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2 sec short circuit - problem? NCR18650Bs

Vega

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Feb 14, 2020
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Hey guys!

I´m finally experimenting with real battery and electricity projects after a long while of learning about it from a theoretical point of view!
I´m aspiring to set up my own off grid power. But naturally I´m starting small, so I just recently got myself 3x NCR18650Bs to power first and foremost an LED strip. The LED strip is 12v.

I have 3 points that I am wondering about, where starting from 1 is my main inquiry!

1. I had a small accident while all the 3 batteries where connected and it short circuited for about 2 seconds. The only apparent damage was that the rubber of the wires partially melted. But I was wondering if there could be potentially any other damage done to the batteries, that I should take into account? What do you think?

2. And regarding the set up for the LED strip - I was wondering what would happen to the battery if I hooked it up to the LED strip alone. I mean not the 3 in series but just one of them. Obviously the voltage would be too low to run the LED, but would it harm the battery?

3. Last thing that puzzled me a little bit. After my experience with short circuiting, I was wondering why don´t the batteries short circuit when I measure it with the multimeter. I figured it must be because there is a resistor in the multimeter, or the multimeter functions as a resistor in itself (I don´t know what would be the right terminology here) - so it led me to the question: Could a battery potentially short circuit while being hooked up to any sort of appliance? Say if the power of the battery is too great for the appliance. Or maybe it would simply just blow like a fuse, either the appliance or the wire?

Any feedback would be greatly appreciated!
Cheers!!
 
1. Possibly. It would be appropriate to compare repeat your tests and compare to pre-short tests.

2. The harm would be leaving it connected and discharging the cell too low.

3. Multimeters have a very high resistance associated with the leads. It allows a very small current to flow, and the current flow determines the voltage, V = I * R. Some multimeters have a 10A ammeter setting that requires you to move the (+) lead to a different port. This is actually a very low resistance circuit and can result in a short if applied to both poles of a cells vs. in series with the load as intended.

Lithium NMC (3.6-3.7V nominal) is a terrible choice for 12V systems. 3S is too low and 4S is too high.

Discharge them too low, and they'll swell and ruin themselves. Overcharge them, and you may get a fire. Hopefully, you have selected an appropriate BMS.

Recommend you shift to LFP.
 
Hey man! Thanks a lot for your feedback!

I´m not sure it´s what you´d call a BMS - I have this thing for charging:
( lithium battery charger module 1a - tp4056 )

Lithium NMC (3.6-3.7V nominal) is a terrible choice for 12V systems. 3S is too low and 4S is too high.
The guy in the shop recommended me to choose 3 instead of 4, as being a little bit under for this particular LED strip (I believe it´s 12w, (I only know its voltage actually -12v)) was to be recommended.
- What does the "S" stand for by the way?

I just read that the nominal charge of LFP is 3.3. So how many batteries would you choose of LFP for this small set up?

Discharge them too low, and they'll swell and ruin themselves
So I guess this is a potential danger with my batteries connected to the LED. What could I do to prevent this? Apart from timing my usage time and checking with a multimeter every so often.

1. Possibly. It would be appropriate to compare repeat your tests and compare to pre-short tests.
Unfortunately I didn´t make any tests on beforehand: Actually what would such a test imply? Voltage check?
Are there any danger concern in continued use of these batteries without having determined any possible damage - or simply a potential performance drop?

Thanks a lot!!
 
No. that's not a BMS, but it will charge a single cell to 4.2V.

S indicates series. 3S means 3 cells in series.

For LFP, 4S gives a great 12V option - essentially a drop-in replacement for 12V lead-acid.

You must provide a means of keeping the cells in a 2.5-4.2V range. Exceed that range and damage occurs.
 
No. that's not a BMS, but it will charge a single cell to 4.2V.
Alright so for 1 cell at a time in charging mode, it kind of serves the same function, right? I mean after 4.2V is reached, it cuts the charge going into the cell - am I right?

You must provide a means of keeping the cells in a 2.5-4.2V range. Exceed that range and damage occurs.
In the case my conclusion above is correct; I suppose my only concern would be for it to deceed below 2.5V - In which case here is where the BMS comes in, am I right?

For LFP, 4S gives a great 12V option - essentially a drop-in replacement for 12V lead-acid.
That´s good to know man! Somehow in my research that skipped me. I didn´t know they existed in such small cells as well.

Thanks for all your answers man!
 
The charger will take care of the upper limit, and a 3S BMS will take care of the lower. Just make sure the BMS supports your chemistry. Some are LFP only.
 
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