Fusing individual 100AH BattleBorn batteries in a 4 battery bank

[rarebear.nm]

I read various opinions about fusing each individual battery in a four battery bank of Battle Born 100AH batteries.


What are some opinions and reasoning why?

[TacomaJoe]

I have three DIY batteries feeding a bus. Each battery has a MRBF on it. My reasoning is that without individual fusing, that wire between the bus and battery post is unprotected. If an idiot (not me) were to drop a wrench across the battery terminals there is nothing to stop the current from the other batteries from feeding a heap of current into the short.

I will also add that the BMS in the other batteries would shut down due to overcurrent, but if I fully trust that, then there is no need for fuses.

[rarebear.nm]

OP here:


I agree with your reasoning for individual fuses.


What I did not mention in the OP, is that all four batteries connect to a 500amp buss bar with 2/0 wire that is then connected via 4/0 wire to a 400a Class T fuse before it goes anywhere else.


I could use 100 amp MRBF fuses connected to the battery posts then feed the first buss bar. Those batteries have a 100 amp BMS.


A Multiplus 120v 3000watt inverter is connected via 4/0 wire off a main buss bar.

[beach house]

An even more important reason to fuse each individual battery, is in case one battery develops a dead short, and the BMS fails to disconnect that battery, the fuses will prevent the energy stored in the other three batteries from entering the shorted battery. All of my batteries are fused using MRBF fuses.

[Stircrazy1]

Originally Posted by rarebear.nm

OP here:


I agree with your reasoning for individual fuses.


What I did not mention in the OP, is that all four batteries connect to a 500amp buss bar with 2/0 wire that is then connected via 4/0 wire to a 400a Class T fuse before it goes anywhere else.


I could use 100 amp MRBF fuses connected to the battery posts then feed the first buss bar. Those batteries have a 100 amp BMS.

you need larger than a 100 amp MRBF, 100 amps is your working output, then you have a over current for a period of time so 200 amp for 30 seconds and 500 amps for 1/2 a second. I would fuse them for say 250amps, but here is the recommendation from battle born.

"For a Battle Born 100Ah 12V LiFePO4 battery, a Class T fuse rated for 200 amps or higher is recommended for the main power conductor on the positive side, within 7 inches of the battery"

[beach house]

The protection needs are different compared to lead acid batteries, and the key point of using MRBF fuses is to prevent thermal runaway of the Lithium battery bank in case the bms fails, especially during and internal battery short situation. Under those situations, every second counts. Attached is trip curve of the MRBF fuses per Blue Sea website. It shows that it takes about 10 minutes to blow an MRBF fuse at 140% load, and about 1.5 minutes to blow a fuse at 200% load. It logically makes sense that the bigger the battery bank is, the lower (with in reason) the MRBF fuse should be. For 4 batteries, the numbers show that a 150 amp fuse would be more that adequate. 140% of 150 amps is 210, so it would take about 10 minutes to blow a 150 MRBF fuse at 210 amps, if the bms fails to shut down. Now multiply that by four. That would be 840 amps combined draw on the system, and if you are drawing that much, something is really wrong. The point being that unless you have a short in the inverter, or an internal short in one of the batteries, the odds of blowing one of these MRBF fuses is extremely low. Even using 100 amp fuses with four batteries would take 560 amps to trip, and that would take 10 minutes to trip. So with a four battery bank, even if you have a spike of 560 amps, it would have to be sustained for 10 minutes to blow 100A fuses. For a four battery setup, a 150 amp fuse is more than adequate, allowing you more than plenty of overhead for surge spikes, but providing more protection, at a quicker rate, in case of an internal battery short, or any short for that matter. On the other hand, if one uses 200A fuses, like suggested, it would take about 10 minutes of a combined draw of 1120 amp to blow those fuses. At 200% load, that would be a combined 1600 amps to blow the fuses, and it would still take about 1.5 minutes to cut off. That is way to much unneeded overhead and does not provide enough protection on the larger battery banks. If it was a single battery, that is a different story, in that case, a 200A fuse would be appropriate. Everyone has their own safety threshold, look over the chart, do the math, and make you own decision.

[Stircrazy1]

Originally Posted by beach house

The protection needs are different compared to lead acid batteries, and the key point of using MRBF fuses is to prevent thermal runaway of the Lithium battery bank in case the bms fails, especially during and internal battery short situation. Under those situations, every second counts. Attached is trip curve of the MRBF fuses per Blue Sea website. It shows that it takes about 10 minutes to blow an MRBF fuse at 140% load, and about 1.5 minutes to blow a fuse at 200% load. It logically makes sense that the bigger the battery bank is, the lower (with in reason) the MRBF fuse should be. For 4 batteries, the numbers show that a 150 amp fuse would be more that adequate. 140% of 150 amps is 210, so it would take about 10 minutes to blow a 150 MRBF fuse at 210 amps, if the bms fails to shut down. Now multiply that by four. That would be 840 amps combined draw on the system, and if you are drawing that much, something is really wrong. The point being that unless you have a short in the inverter, or an internal short in one of the batteries, the odds of blowing one of these MRBF fuses is extremely low. Even using 100 amp fuses with four batteries would take 560 amps to trip, and that would take 10 minutes to trip. So with a four battery bank, even if you have a spike of 560 amps, it would have to be sustained for 10 minutes to blow 100A fuses. For a four battery setup, a 150 amp fuse is more than adequate, allowing you more than plenty of overhead for surge spikes, but providing more protection, at a quicker rate, in case of an internal battery short, or any short for that matter. On the other hand, if one uses 200A fuses, like suggested, it would take about 10 minutes of a combined draw of 1120 amp to blow those fuses. At 200% load, that would be a combined 1600 amps to blow the fuses, and it would still take about 1.5 minutes to cut off. That is way to much unneeded overhead and does not provide enough protection on the larger battery banks. If it was a single battery, that is a different story, in that case, a 200A fuse would be appropriate. Everyone has their own safety threshold, look over the chart, do the math, and make you own decision.

so given that, you are using the wrong type of fuse to protect a LiFePO4 battery. what should be on each battery is a proper sized "T" classed fuse which is a fast acting fuse not a slow one, which is the perfect match for LiFePO4 batteries. it should be sized at 1.2 to 2X the maximum current through the battery.

sorry I missed that you were using a MRBF in the original post or I would have mentioned it then. LiFePO4 are not subject to thermal runaway like "other" lithium batteries a MBR isn't going to protect it in the way it needs to be protected, or in the way you think it will.

[beach house]

Originally Posted by Stircrazy1

so given that, you are using the wrong type of fuse to protect a LiFePO4 battery. what should be on each battery is a proper sized "T" classed fuse which is a fast acting fuse not a slow one, which is the perfect match for LiFePO4 batteries. it should be sized at 1.2 to 2X the maximum current through the battery.

sorry I missed that you were using a MRBF in the original post or I would have mentioned it then. LiFePO4 are not subject to thermal runaway like "other" lithium batteries a MBR isn't going to protect it in the way it needs to be protected, or in the way you think it will.

Attached below is a link to Class T fuse trip curve (scroll to page 3), it shows a similar deal, but not as extreme. So, yes, they are faster acting, but still have a curve built in to prevent the fuse from blowing during reasonable spikes. The issue comes in when one has a large battery bank, of 4 or more batteries. Although LFP batteries are fairly stable and safe, they still can achieve thermal runaway, even though it is not as violent as some of the other Lithium chemistries. With either type of fuse, if one has a large battery bank, it is prudent to do the math and calculate what total spike you would expect in your system, and build in a bit of overhead, and then under-size the fuses for each individual battery, essentially building in an additional safety measure, in case something goes wrong. Many times, in RV applications, there are room constraints, where one does not have the room to place 6 individual class T fuses, but does have room for MRBF fuses. In those situation, especially if one moderatley under-sizes the fuses, it will provide protection, specifically in the case of a single battery shorting out, causing an inrush of power from the rest of the battery bank. In the case of a short, the inrush will be enough to easily and quickly trigger an MRBF fuse, but it can't hurt to look at the charts to fine tune things a bit more for a quicker trip. Basically my point is, one can build in additional safety, even when the choices are limited, all they have to do is some homework and math, and it is much better (provides much more safety) than having just one master fuse and no individual fuses for each battery. Put another way, is it perfect? No, nothing is ever 100% when it comes to safety, but it sure is better than nothing. It never hurts to get creative and to improvise, when working with certain constraints and still wanting to provide an additional safety cushion.


Link to Class T fuse graph: https://www.eaton.com/content/dam/ea...s-1025-jjn.pdf