AIC Ratings for LiFePO4 - Class T - MRBF

Q&A and troubleshooting
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Hi,

I’m trying to get some information surrounding AIC (Ampere Interruption Rating) ratings and appropriate fuse types for LiFePO4 installations.

I’m still working this all out so please let me know what I’ve gotten wrong.

It seems like the extremely low internal resistance of lithium batteries allows them to drop huge amounts of current very quickly when something goes wrong. This necessitates the use of fuses with much higher AIC ratings.

I have a couple of installations that are fused with MRBF fuses (One is 12VDC @ 2.5KWH) and the other 48VDC @ 10KWH) so I’ve been trying to work out whether they are adequately protected or need to be fixed.

There doesn’t seem to be a huge amount of information around and it would seem like lithium batteries are relatively new so information, guides and hardware hasn’t kept up. It was only this year that Victron started offering anything other than MEGA fuse holders for batteries.

Anyway, hope I can find some guidance. Cheers.

Why do make a distinction between ‘battery fuses’ and ‘device fuses’ What are their different roles?

Battery fuses
a) Doesn’t fail in such a way as to continue conducting electricity.
b) Protects the battery from over-discharge
c) Protects everything upstream from catastrophic battery failure

Device fuses
a) Protects wires and devices from over current conditions.

Class T – Most often used as battery fuses
MRBF – Used as both battery and device fuses
MEGA – Should never be used as a battery fuse. Great as device fuses.

  1. What types of failure modes are we trying to mitigate, how likely are they to happen and how catastrophic can they be?

Battery fuses:

Catastrophic battery failure (What does this look like)
Battery over-discharge conditions (Which the BMS shouldn’t allow in the first place)
Wire protection

Device fuses:
Shorts
Devices drawing too much current
Wire protection

As far as I know, AIC ratings are affected by:
I’m not sure of the correct cell/battery/pack terminology here

a) Internal resistance of the battery
b) Resistance of the upstream wire
c) Voltage of the battery
d) Capacity of the battery?

What other systems could be in place as secondary protective measures?

  1. The BMS itself should have over-current protection – How is this normally implemented and how can it fail? Do more expensive batteries often have more/better protection?

I’ve seen recommended AIC values of: 10,000A – 20,000A short circuit current thrown around for LiFePO4’s

Would MRBF fuses ever be appropriate for use with LiFePO4 batteries? Even at 12VDC or are Class T fuses the only reasonable option?

It would seem that 12VDC setups are PROBABLY OK with MRBF fuses but anything higher voltage (e.g: 24VDC, 48VDC) will absolutely need class T fuses.

Any other advice while we’re here?

  1. Remember to pre-charge the inverter capacitors with a resistor or the capacitor inrush current might pop fuses and cause damaging arcs.

  2. Be careful hanging fuses off battery posts. Try to reduce the physical stress on it.

  3. The closer the fuse is to the battery, the better.

  4. Impedance/length match battery cables so they don’t go out of balance.

Thanks

1 Like
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Correct

I can tell you what we say on diysolarforum, but it just consensus opinion.
There is very little manufacturer guidance out there, and lots of incomplete information.

I am only addressing 48V nominal battery systems in these comments.
12 and 24V require significantly less protection because arcing is so much harder.

(a) is DC arcing, which is super dangerous and will eventually burn down something
(b) fuses only protect is the max discharge rate being exceeded, normal discharge rates will result in a flat battery eventually
(c) catastrophic or doomsday scenario protection are for if everything else fails

Other options for Class T are NH00 (UK, Europe, AUS) and EF3.
Anything with an interrupt rating above 10 kA, preferably 20 kA rated at least at 60V.
I’ve seen some new automotive fuses rated 20 kA at 125 VDC.
Now that is protection.

Catastrophic battery failure can occur when a cell develops a bad short internally.
In this case, depending on the topology of the battery system, an array of fuses or
circuit breakers can help mitigate having every battery dumping all of its energy into the shorted cell. DIY batteries have burned down people’s houses because of this.

Or when a dead short occurs on the output wiring, or when an inverter dead shorts, etc.

You cannot rely on a BMS to protect you. If it has a contactor, it can get welded shut.
If it has MOSFETs, they typically fail in a dead short. Some rack mount batteries have
an inexpensive DC breaker. The reliability of those are questionable at best.

(a) and (d) go hand in hand, because as you parallel more batteries together the effective internal resistance goes down (e.g. 3 batteries in parallel will have 1/3 the resistance).
(c) matters for 48V and above, and you will notice that some fuses have really high interrupt ratings at 12V, but only 2 kA at 48V.

Covered above (contactors, MOSFETs, circuit breakers).
There is likely a weak correlation of price with quality.
The best thing to do is to look for torture test videos on YouTube for the battery in question.

I shoot for 20 kA. I don’t have time to entertain a doomsday scenario more than once.

Class T is overkill at 12V, MRBFs are great for 12V and 24V systems (IMO).

Put your Class T fuses in a proper fuse holder, which may cost more than fuse.
When it goes, you want mechanical strength and containment.

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The biggest issue with lithiums being able to let out huge amps in short periods is actually arc faults, so you need a fuse that can both break the path and quench the arc.
Batteries have a high KA rating. Most breakers and fusing you see have a ka rating.
So you choice really depends on if that has the potential to happen.

4

@ricardocello
@lxonline

Thank you for the input. I guess I’ll rip out the MRBF’s on my 48V Pylontech setup and get some class T’s in there. Is there just the one form factor for class T’s? You mentioned NH00 and EF3, what are these distinctions?

What’s the best way to attach an M8 Lynx distributor to an M10 Class T power-in?

It’s pretty frustrating to have two different incompatible lines of Lynx products. Are there any plans to release an M8 Class T power-in?

Thanks

5

Class T sizes are not typically standardized, unfortunately.

Pick a vendor (e.g. Littelfuse JLLN or bussmann/eaton JJN), a fuse rating in Amps, and find a matching fuse holder. The size gets bigger over 200A.
Sometimes fuse/holder are sold as a set.
There are off-brands on Amazon, I have no opinion on those. BlueSea uses the main brands.

https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/bussmann-series-branch-circuit-fuses/class-t/bus-ele-ds-1025-jjn.pdf

https://www.littelfuse.com/assetdocs/jlln-datasheet?assetguid=2925cc6d-2278-44e0-8fdd-106f087f707e

NH00 and EF3 are different classes of fuses with similar high DC interrupt capability.
Not Class T, but just as good, easier to find in non-US markets and cheaper.

I don’t keep up with the Lynx developments, so perhaps someone else can help.

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Hello @confusion , interesting question and thoughts in your post. Why should a MEGA fuse never be used as a battery fuse in your opinion?