Which is preferred: BMS limiting amps or fuse limiting amps?

[neil_99]

My battery bank will be 4 sets of 4 EVE batteries with 100A JBD BMS on each set. The batteries will be arranged with 2 packs paired in parallel to a class T fuse block and another 2 packs in parallel to a second class T fuse block. After the fuse the two pairs are connected in parallel to a disconnect switch. I had planned on 200A fuses on each block, but I have now realized that the fuse block will only work down to 225A. My wires outside the BMS should be fine at 225A. My question is which is a better/safer failure point, the BMS or the fuse? Should I trust the BMSs to be okay with current between 200A and 225A, or should I be protecting it from that potential of failure with the class T fuse? Basically, should I buy smaller fuse blocks or bigger fuses? Thanks!

 

[Samsonite801]

The fuses or circuit breakers are intended to protect the wires (from fire hazard) should they hit an overcurrent situation, so they should be rated for the cable gauge.

There are calculators online to help determine max safe current based on different cable gauges.

 

[neil_99]

So, it sounds like I should let the BMS stop current before the fuse. My wires are sized large enough to handle more than 200A.

 

[TomC4306]

In operation I let the inverter be the bottleneck. Batteries capable of 1c. 150amps. 100 amp bms. 250 amp switch. 250 amp buss bar. 150 amp CB. All 2/0 wire. 3000 watt inverter max draw 67 amps. HF so 20% surge 3600. 80amps.
If something trips or blows, 1 of 2 things is true. Either we've had a catastrophic failure somewhere or/and my design was inadequate.
I don't ever intend for the BMS to trip.

 

[neil_99]

In operation I let the inverter be the bottleneck. Batteries capable of 1c. 150amps. 100 amp bms. 250 amp switch. 250 amp buss bar. 150 amp CB. All 2/0 wire. 3000 watt inverter max draw 67 amps. HF so 20% surge 3600. 80amps.
If something trips or blows, 1 of 2 things is true. Either we've had a catastrophic failure somewhere or/and my design was inadequate.
I don't ever intend for the BMS to trip.

It sounds like you are running at 48v. I'm running at 12v, so I have a higher likelihood of having higher currents. But in your example, if you had some random large DC load pulling 30amps while your inverter is pulling 80a, your bms would trip without a fuse blowing, right? It doesn't sound like you have a fuse that is covering the gap between the 100a bms and the 150a fuse. I'm asking if that is the preferred setup, or should the fuse blow before the bms reaches its max.

 

[TomC4306]

Yes, I am running 48v.
80 amps is max surge at low voltage disconnect. This decidedly not normal ops. I have no dc loads except for some instrumentation. BUT, if I did, it certainly would not be random unknown 30 amp draw.
If anything is on this system, I put it there.
Whether that is DC hanging directly off the batteries or AC loads on the inverters.
And I actually described half of my system. I'm running parallel batteries and parallel inverters.

 

[neil_99]

I'm not trying to argue with you about whether you did a good job on your design. Sorry if my message gave you that impression. I'm trying to get back to my question, which I don't think got answered. Which should be sized to trip first, the BMS or the Fuse?

 

[toms]

The BMS current limit protects the batteries from exceeding their C rate.

The fuse protects the wire from burning in the event of a short circuit.

In my case the BMS will open the contactor before the fuse blows. If the BMS fails during a short circuit event, the fuse will prevent the wiring burning.

 

[neil_99]

The BMS current limit protects the batteries from exceeding their C rate.

The fuse protects the wire from burning in the event of a short circuit.

In my case the BMS will open the contactor before the fuse blows. If the BMS fails during a short circuit event, the fuse will prevent the wiring burning.

Thanks!

 

[Dzl]

Oversizing the Class-T fuse (relative to the max expected load, not relative to the wire size) seems like a solid approach considering that those suckers are $$ and not something you want to replace because your numbers/assumptions were a little off. And because its main role is catastrophic protection, you can generally afford to give it a bit more breathing room so long as it still protects the wire or a second fuse or breaker is used in series.

That doesn't answer your question, but I don't have an answer I'm confident in.

One further thing to think through. Is how your arrangement of 4 x 100A BMSwill effect things. Think through the different possibilities of packs disconnecting and how that would effect the assumptions about max currents you've made.

 

[neil_99]

Oversizing the Class-T fuse (relative to the max expected load, not relative to the wire size) seems like a solid approach considering that those suckers are $$ and not something you want to replace because your numbers/assumptions were a little off. And because its main role is catastrophic protection, you can generally afford to give it a bit more breathing room so long as it still protects the wire or a second fuse or breaker is used in series.

That doesn't answer your question, but I don't have an answer I'm confident in.

One further thing to think through. Is how your arrangement of 4 x 100A BMSwill effect things. Think through the different possibilities of packs disconnecting and how that would effect the assumptions about max currents you've made.

Thanks! Yeah, you are right, that if one pack shuts down (via bms or blown fuse) the other packs are pretty likely to shut down due to going over current. I guess by going to a 225 or 250A fuse, I probably eliminate the chance of blowing one of those fuses (which are expensive and hard to find). Theoretically, I will be around the nominal wattage of my inverter with two bms working, so I could probably limp along if I had a prolonged problem with one pair of batteries.

 

[JWLV]

Fuses are cheap. It's good insurance.

 

[wholybee]

In the event of a short circuit, the BMS should react faster than a fuse. So regardless of fuse size, in that event the BMS will be providing the protection.

If i have a 120A BMS, i want to protect it from ever seeing 120A. A BMS is more expensive and more difficult to replace than a fuse. So my fuse is smaller than the BMS rating. However, i then have the BMS protection set lower than the fuse, at the max current i expect to ever use.

Example, 120A BMS. 100A fuse. Protection set to 75A. I never exceed 50A during use.

 

[John Frum]

So regardless of fuse size, in that event the BMS will be providing the protection.

I would say that the BMS will be the first line of protection.
It may weld closed during the attempt.
Or... it may have welded closed during some previous event.
Breakers and contactors can also weld closed.
I have a class-t fuse as close as possible to the positive battery terminal out of consideration for these eventualities.

 

[neil_99]

Fuses are cheap. It's good insurance.

Compared to everything else, yes. But $40-60 and a week or so waiting for another fuse in the mail (if I can find one...) isn't something I want to do needlessly. If $40-60 for a fuse is cheap, then $80 for the BMS is cheap too. I guess it is all relative.

 

[Zil]

Size the fuse to the ampacity of the cable you are using. Fuse as near as possible to the battery positive terminal. If you have three battery banks feeding one positive BusBar by keeping the first cables short it is possible to use one catastrophic fuse from that BusBar. Only you can see if there is danger of a short in the first cable from the battery.
The BMS is not relative to protecting wires from overheating or battery from exploding in a dead short. A dead short may even arc the BMS and be in a very bad way.

 

[HRTKD]

Which should be sized to trip first, the BMS or the Fuse?

None of the above? Not really a good answer, but the BMS should be the last line of defense. This is the answer we give when it comes to voltage. Your charging devices should be configured to never exceed what the BMS (and battery) can handle. Discharge devices should be configured the same.

I bought solar charge controllers that fit within what my batteries and BMS are willing to accept. The Inverter could be programmed to limit the amperage, but I think it's going to be OK with the OEM setting.

 

[time2roll]

I went with a low cost MEGA fuse on each battery just below the BMS rating. Between the two something should stop the short circuit.
I carry spare fuses.

 

[dweick]

Fuse/breakers should protect the circuitry/wiring in overload conditions.

BMS should be rated for the max expected load with some headroom. You get a higher than expected load I wouldn't want to rely on a BMS to shut it off, I want something reliable and guaranteed to shut things off according to a precise data sheet.

 

[rmaddy]

I went with a low cost MEGA fuse on each battery just below the BMS rating. Between the two something should stop the short circuit.
I carry spare fuses.

LiFePO₄ can have a huge amp surge during a short circuit. The AIC (ampere interrupt current) of a fuse is important for a LiFePO₄ battery fuse.

MEGA fuses - 2,000A
ANL fuses - 6,000A
Class T fuses - 20,000A

MEGA fuses may not be the safest choice for LiFePO₄ batteries. There are several threads on this site discussing the best fuse for LiFePO₄. Probably worth a look.