Begginers Guide to Processing 18650 Cells | Cell Database


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Are Tesla Style Fuses Enough?
#1
I question this because I have heard of 4P 18650 charging at 1 amp (divide by 4 per  cell) 3.7v get so hot you couldn't touch it. The fuse didn't blow because it didn't break the amp threshold but still got super hot. What about using Thermal fuses? They break at a certain amp AND temperature. What's your thoughts? Link below:
Thermal fuse 72C Degrees 250V 10A Thermal Cutoff
 http://s.aliexpress.com/6fEJjuAZ

This is a quote by Steve McMechan on YT:
I personally tested using the fuses this style (resistor) and I don't believe it offers enough protection personally. I setup a simple 4P 18650 battery holder that I'd soldered cell fuses onto (using the same ones you used that Jehu recommended), and there were 2 bad cells out of 4 in the 4P set, and I was only charging at 4v 1amp current, and within 5 minutes the 2 bad cells got so hot I could hardly touch them without burning my fingers, yet the cell fuses DID NOT BLOW (as the cells were not sucking more than 5amps to blow them). So I replaced the cell fuses with mini 10a 70C thermal fuses and that did the job perfect, protecting the pack from the bad cells when they over-heated above 70C they blew the fuses nicely (even though they were only sucking around 1amp of current). I strongly recommend needing to use thermal fuses not just the resister legs as you've used, as that's a fire hazzard as I found from my own personal testing sorry. The Thermal fuses are super cheap anyhow, and just as easy to solder in place but offer much better protection for each cell.

What are your thought everyone?
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#2
I agree that the fuses probably won't do much in a 4p pack. I think most of us are building much larger packs though. I'm using 120p packs, many here are using 80p packs. If you charge the pack at 20A (which is common in my setup), that is 0.16A per cell. The fuses shouldn't get warm at all. Now let's say each cell can supply 2A of current, admitting that this is probably the extreme low end since most can do 5A+ very easily. If you assume 2A times 119 cells, that fuse on the bad cell is now going to carry 238A and will surely blow. The idea of fuses in my circuit is supposed to prevent a short cell, not a hot cell.

The more strings you add, the less the load per cell is. The goal in my setup is to get this as low as possible. If I have 4 strings of 120p packs charging at 20A, now that's 0.04A per cell. There is certainly no problem with fuses there Smile

A good test would be to have some bad cells that get hot and put in a large pack to see what happens. Maybe I'll try to get some test together considering I have a huge case of bad cells.
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#3
Fuses will do their job whatever size pack you have - big or small.

But if folks are just cutting legs off resistors and expecting them to "work" they might be in for a unpleasant supprise - any wire that is intended to be used as a fusable link must be tested - properly - including work to determine what length is to be used, and when installed that length must be kept consistent with your test results.

Thermal fusable links are a good idea, alongside traditional individual cell protection fusing.
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#4
This is why i dont use fuses at all, after testing over 3000 second hand cells from over 500 laptop batteries, none of the bad ones were short circuit. Less than 50 cells got hot when charging (even cells that tested 0v)  The only cells that got super hot (70c+) were about 8 of the red sanyo cells, and these got really hot @ 1000mAh charge, and barely warm at less than 500mAh, at the 100-200odd mAh in the power wall, they didnt do anything abnormal.  BTW, the evil red sanyos were not <3v when initially recycled, but they did consume abnormal current without going much higher than nominal.

(01-02-2017, 04:06 PM)mike Wrote: A good test would be to have some bad cells that get hot and put in a large pack to see what happens. Maybe I'll try to get some test together considering I have a huge case of bad cells.

Having done this, I can confirm that your initial hypothesis is confirmed, when charged at <500mAh cells didnt get hot, but some never got to 4.20v, they hovered at 4-4.1v and consumed 50-100mAh, warm but not hot.
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#5
(01-02-2017, 05:15 PM)3nergE Wrote:
(01-02-2017, 04:06 PM)mike Wrote: A good test would be to have some bad cells that get hot and put in a large pack to see what happens. Maybe I'll try to get some test together considering I have a huge case of bad cells.

Having done this, I can confirm that your initial hypothesis is confirmed, when charged at <500mAh cells didnt get hot, but some never got to 4.20v, they hovered at 4-4.1v and consumed 50-100mAh, warm but not hot.

So they were just burning off the energy as heat, thus reducing the pack's capacity, not actually making it catch fire. Interesting. I guess it depends on the charge per cell and if it can dissipate the amount of heat it's generating.
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#6
(01-02-2017, 04:06 PM)mike Wrote: So they were just burning off the energy as heat, thus reducing the pack's capacity, not actually making it catch fire. Interesting. I guess it depends on the charge per cell and if it can dissipate the amount of heat it's generating.

Yes indeed, and at 4v ive yet to see a cell discharge at more than 50odd mA, that equates to less than a quarter watt, not enough to get warm.  The sanyo red devils were the exception, they were dissipating around 3 watts each being held at 4v, thats enough to get too warm.
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#7
(01-02-2017, 05:15 PM)3nergE Wrote: This is why i dont use fuses at all, after testing over 3000 second hand cells from over 500 laptop batteries, none of the bad ones were short circuit. Less than 50 cells got hot when charging (even cells that tested 0v)  The only cells that got super hot (70c+) were about 8 of the red sanyo cells, and these got really hot @ 1000mAh charge, and barely warm at less than 500mAh, at the 100-200odd mAh in the power wall, they didnt do anything abnormal.  BTW, the evil red sanyos were not <3v when initially recycled, but they did consume abnormal current without going much higher than nominal.
I had the same thoughts as you, INITIALLY ! but found that fuses clearly save me from memory I've had 5 Fuses blow on me with three cells going dead short. These cells WOULD have damaged my pack either by creating a huge amount of heat having 79 other cells dumping every available amp into it trying to balance the cell, or just vented.
So not only will I use fuses, personally from my experience & what happened to Rohan I'll be double fusing every cell from now on. If Rohan's had of been double fused it would have been a non-issue.
http://www.hbpowerwall.com for a full video diary of my project.... 
https://www.amazon.com/shop/hbpowerwall to buy stuff I use Smile 
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#8
With the thermal fuses is it visible that they have failed when they fail? My fuses even tho it takes a long time I can visually check every fuse in the pack with out taking it out. Does that little cylinder break showing you there is an issue?

I've also seen the resettable thermal fuses - same issue with them you don't know if there is an issue.
http://www.hbpowerwall.com for a full video diary of my project.... 
https://www.amazon.com/shop/hbpowerwall to buy stuff I use Smile 
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#9
(01-02-2017, 04:06 PM)mike Wrote: I agree that the fuses probably won't do much in a 4p pack. I think most of us are building much larger packs though. I'm using 120p packs, many here are using 80p packs. If you charge the pack at 20A (which is common in my setup), that is 0.16A per cell. The fuses shouldn't get warm at all. Now let's say each cell can supply 2A of current, admitting that this is probably the extreme low end since most can do 5A+ very easily. If you assume 2A times 119 cells, that fuse on the bad cell is now going to carry 238A and will surely blow. The idea of fuses in my circuit is supposed to prevent a short cell, not a hot cell.

The more strings you add, the less the load per cell is. The goal in my setup is to get this as low as possible. If I have 4 strings of 120p packs charging at 20A, now that's 0.04A per cell. There is certainly no problem with fuses there Smile

A good test would be to have some bad cells that get hot and put in a large pack to see what happens. Maybe I'll try to get some test together considering I have a huge case of bad cells.

While I agree with everyting you said, I still think it's possible for a cell to overheat (and potentially catch fire) even when charging at 4V 250mA (1A/4cells) - it has been tested by one person, I would like to see of we can reproduce this occurrence.

(01-02-2017, 09:03 PM)hbpowerwall Wrote: With the thermal fuses is it visible that they have failed when they fail? My fuses even tho it takes a long time I can visually check every fuse in the pack with out taking it out. Does that little cylinder break showing you there is an issue?

I've also seen the resettable thermal fuses - same issue with them you don't know if there is an issue.

This is a very valid question. I am not sure if there is visually. I will contact the manufacturer.
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#10
(01-02-2017, 11:54 PM)cstanley Wrote: While I agree with everyting you said, I still think it's possible for a cell to overheat (and potentially catch fire) even when charging at 4V 250mA (1A/4cells) - it has been tested by one person, I would like to see of we can reproduce this occurrence.

Its not about belief friend, its physics, originally you said 4p@1000mA, so the cell getting hot was likely 1 cell getting 1000mAh, thats about 3-4 watts, thats enough to get hot.  Divide this by 4 and you get less than a watt, which just cant really get hot, thats why I showed my math earlier, which was also reflected in my testing.

While I agree fuses are a great way to increase safety, its also a way to decrease reliability / safety if you cascade the failure and all of a sudden cells are getting overcharged at an over-current rate / situation.

Keep in mind that most people fuse for draw rate, which is double the rate that most cells can be charged at.

Horrible things can go wrong if you overcurrent cells, even good ones

This is why I dont fuse.

kind regards
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