Finishing DIY Packs - Dead Shorts and Fuses

[undercrust]

Hello,

About to begin assembling a 12V pack right now with 4x EVE LF280K battery cells. This weekend's rabbit hole topic was fuses and master disconnects. After distilling a few dozen (hundred?) of previous comments, noting the excellent input of @John Frum in particular, I just wanted to check the following is true.

1a) For this 12V battery pack, an adequate and suitable main battery fuse would be a Blue Sea (or similarly high-quality) MRBF terminal block fuse, rated at 125% of the expected amp draw of the system. For example, on my system I expect to have a 120A JBD BMS, and the highest expected draw is a 1200W Giandel inverter. Therefore, after building in inverter inefficiency, a 150A MRBF terminal block fuse would be sufficient?

1b) Continuing on this thought, a similarly sized Class T fuse would be considered the best-in-class fuse option, but is not necessarily required for this battery bank at only 12V? Conversely, using a similarly rated ANL fuse is insufficient as it does not have a high enough AIC rating for even a 12V LiFePO4 battery dead short?

1c) A proper fuse on the main positive terminal of the 12V battery will not save my bacon if something causes a dead short between the positive and negative terminals of one of the individual component cells and therefore I should wrap the individual cell terminals / exposed metal on terminal bus bars in electrical tape or kapton tape? [Try as I might I couldn't find a clear answer to this one, it seems I'm seeing pictures of a lot of people's finished systems where they leave the individual cell terminals exposed so I'm wondering what I'm missing?]

2a) In wiring the master battery disconnect, I should place the master disconnect on the positive wire before the positive bus bar? I will also need to be aware of the solar charge controller or inverter capacitors maintaining system power even after I turn the master disconnect to the off position. This is a battery master disconnect only.

2b) It seems some people opt to skip the master disconnect and just disconnect the battery cables directly? I have seen this particularly on portable DIY battery builds, where the battery is being brought to and from an RV or teardrop trailer or fishing boat and hooked in as a 12V DC system power source, using either SAE or Anderson quick-connect plugs. This will also be my use case, so I'm wondering if I need a master disconnect at all?

Thanks as always in advance to the pros that haunt this forum to keep us newbies out of trouble.

 

[John Frum]

Hello,

About to begin assembling a 12V pack right now with 4x EVE LF280K battery cells. This weekend's rabbit hole topic was fuses and master disconnects. After distilling a few dozen (hundred?) of previous comments, noting the excellent input of @John Frum in particular, I just wanted to check the following is true.

1a) For this 12V battery pack, an adequate and suitable main battery fuse would be a Blue Sea (or similarly high-quality) MRBF terminal block fuse, rated at 125% of the expected amp draw of the system. For example, on my system I expect to have a 120A JBD BMS, and the highest expected draw is a 1200W Giandel inverter. Therefore, after building in inverter inefficiency, a 150A MRBF terminal block fuse would be sufficient?

1b) Continuing on this thought, a similarly sized Class T fuse would be considered the best-in-class fuse option, but is not necessarily required for this battery bank at only 12V? Conversely, using a similarly rated ANL fuse is insufficient as it does not have a high enough AIC rating for even a 12V LiFePO4 battery dead short?

1c) A proper fuse on the main positive terminal of the 12V battery will not save my bacon if something causes a dead short between the positive and negative terminals of one of the individual component cells and therefore I should wrap the individual cell terminals / exposed metal on terminal bus bars in electrical tape or kapton tape? [Try as I might I couldn't find a clear answer to this one, it seems I'm seeing pictures of a lot of people's finished systems where they leave the individual cell terminals exposed so I'm wondering what I'm missing?]

2a) In wiring the master battery disconnect, I should place the master disconnect on the positive wire before the positive bus bar? I will also need to be aware of the solar charge controller or inverter capacitors maintaining system power even after I turn the master disconnect to the off position. This is a battery master disconnect only.

2b) It seems some people opt to skip the master disconnect and just disconnect the battery cables directly? I have seen this particularly on portable DIY battery builds, where the battery is being brought to and from an RV or teardrop trailer or fishing boat and hooked in as a 12V DC system power source, using either SAE or Anderson quick-connect plugs.

All correct so far.

This will also be my use case, so I'm wondering if I need a master disconnect at all?

Thanks as always in advance to the pros that haunt this forum to keep us newbies out of trouble.

My opinion is to keep the number of disconnects to a minimum.
If an inverter has an on/off switch that generally counts as a disconnect.
The exception is for inverters that prime their capacitors regardless of the position of the on/off switch.
Even then its probably sufficient to use a suitable resistor to prime the resistors when you first connect the inverter.

Solar charge controllers don't generally require a disconnect on the battery side because they only draw milli-amps when the panels are disconnected. Its become very common practice to use a double pole single throw switch to isolate he pv array from the solar charge controller.

The secondary dc distribution(fuse_block) can benefit from having a disconnect switch but its not absolutely required because the current on to the fuse_block is usually relatively low.
Another alternative for this circuit is a victron battery protect which provides configurable low voltage disconnect and administrative disconnect.

A dc2dc charger generally doesn't require an disconnect switch on the house battery side or the starter battery side because turning off the engine usually serves to disconnect/deactivate the charger.

If a battery has a smart BMS that can be used to disconnect the charge/discharge paths through the battery.

 

[John Frum]

TL;DR
My preference is to have all but one endpoint administratively dis-connectable.
In many systems that can be done with 0 disconnect switches.

 

[John Frum]

In the cases where an MRBF has suffficient breaking capacity I prefer it because....
1. Its compact
2. Its inexpensive
3. It is fitted almost directly on the battery positive terminal which minimizes the chance of an upstream short.
4. they can also be used on the positive busbar which can consolidate the bill of materials and required spares.

 

[undercrust]

All correct so far.

Well holy cow, that's a first.

Just to triple-check one point, after I've assembled my 12V cell, I should tape off the exposed metal on the bus bars and terminals for each cell?

If a battery has a smart BMS that can be used to disconnect the charge/discharge paths through the battery.

So, because I can turn off the charge/discharge paths via Bluetooth on my BMS, I could actually just use this feature as a master disconnect switch when I want to ensure a safe way to remove or connect my battery and the rest of my system? Well crap, if so that's easier for sure...

 

[John Frum]

Well holy cow, that's a first.

Just to triple-check one point, after I've assembled my 12V cell, I should tape off the exposed metal on the bus bars and terminals for each cell?

Putting Katon tape on the busbars impedes their ability to dissipate heat, If there is physical security like a box around the battery then its probably not worth it.
It there is any chance that a bit of metal could fall across the terminals then Kapton or heat shrink or a terminal boot is probably a good idea.

So, because I can turn off the charge/discharge paths via Bluetooth on my BMS, I could actually just use this feature as a master disconnect switch when I want to ensure a safe way to remove or connect my battery and the rest of my system?

Yes but remember about the inverter caps.

 

[John Frum]

I would be glad to have a look at your full system topology if you want.

 

[rhino]

Yes but remember about the inverter caps.

Yes, remember this. I blew an undersized T-fuse when the inverter was "off" and connected the battery since it still charged the caps even when on the "off" position.

 

[rhino]

In the cases where an MRBF has suffficient breaking capacity I prefer it because....

I think that should be clarified since there is always a lot of confusion and/or lack of knowledge about a fuse's AIC and impacts related to your nominal battery voltage. I know even Victron uses those in 48V systems but I thought the interrupt current rating is relatively low 1000's whereas a T-fuse is in the 10k or more even on a 48V system. For others reading, if the short allows more current to pass through than the fuses AIC rating then the fuse would not be be able to break the current flow.

 

[John Frum]

I think that should be clarified since there is always a lot of confusion and/or lack of knowledge about a fuse's AIC and impacts related to your nominal battery voltage. I know even Victron uses those in 48V systems but I thought the interrupt current rating is relatively low 1000's whereas a T-fuse is in the 10k or more even on a 48V system. For others reading, if the short allows more current to pass through than the fuses AIC rating then the fuse would not be be able to break the current flow.

If you have specific questions I will try to answer them.

 

[Powderday]

So, because I can turn off the charge/discharge paths via Bluetooth on my BMS, I could actually just use this feature as a master disconnect switch when I want to ensure a safe way to remove or connect my battery and the rest of my system? Well crap, if so that's easier for sure...

It depends what you really want to use the "master disconnect" for. Are you using it to just prevent the batteries from discharging when you are not using the system or are you relying on it to remove power from the system so you can work on it? I like to utilize a physical switch to isolate the battery (or other sources) if I'm going to work on the system.