and should not be used as primary protection on lead acid or lithium battery banks.

Hope this helps.

Excellent information

[Mike Lewis]

I have four lithium batteries in parallel, each protected with MRBF fuses. You are saying I need to replace these fuses with Class T fuses. Is that right? Just making sure.

[tdf-texas]

Originally Posted by Mike Lewis

I have four lithium batteries in parallel, each protected with MRBF fuses. You are saying I need to replace these fuses with Class T fuses. Is that right? Just making sure.

Since you have fuses installed on each battery, the set can be evaluated as individual lithium batteries and not in parallel. So the question is: is a MRBF fuse rated for the interrupt current of a single lithium battery.

A MRBF fuse has a current interrupt rating of 10,000 amps. Since a lead acid battery typically can produce about 5000 amps in a short circuit, the MRBF fuse is acceptable. A lead acid battery is limited in producing short circuit current due to the battery's internal resistance.

A lithium battery has very little internal resistance and, when short circuited, can produce over 10,000 amps. That much current could cause a MRBF fuse to vaporize the fuse metal and continue to supply a path for current.

So, even though each battery has individual MRBF fuses, they do not meet the current interrupt specification for a lithium battery.

One caviot to all this: Some lithium batteries have a built in BMS that protects from short circuit currents. If your batteries have this feature, the MRBF fuses may be OK.

[Mike Lewis]

Originally Posted by tdf-texas

One caviot to all this: Some lithium batteries have a built in BMS that protects from short circuit currents. If your batteries have this feature, the MRBF fuses may be OK.

Thanks. Mine are the BattleBorn batteries that have some kind of BMS. I will investigate further. I don't want you all to read about me in the news. After all, I am Florida Man.

[Brian B-P]

Originally Posted by tdf-texas

Since you have fuses installed on each battery, the set can be evaluated as individual lithium batteries and not in parallel. So the question is: is a MRBF fuse rated for the interrupt current of a single lithium battery.

I agree that's the situation.

Originally Posted by tdf-texas

A MRBF fuse has a current interrupt rating of 10,000 amps. Since a lead acid battery typically can produce about 5000 amps in a short circuit, the MRBF fuse is acceptable. A lead acid battery is limited in producing short circuit current due to the battery's internal resistance.

A lithium battery has very little internal resistance and, when short circuited, can produce over 10,000 amps. That much current could cause a MRBF fuse to vaporize the fuse metal and continue to supply a path for current.

So, even though each battery has individual MRBF fuses, they do not meet the current interrupt specification for a lithium battery.

I understand the logic, but is someone can take a BattleBorn LFP battery, disable the internal overcurrent protection, connect the negative to the frame with the cables and terminals used in the trailer, connect the MRBF holder and fuse to the positive terminal, connect the fuse to another point on the trailer frame by a cable with a switch, and throw the switch on and get over 10,000 amps to flow before the MRBF opens... I'll buy them another battery.

While lithium-ion cells have relatively low internal resistance, there are a lot of parts to this system, with substantial resistance in each of them. The internal BMS alone is a current bottleneck, with substantially thinner wire than typically used externally to the battery case in these installations.

Originally Posted by tdf-texas

One caviot to all this: Some lithium batteries have a built in BMS that protects from short circuit currents. If your batteries have this feature, the MRBF fuses may be OK.

All of the lithium-ion drop-in replacement battery units mentioned in discussions here as installed in Escapes have built-in BMS with a contactor which is supposed to open at a much lower current than the fuse rating; the external fuse is a "disaster fuse" for situations such as a wiring short of extreme equipment failure, rather that for overloads due to turning too many things on at once. I think the scenario of concern is that a short circuit causes current to rise so quickly that the contactor fuses closed, leaving no working overcurrent protection internally.

[tdf-texas]

Originally Posted by Brian B-P

I agree that's the situation.

I understand the logic, but is someone can take a BattleBorn LFP battery, disable the internal overcurrent protection, connect the negative to the frame with the cables and terminals used in the trailer, connect the MRBF holder and fuse to the positive terminal, connect the fuse to another point on the trailer frame by a cable with a switch, and throw the switch on and get over 10,000 amps to flow before the MRBF opens... I'll buy them another battery.

It's not that the fuse blows on the overcurrent, it's that the instantaneous current is so high that the fuse material vaporizes, then forms on the fuse substrate material and allows current to continue to flow.

And yes, the Battleborn batteries have built in protection circuits which is why I stated that the MRBF fuses may be fine to use.

[Brian B-P]

Originally Posted by tdf-texas

It's not that the fuse blows on the overcurrent, it's that the instantaneous current is so high that the fuse material vaporizes, then forms on the fuse substrate material that allows current to continue to flow.

Yes, that's the failure mode of a fuse with inadequate circuit-interrupting current capacity. And it's only a concern (meaning the MRBF is unsuitable) if the current rises to more than the current-interrupting capacity of the MRBF (10,000 A was given earlier) before the MRBF interrupts the current... which is why I think such a failure is exceptionally unlikely in a realistic scenario.

Of course if you connect the positive and negative terminals of the battery through nothing but 4/0 gauge cable, a MBRF, and a switch, you might get that 10,000 amps and my replacement battery offer is void.

[tdf-texas]

Originally Posted by Brian B-P

Of course if you connect the positive and negative terminals of the battery through nothing but 4/0 gauge cable, a MBRF, and a switch, you might get that 10,000 amps and my replacement battery offer is void.

Engineering rule of thumb: Always design for worst case scenario. Murphy's law says it will probably happen.

[Centex]

Originally Posted by Brian B-P

... which is why I think such a failure is exceptionally unlikely in a realistic scenario.

Accidentally dropping a wrench while working in the often confined space of a battery-bay may not be common, but IME (thankfully not first hand) it's a realistic scenario that can and does occur with perhaps surprising regularity.

Lacking best-practice protective devices the consequences can be very expensive if nothing else. Granted, those best-practice protective devices may not always preclude that expense, but like a vaccination, they significantly improve one's chances of better outcomes.

IMO, YMMV of course.

[Brian B-P]

Originally Posted by Centex

Accidentally dropping a wrench while working in the often confined space of a battery-bay may not be common, but IME (thankfully not first hand) it's a realistic scenario that can and does occur with perhaps surprising regularity.

But that wrench will not be firmly bolted to either terminal, and the contact resistance alone will be substantial. The sparks are impressive, but it's not likely to pass 10,000 amps. And it's more likely to bridge the battery terminals directly than the bridge the negative terminal to the protected side of a separately mounted fuse. Indeed, this scenario is an excellent argument for using a terminal-mounted MBRF instead of a remote fuse. Yes, I did think of that scenario...

By the way, the rational precaution if concerned about this is to have non-conductive (any plastic) caps over terminals. Cars typically have this, and electric cars normally have covers over every terminal of the modules even within a sealed battery case; however, in RVs it seems common to just leave everything exposed. After all, it's a dollar or two per terminal saved, and that's the level where most RV construction decisions operate... which makes the choice of ultra-expensive Class T fuses ($35 USD and up, $54 CAD and up) and holders interesting. I support using Class T fuses, which are clearly appropriate for this duty, but I think keeping some perspective is in order.

[richm]

Is the class T fuse intended to protect only the battery? not the shunt or BMS?

I'm curious, because typically, there is a BMS and shunt in the circuit near the battery. I'm just curious whether they would be an issue for 10,000A even if for only a few hundred milliseconds.

[oldwave]

Fuses protect wire

[tdf-texas]

Originally Posted by Mike Lewis

I have four lithium batteries in parallel, each protected with MRBF fuses. You are saying I need to replace these fuses with Class T fuses. Is that right? Just making sure.

Just a thought. If the batteries are paralleled after the fuses, the sum of all four fuses is the current that could pass from the batteries to the trailer. Let's see, with inverter - 200a x 4 = 800a. Ouch!

They would have to be connected as in the attached diagram to keep this from happening.

[M_canada]

Originally Posted by tdf-texas

Just a thought. If the batteries are paralleled after the fuses, the sum of all four fuses is the current that could pass from the batteries to the trailer. Let's see, with inverter - 200a x 4 = 800a. Ouch!

They would have to be connected as in the attached diagram to keep this from happening.

I think you'd be best to use a MRBF on each battery terminal and then just wire the batteries in parallel to a busbar. The inverter would be wired to the busbar through another fuse which would be rated to protect that wire. Any other connections from the busbar would also need to be fused to protect each wire.

[Brian B-P]

Originally Posted by tdf-texas

... 200a x 4 = 800a. Ouch!

They would have to be connected as in the attached diagram to keep this from happening.

The top fuse would carry the combined current from all four, the next one down would carry the combined current of the three below it, and so on. You could do this, choosing a different rating for each fuse (800 A, 600 A, 400 A, 200A), but it seems pointless to me, as only the bottom battery would be protected from excessive current through that individual battery.

If batteries are connected in parallel, each with a fuse in series with only that battery, the cable that carries the combined current could indeed carry the total of the ratings of all paralleled fuses. That cable needs to be large enough for that combined current, or there needs to be another fuse after they are combined. A 200 A fuse for each battery plus an 800 A fuse for the combined cable would work.

[Brian B-P]

Originally Posted by M_canada

I think you'd be best to use a MRBF on each battery terminal and then just wire the batteries in parallel to a busbar. The inverter would be wired to the busbar through another fuse which would be rated to protect that wire. Any other connections from the busbar would also need to be fused to protect each wire.

Yes.