PPTC self-resetting fuses for the balance wires

i believe em3ev uses them. we should too. any thoughts on component selection? how fast do they work?

I've used PTC fuses in radio battery packs that I've rebuilt in the past, because that is what they used in the OEM pack. I've tested them with a bench power supply and electronic load.

How fast do they work really depends on the current. They have a rated range where they are guaranteed not to trip. Then exceed it by say 20-50% it may still never trip. Exceed it by 100% it might take a minute or two. Exceed the current trip rating be several hundred percent and it will trip in seconds. The type I used I can spot weld directly between cells. But its only good for lower current applications (acts as a protection device). They can have pretty high resistance.

thanks redilast. the highlighted seems like it would do for balance wire applications where balance currents don't exceed 500mA. has a 1A trip current and trip time of 0.8.

I think its good idea. I use them. Tested @ 72v dead short = tiny like electrostatic spark.
Prevents fire if balance harness gets shorted.

I've used those before too, as well as fuses. One possible issue is when the BMS is balancing, there will be a little voltage drop across the PTC which might make the balancing shunt oscillate. Not a huge deal in most cases, but the effective balance voltage will be off a little.

They get hot when shorted. So if you dont notice, it drains you cell. Im thinking using standard fuse instead.
Pico fuses are nice and small. BMS wire is silicone 22AWG so 1-3A should be perfect.

How could it drain the cell? It is simply in series with the balance wire.

Unless (like anything else could, including fuses or even the balance wires themselves) it mechanically shorts to something else, because of inadequate insulation, there's nothing to short to drain anything thru.

amberwolf said:

How could it drain the cell? It is simply in series with the balance wire.

Unless (like anything else could, including fuses or even the balance wires themselves) it mechanically shorts to something else, because of inadequate insulation, there's nothing to short to drain anything thru.

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what he meant was that in case of unattended short (usually against an adjacent balance wire/pin) the PPTC will gradually reset then heat up again, eventually draining the cells

i guess i'll just have to play w/ them to determine their suitability

those "pico" fuses sure are viable, especially since balance wire shorts are pretty rare so being non-resettable shouldn't be a problem

Ah, that makes sense.

Im sorry to dredge up an old thread but this seems like exactly the information I need

Amberwolf I did the unthinkable and let two wires of different voltages resolve their differences I apparently missed reinforcing a spot on one of my balance wires and it shorted on the main positive lead of the 7th battery in my 14s string lots of heat and smoke, vaporized wire but no fire and the batteries are all testing at the proper voltage. BMS appears to be functioning just fine...

This here fuse looks like it would cut things off prior to the wire vaporization though?

How would one go about wiring this into series with the balance wires? I can't quite wrap my head around it...

The way I've seen them used is there is one for each balance wire and it's attached to the spot on the pack where the balance wire goes.

Klauts said:

This here fuse looks like it would cut things off prior to the wire vaporization though?

Click to expand...

Probably, but it depends on the current created during such a short--if only two balance wires shorted on cell groups right next to each other, it might be low enough to not cut the current. You'd have to make sure, like the second post in the thread implies, to pick one that will trip on the mildest current that you want to protect against, but *not* on any actual desired current.

Balancing currents are typically in the few dozen mA range at most (some BMS are much less than this), so check what yours can do and pick a PTC that is rated for just that amount or a tiny bit more, but that according to it's spec sheet *will* trip on the lowest current you might see during a shorting event to the next cell group over.

The next cell group over will be at most 4.2v difference for the lowest-potential short, or 8.4v for the highest-potential short. The least difference would be whatever the LVC on your cells is in the BMS. To find out the least-case short current, you take that least difference (probably 2.8v?) and divide it by the resistance of one full parallel cell group, which would be the resistance of the balance wire plus that of the interconnects (both probably negligible), and that of the cells themselves at LVC (cell resistance divided by the number of cells in parallel--so if they are say, 100milliohm at that voltage, and there are four in parallel in the group, that's 100 / 4 = 25 milliohm cell resistance. Round it up to 30 to include the wires/etc if you like. (but you'll need to look at the cell spec sheet or testing data to find out it's resistance at LVC, which is different than the average resistance published for the cell in it's spec sheet (though you can use that if you have to).

So in that example, 2.8v / 30milliohm = 93.34amps. Meaning the PTC would be required to trip "instantly" below that current (preferably below the current at which the balance wires would begin to be damaged), but *not* trip at the balancing current itself (call it 50mA unless your BMS has a different spec).

Klauts said:

How would one go about wiring this into series with the balance wires? I can't quite wrap my head around it...

Click to expand...

That is the simplest part--you disconnect the balance wire from the cell, and connect it to one leg of the PTC. The ohter PTC leg connects directly to where you disconnected the balance wire from so the PTC is actually at the cell or cell interconnect, with no wire between it and the cell/interconnect, since it is there to protect the wire you don't want it to have any unprotected wire. .

Mounting the PTCs so they won't have legs break off from stress or vibration, or short to anything, is a good idea.