resistors as fuses

With carbon film, just before they open up, they catch fire. Put the little sucker inside of a glass tube and you will probably be OK.

Jim

Hmm I have no idea, but what's wrong with a fuse? It's made for the job.

Make sure you use a fuse rated for DC volts. (google fuses + DC volts) (What happens if the resistor develops a nice DC arc as it fails?)

George H.

Unless you specifically but a fuse type resistor (fusister?) you can't be assured that it will blow the same way every time. I had some resistors actually short to a lower value when overloaded heavily.

Vishay, RCD and others make a fusable resistors.

If you're concerned about the peek current causing the lithium batteries to explode, you want to put some custom designed circuitry to prevent that situation.

nice thing about fuses is the higher the peek current the faster they blow.

Google "fusible resistors", Vishay and others make resistors which are guaranteed to raise resistance and eventually fail open-circuit without flames when overloaded.

Usually Li-ion batteries have internal circuitry to limit the current. Check that first before jumping through all kinds of hoops.

So a resistor is a bad choice. Just use a fuse.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

I slapped an assortment of 0805 resistors across the AC line. Some went *pip!* and opened up, some actually caught fire.

I'd go for two or three surface-mount resistors in series, each capable of handling the full voltage. 20K or so each. It shouldn't be hard to measure the battery voltage behind 60K or so of resistance.

John

Noise??? Where from?? In the "good old daze" some circuits were protected with resistors like you suggest; they were (naturally) carbon composition. At some point, instead of getting slowly brown and then black, more power dissipated caused them to burn, smoke and finally break open, At a higher power,they would crack showing less blackening than a lower power would cause.. It is that cracking open that is ideal condition to engineer into the circuit, in that one gets a very visible indication. But..these daze, a fuse might be better especially if a resistance in series is not needed for some other purpose.

Size. Need holders. Cost Unless designed for such, they fail from vibration. [BTDTGTTS] Most auto fuses are rated 32V. Don't offer any current limiting.

Goal is to protect the wiring/car from fire, not the batteries. (They furnish many amps in ordinary service; ISTM the controller will limit to 150A.)

I'd think it hard to get self-sustaining arc with 100V.

Would use "flameproof" resistors.

--
A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

The large DC motor driven by those batteries. The PWM controller is hardly Part 15 conforming...

We'd know about any open; the sensed voltage would go to zero.

Be nice to have that R to prevent [not just terminate] excess current.

--
A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

If the fuse catches fire, doesn't that defeat the purpose of the fuse?

I think I've made a mistake. I thought you were fusing the whole 100 amps of current. You are 'only' worried about shorts between the monitor leads? Then why not put the resistors right at the battery? With R choosen to limit the power to 1/4 W.. ~100k ala JL.

George H.

PTC fuses would be more appropriate, they are spot welded right on the cell.

(mind the wrap)

Cheers

Hmmm, let me think that over.

John

Firstly, a 50-cell Li-Ion battery string is not 100-odd volts, try

200V in your calculations. Makes a difference to arcing.

Have you ascertained from the manufacturer whether these cells have internal fusing? As an aside-type question, have the cells passed any puncture tests?

BTW, fuses ARE resistors. They are just ones designed to fail open safely. Seems to me that is what you are trying to do ....

The risk of cell-to-cell shorting can be dramatically reduced by consideration of battery layout, inter-cell connection arrangement etc.

I noted that you intend monitoring individual cell voltages, presumably as under/over-voltage protection. Most of the manufacturers of pack protection modules that I am aware of don't go anywhere near 50-cell configurations in their off-the shelf range, but you *may* be able to use several of their modules with each spanning a subset of your string. (This is obviously dependent on locating modules designed for your maximum working current.)

I've seen it done with independent modules bolted on each cell, modules communicating via some isolated serial bus, optical maybe?

-Lasse

I know someone who has developed some battery monitor/charger/controller =

devices that may be suitable for such an application. Have a look at:

Paul=20

• Incorrect on many counts: 1) there are picofuses (tradename i think), there are numerous sizes that have leads, there are SMT fuses,and auto fuses are rated BY CURRENT and thus _do_ offer current limiting. Fuses do not normally fail due to vibration unless they nominally handle a current "near" the rating.

• A "flameproof" resistor may be a very poor choice if you want fuse type action.

..so put the fuses under the designer's backside so he automatically gets.. .. .. .. .. .. fired.

"Robert Baer" " David Lesher"

** Flameproof resistors are also know as " fusible " resistors - typically metal film types with a non flammable coating.

When overloaded by about 10 times power, the coating smokes and the surface glows bright red for a few seconds - then it goes open. When overloaded by larger factors, opening is much quicker and may leave no visible trace.

Readily available in values from 0.1 ohms to 10 kohms and 0.5W and 1W ratings.

Here you will see a pic of one being tested on my bench.

... Phil