are fuses a good idea?

daromer

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Yes that works out on matched packs :) but not not in same degree on the packs we build where the initial resistance easy can be 3* om One cell comparing to another.

The cofficient does not change that much last time i checked.
 

Korishan

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yes, a cell that gets warmer will have higher resistance. But a cell that is going dead short will not stop increasing in heat as long as current is being supplied. A dead short means that there are physical connections inside the cell shorting it out. Just because the temperature changes doesn't mean the cell will stop shorting. Ever put a wire across a charged 12V lead acid battery? Does it stop glowing because of increase in resistance? Nope! It'll continue to heat up till either the battery can't deliver any more amps, or the wire vaporizes.

In a pack where a cell goes dead short is just like that wire. The rest of the cells in the pack is like that lead acid battery giving all it's got to that short. Put a cell-level fuse between the cell and pack, and that short circuit condition can't heat up the cell.

This is a method that is highly recommended for "USED" cells because they are NOT balanced or matched to each other. If someone takes the time, effort, and money, to build a pack of used cells that are "only" matched with capacity and resistance, then they probably won't need cell-level fuses. But how is one going to accomplish that? Better chances of winning the lottery.

All cells connected in parallel will have the exact same "voltage". If current load is high enough where internal resistance starts to become a factor, then each cell in the parallel group could have different current dissipation. One cell could be delivering 20% more current than another, for example. The current that doesn't flow out of the cell down the wire is converted to heat. So from that perspective, the energy dissipation may be equal, just dissipated in different manners.
 

MattsAwesomeStuff

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Korisahn said:
But a cell that is going dead short will not stop increasing in heat as long as current is being supplied

Yes, that's what I said 2 posts back.

The fuse isn't there to protect the cell from over discharge, it's there to protect the pack from flooding through it if it shorts.

But my side note to that is, with any decently sized pack, *any* conductor you use through a short is going to melt.

In fact, in Dewalt tool packs, the fuse is the nickle strip itself, by design. The fuse is a specifically-thin amount (about 1/2 the normal conductor), and that's only for a 2p or 3p pack.


image_ivgxpq.jpg


In a 100p pack, a short is going to vaporize anything you'd use as a conductor. So, I kinda think, don't bother, just use whatever you'd use to connect them (nickle strips, copper wire, whatever), and that connection is surely plenty thin enough to fuse in the event of a cell shorting (which itself is an extraordinarily rare event, I've never seen one).
 
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