DIY Battery Safety Questions

Hi there!

After seeing the recent posts in the up-in-smoke section, I realised that I've got a few holes in my own knowledge so I'm hoping that some of you smarter people can help teach me. This is specifically about the safety features when designing a DIY battery.

Firstly, I'm curious about the best position for a T-Class Fuse, when it comes to safety. Is it "better" to put it between the cells and the BMS, or, is it better to put it after the BMS? (In this case, let's say that I've got 2 batteries in parallel connected via the Victron Lynx Distributor).

I've tried to search around these forums however I could not find a definitive answer, as people have argued for both giving the reasons: Fuse after the BMS to protect the wires, and fuse before the BMS to protect the BMS.

However, what I cannot find (or perhaps I cannot understand) is a clear-cut answer on which position offers the best safety. Is it really just down to personal choice, or is there something that I've missed entirely?

Secondly, the use of breakers instead of a fuse, specifically between the cells and the BMS.
I've been taught that breakers are not the best substitute for a fuse, especially when compared to a T-Class fuse. However, looking at some of the better batteries, for example, the SOK 48V/100A server rack battery (using this as an example as I have one and it works brilliantly) it seems to use a DC breaker instead of a fuse.

For context, I've not opened mine up yet as I've not had a reason to, but looking at this video seems to show that it's just a breaker, no fuse.

If this were a random battery off Aliexpress, I'd not even think about it twice and just ignore the breaker, but, because this is from SOK and Current Connected (who have a really good reputation), I'm wondering if perhaps my way of thinking is either outdated or perhaps even wrong?

Finally, I'm curious if people who make their own batteries also design any form of fire suppression systems for "just in case".
I do know that LiFePO4 does not burn as easily as other battery chemistries, however, nightmare situations where things go up in smoke do happen (whether the cells catastrophically fail in a short, or if they do something to set fire to the surrounding materials). So, do you/anyone here design any systems to kick in if that happens? If so, any advice on what to do?

tl;dr, my questions are:

1. When putting a T-Class fuse in a DIY 48V (16S) battery, do you:
   1. Put the fuse before the BMS
   2. Put the fuse after the BMS
   3. Both
      
2. Breakers instead of a fuse between the BMS and the cells (like some server rack batteries do)
   1. Is this safe/ok to do?
      1. If so, in these situations, would you still recommend an MCCB, or would a DC-rated MCB be enough?
   2. Or, regardless of whether it's safe, is a fuse still the best?
      
3. Fire suppression systems for DIY packs, do you design your own, or are there off-the-shelf solutions, or...do you do nothing?

*Oh, I should add that when it comes to fuses/protection, I say fudge the costs, I only care about being safe.

Class T fuse after the BMS.
Class T fuse is last line of defense. It had a higher interrupt current rating than the breaker in a worst case scenario.

The most common DIY setup is Cells with Smart-BMS in a "box" which then goes to Fuse to busbar to switch to AIO./Inverter/Charger.
Most use Class-T fuses but some of us also use MRBF (12/24V) or MegaFuse (Victron uses these too).

Those who use Steel Cases most often get those with DC Breakers, some have an option for a Fuse internally as well, some of our members add their own Class T or Mega between.

Rack-mounted battery packs usually come with a DC Breaker, but not all. In such cases many will add a fuse and that can be tricky so different solutions are applied.

I've DIY'ed packs with my own DIY Boxes, no fuses or breakers internally but each get's a fuse @ the busbar. I am in process of switching all my packs over to ew Steel Cases and adding new packs as well. Each has a Breaker internally and will also be fused via an MRBF Fuse (I am 24V). NOTE the Victron Lynx Distributor uses Mega's and this is a Top Notch way of paralleling your battery packs.

As for BMS: I was using the Standard JKBMS with Active Balancing but I am Upgrading all to the new JKBMS Inverter Edition which has a great deal more features/functions.

Internal Fusing of cells with LFP is RARE, most especially for ESS. That is not something you will see in Server/Telecom rack battery packs nor other LFP packs. You will see this with different chemistries which have several cells in parallel internally. Think EV Batteries (NMC/NCA). I should also note that a few have made such attempts only to get overly complicated, frustrated at the quirks & issues that resulted and I'd say, most ended up ripping it all out and reverting to the 1st & Most important rule of K.I.S.S. Hint: BMS' are Millivolt & Milliohm sensitive.

Fire Suppression systems internal for ESS: No, no one offers that and never seen anyone DIY that. Grid ESS systems have that but those are another beast altogether.

My own system is a fairly common setup (I do not use an AIO (All in One)).
Battery Packs (with breakers) to Fuse @ Common DC Bus, to Midnite E-Panel with Master DC Breaker which then handles SCC input & DC out to Inverter.

The MOST IMPORTANT THING !
Never EVER Cheap out on Fuses or Breakers !!! This is one thing to NOT to screw around with !!! Use only High Quality Known Name Brand Fuses/Breakers. Eaton Corp has bought up several and kept them "Good". Some folks even question the Breakers supplied from China and just replace them, fortunately, they are a standard size (dimensionally)

Hope it helps, Good Luck

To tag along. I have 4 12 volt batteries in a mobile application. These are manufactured batteries sealed with internal BMSs.
They will each be wired to common bus bars with 5 to 8 foot cables total length + & - . I was thinking it would be safest to install MRBF fuses/holders at the positive battery terminal at each battery therefore protecting the cables at the source of the battery.
Then I realized that once I added the solar charge controller they would be at the end of the cables.
Which is better, safer or standard?

@Nightguest I am with you on being safe. Money is not the concern!

Partimewages said:

Then I realized that once I added the solar charge controller they would be at the end of the cables.
Which is better, safer or standard?

Click to expand...

The biggest risk is unlimited power OUT of the Battery. So fuse closest to the battery.

For added safety, put a fuse at the SCC incase it decides to output 600 amps. Probably can be a cheaper fuse because it is incapable for putting out 20,000 amps.

Steve_S said:

Fire Suppression systems internal for ESS: No, no one offers that and never seen anyone DIY that. Grid ESS systems have that but those are another beast altogether.

Click to expand...

Actually there are several server rack vendors including fire supression inside the packs.

I forget which I read about the other day, it was either power queen or AOlithim I think. From the teardown it looked like a thin pack attached to the lid that burst from abnormal heat and smothered the fire with some chemical.

When installing a fuse for a DIY battery, it is common to place it as close as possible to the positive terminal of the battery cell.

While installing a fuse on both the positive and negative sides can provide redundancy, typically, a single high-quality, reliable fuse installed on the positive side is sufficient.

T-class fuses, which blow at an extremely fast rate, are used for short-circuit protection, while circuit breakers are used for overcurrent protection. Therefore, if safety is a top priority, it is recommended to install both.

Regarding fire suppression systems for battery packs, some high-end home energy storage systems incorporate Aerosol Fire Extinguishing devices. These are fire suppression systems that release aerosolized particles containing potassium ions and other radicals to inhibit the combustion chain reaction.

I have never seen these devices sold to general consumers in Japan where I live. However, I found similar fire extinguishing devices available on Aliexpress, and I am considering purchasing one.

The manufacturer is likely JiAnDun (及安盾).

According to the product page, the main ingredients appear to be potassium nitrate and strontium nitrate. When exposed to the heat of a fire, these substances burn rapidly inside the device, releasing potassium ions and other radicals in the form of aerosolized particles. It appears that the released particles work to inhibit the combustion chain reaction.

The product is claimed to comply with the Chinese national standard GB/T GA499.1-2010 and is widely sold on Chinese shopping sites like Taobao.

Based on my search, the company seems quite reputable; however, due to limited information, the reliability of these fire suppression devices is unknown.