Should I add a fuse to my battery pack?

[JanC]

Hi,

I'm upgrading my solar system after about 8 months since I built it with one extra lithium battery that I will wire in parallel with the older one. The battery is Elkersolutions lifepo4 24v 100Ah. Currently I have one 300A MRBF fuse right on the positive terminal.

What I researched so far is that it's recommended to fuse each lithium battery when connecting in parallel. But they were usually talking about systems that have 4 or more batteries connected in parallel. My system is rather small, so I'm trying to find out if it is that dangerous to not buy one extra fuse and only use the main MRBF fuse on one battery terminal as I have been using so far. It's not really about me not wanting to spend extra money on an additional fuse but the problem is, it takes 2 weeks at least to ship that fuse to my place and we're currently in the process of building a small house so we use a lot of tools and we kind of need that battery working as soon as possible. So I need to know if it's okay to wire it without a fuse at least for now.

I will just explain my thinking here, so please correct me if I'm wrong. The reason why there need to be a fuse between each battery is that in case there is a dead short in one of them, the wire connecting them can get hot and catch on fire. Is this the only reason why I should have a fuse in between those 2 batteries? And if I understand correctly, it all depends on how much I trust the BMS in this battery, because the BMS should take care of this problem unless it fails for some reason. But isn't the BMS built specifically for taking care of this situation? And in case I really need to fuse it, would the same 300A MRBF fuse be suitable? I will attach a drawing of how I would fuse it. The cables used are 2/0 AWG or in my case 70mm2.

Thanks

Jan

 

[stromy]

Hi Jan,

the fuse protects the battery, the wires, the inverter, and anything else connected. Depending on how it is sized, the protection can be against short circuit, fire, electrocution (not relevant here), overload, etc.
So each battery should have a fuse. They should be the same because of balancing.

The reason to use a fuse in addition to the BMS is, they have different properties:
Fuses can safely break huge currents, but they are very coarse, have big tolerances, like 50% or more. They are also relatively slow.
A BMS can be much more precise, faster, can be configured to the exact needs, with very low tolerances, can be reset. However BMS use semiconductors as switching devices. Semiconductors typically become better conductors if their temperature rises. Which leads to even more current and more heat (thermal runaway). If semiconductors overheat, the PN junctions will melt, and the semiconductor becomes permanently conductive on all its connections (the opposite of a fuse). Because of this, semiconductors cannot be used alone as protective devices.

The solution is to use both, a BMS and a fuse.

The exact sizing of the fuse depends on many parameters (environment temperature, type of cable isolation, PSSC, type of installation, etc.). If the fuse has been correctly sized for your existing battery, use the same for the new one.

Best Regards

Matthias