AIC Ratings for LiFePO4 - Class T - MRBF

[totalconfusion]

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

 

[stromy]

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

The fuses have different dimensioning, according to the needs of the battery or the device.

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?

Every electrical system needs at a minimum protection against these five threats:
1 short circuit
2 protection of humans from electrocution
3 protection against electrical fire
4 protection against overload
5 protection of equipment

If you use fuses for this:
For 1: these are short events. All the energy of the short circuit current heats the conductors (temp. increase). No heat is conducted to the environment. Your fuse has to interrupt before the insulation melts (150C for PVC). See fuse I²t characteristics from datasheet.
For 2: not needed if voltage <60V DC.
For 3: if your error current is >300mA, your fuse has to interrupt within 5sec.
For 4: you look at your conductors after 1hr continuous use. The energy heating your conductors is the same as the heat being transfered to the environment. You select your fuse for the conductors to stay inside safe temp range (75C for PVC).
For 5: you select a fuse based on the requirements of the equipment.
Normally you want to use one fuse, that fits all of these requirements. However that is not possible with the parameters of LiFepo. So you use a combination of fuse + BMS.

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

Primary reason for battery fuse is to handle BMS failures.
Such failures "look" like an arc welder, possibly combined with an "metal fire". People have put this on youtube.

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

AIC has to be higher than the short circuit current. Typically you measure the internal resistance (see datasheet of battery) and compute the Prospective Short Circuit Current PSSC (using ohms law).
The fuses voltage rating has to be higher than the open circuit voltage.
Also very important: these ratings are different for AC and DC. Your fuses rating must match the type of current (AC or DC)!!! DC is much harder to interrupt for fuses.

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?

The BMS is based on semiconductors. Advantage is fast response and very tight tolerances.
Disadvantage is the failure mode in case of overheat or voltage pulses (ESD). Semiconductors may become conductive in all directions in case of a failure. Also they depend on auxiliary power supply, software, etc.

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.

Compute the PSSC from your battery datasheet and compare with the fuses datasheet AIC.

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.

Fuses need fuse holders. They sometimes break mechanically. You do not want to have cables flying around because they used a fuse as a terminal.

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

Fuses protect only stuff that is "downstream" to them.

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

Thanks

 

[totalconfusion]

The fuses have different dimensioning, according to the needs of the battery or the device.

Every electrical system needs at a minimum protection against these five threats:
1 short circuit
2 protection of humans from electrocution
3 protection against electrical fire
4 protection against overload
5 protection of equipment

If you use fuses for this:
For 1: these are short events. All the energy of the short circuit current heats the conductors (temp. increase). No heat is conducted to the environment. Your fuse has to interrupt before the insulation melts (150C for PVC). See fuse I²t characteristics from datasheet.
For 2: not needed if voltage <60V DC.
For 3: if your error current is >300mA, your fuse has to interrupt within 5sec.
For 4: you look at your conductors after 1hr continuous use. The energy heating your conductors is the same as the heat being transfered to the environment. You select your fuse for the conductors to stay inside safe temp range (75C for PVC).
For 5: you select a fuse based on the requirements of the equipment.
Normally you want to use one fuse, that fits all of these requirements. However that is not possible with the parameters of LiFepo. So you use a combination of fuse + BMS.

Primary reason for battery fuse is to handle BMS failures.
Such failures "look" like an arc welder, possibly combined with an "metal fire". People have put this on youtube.

AIC has to be higher than the short circuit current. Typically you measure the internal resistance (see datasheet of battery) and compute the Prospective Short Circuit Current PSSC (using ohms law).
The fuses voltage rating has to be higher than the open circuit voltage.
Also very important: these ratings are different for AC and DC. Your fuses rating must match the type of current (AC or DC)!!! DC is much harder to interrupt for fuses.

The BMS is based on semiconductors. Advantage is fast response and very tight tolerances.
Disadvantage is the failure mode in case of overheat or voltage pulses (ESD). Semiconductors may become conductive in all directions in case of a failure. Also they depend on auxiliary power supply, software, etc.

Compute the PSSC from your battery datasheet and compare with the fuses datasheet AIC.

Fuses need fuse holders. They sometimes break mechanically. You do not want to have cables flying around because they used a fuse as a terminal.

Fuses protect only stuff that is "downstream" to them.

Thanks for reply.
I think I'll replace the 48V setup with class T's and leave the 12V setup as is.
Just need to find a way to attach a Victron M10 Class T power-in to a Victron M8 Lynx distributor