Points of clarification, Unprotected cell in series

Hi all,
Ive been reading around the forum about battery safety. However I am very confused as there are a lot of contradicting information. I just purchased my first in series unprotected 26650 flashlight with unprotected batteries (The Sofirn SP70). Without even knowing they were unprotected. I would like to have protected 26650s. However, The 20A is needed to push the light up to almost 7000lm. Since this is a "toy" light Id like to keep that capability. Digging deeper into the batteries i find out that they are made by a Chinese company called PLB. The battery model is either NiCoMn(NCM) chemistry lets call this battery A. And a battery that should not be used without "PCM/BMS" lets call this battery B. I have a couple of points I would like to clarify.
links below.

1. Is PLB a good brand?

2. Does anyone here have experience with the PLB or Sorifn 26650 5000mah 25A batteries?

3. Everyone says max discharge voltage is 2.8 volts. However, A thread here on CPF some mentioned 3.0, 3.2, 3.4v. While cellphone batteries shut off at 3.5V. Chinese specs say its 2.75v Im just trying to not get blown up. What is the actual max discharge voltage?

4. I've heard of the 2 main risk of running unprotected cells in series. This makes the flashlights internal voltage safety useless. Am I right? And one battery discharges faster which causes the other one to charge it. Is this inherently bad or only bad if the voltages get too low due to one battery draining the other really fast?

5. How do I safely run unprotected cells in series?

6. How do I test if the batteries are still healthy during its life span?Something about resistance and capacity?

7. Which battery do you think I have A or B? Im concerned because I don't have "PCM/BMS"

8. Is there a LiFePO4 with the same specs I can run on this light? I keep finding 3.2v 5C cells.

9.Should I be scared?

+

I run my non pcb cells in series without a thought.

There's lots of hysterical threads & posts from people who never tried it, or are too scared to!!!


The 70.2 emitter will likely be pulling less than 10A @ 8.4V (26650 in series) in that light, which would be over 80Watts to a 70.2.

, so it's likely far less than 10A done in series, probably closer to 5A in reality

4(ish) the only possible way they could have any form of LVP would be to Add the lowest discharge voltage of a cell to 4.2v and call that the cutoff point, so for instance if we say that 3v is the lowest we`ll allow a cell to discharge, and there`s 2 cells then that`s 7.2 volts, then set the cutoff point at 7.2v, Anything above that and you`re good. that way you ensure that even in the worst case scenario (one full and one flat) the light won`t fire up and cause damage, but you still have from 7.2v to 8.4v to work with, and since these cells spend most of their dischage cycle at 3.7v you`re still good to go at 7.4v

that`s the only way they could do it really and still be affordable and not have to use separate monitored compartments for them.

for point 3. you really need to look at the manufactures data sheet to get an accurate answer, and it`s a different answer for each brand too!

First, personally I would not use a multi cell unprotected lithium rechargeable light.

But to comment on your overall questions...

2.7 is more or less considered the lowest safe cutoff voltage. Remember that at high loads voltage depression happens. Once the light is OFF, your cell voltages will rebound.

From my understanding, discharging too low is not the dangerous action... the recharge following it is.

So maybe a) don't press your luck on runtime and b) check cell voltages before recharging.

Given the spectacular way in which lithium can spontaneously decompose with thermal release (catch fire and blow up) I ONLY run major name brands, usually panasonic.

advcat said:

...I just purchased my first in series unprotected 26650 flashlight with unprotected batteries (The Sofirn SP70)...

Click to expand...

+

Your new light has Low Voltage Protection built in (just like the 99.99% of all lights made)

So,, go enjoy your new light :thumbsup:

orbital said:

+

Your new light has Low Voltage Protection built in (just like the 99.99% of all lights made)

So,, go enjoy your new light :thumbsup:

Click to expand...

But if a person runs mismatched cells (or cells at different charge levels) one (or more) can discharge below safe levels before the protection circuit catches it.

Would recommend cells from same batch and don't run down till protection kicks in.

This is not theoretical stuff... CPF has various threads with multi-cell lights that failed. The r/c hobby world is full of evidence as well.

turbodog said:

But if a person runs mismatched cells (or cells at different charge levels) one (or more) can discharge below safe levels before the protection circuit catches it.

Click to expand...

True if the LVP didn`t kick in at 7.2v (or less), as in my first post, it`s not in the least bit dangerous above this voltage.

Don't know about that. In an extreme case... putting a nearly dead cell along with a 100% charged one. Setup such as that would allow too low discharge masked by the high voltage of the fresh cell.

Lithium is like a gun, be careful, even with primaries.

+

How much of an airhead would you have to be to put in one full & one discharged cell??


____________________________
LiPo units have a balance board, so that takes out the airhead factor.

turbodog said:

Don't know about that. In an extreme case... putting a nearly dead cell along with a 100% charged one. Setup such as that would allow too low discharge masked by the high voltage of the fresh cell.

Lithium is like a gun, be careful, even with primaries.

Click to expand...

at 7.2v cutoff there are Zero possible scenarios of that happening. worst case of mismatch 3v and 4.2v, best lowest case 3.6v and 3.6v for perfectly balanced. there`s no mathematical possiblity of any danger with 7.2v as cutoff.

+

A prerequisite to becoming a CPF member is having a multi-meter & knowing how to use it.

I have no issues using unprotected cells in series. I just make sure to be careful. Shut the light off when you get a low-voltage warning, to prevent damaging cells. Check each cell's voltage before recharging, and you'll be perfectly safe.

7.2v would be an insane low-voltage cut-off to use. Something around 6v makes much more sense. That allows for some difference in cells, but still gives you most of the usable capacity. The user has to take some responsibility for safety; otherwise he should just buy a AA alkaline light from Walmart.

WalkIntoTheLight said:

7.2v would be an insane low-voltage cut-off to use. Something around 6v makes much more sense.

Click to expand...

7.2 has Zero danger, your 6v worst case is 4.2v and 1.8v, 1.8v isn`t a good voltage to take Li-Ions down to.

Katherine Alicia said:

at 7.2v cutoff there are Zero possible scenarios of that happening. worst case of mismatch 3v and 4.2v, best lowest case 3.6v and 3.6v for perfectly balanced. there`s no mathematical possiblity of any danger with 7.2v as cutoff.

Click to expand...

Yup. I must have carried a zero in my head when I did quick math on it. Somewhere in the range of 7.0 +/- .1

orbital said:

+

I run my non pcb cells in series without a thought.

There's lots of hysterical threads & posts from people who never tried it, or are too scared to!!!


The 70.2 emitter will likely be pulling less than 10A @ 8.4V (26650 in series) in that light, which would be over 80Watts to a 70.2.

, so it's likely far less than 10A done in series, probably closer to 5A in reality

Click to expand...

So I can run basically run any protected cell without sacrificing lumens since the amp rating is so low. Great to hear

Katherine Alicia said:

7.2 has Zero danger, your 6v worst case is 4.2v and 1.8v, 1.8v isn`t a good voltage to take Li-Ions down to.

Click to expand...

That assumes the user is so dumb that he's stuck a fully charged cell and an empty cell together. That's why I said you have to assume the user is somewhat responsible. They shouldn't be using lithium-ion cells, if they're not responsible. In any case, it's not discharging that is unsafe, it's charging. As long as you check cell voltages before charging, you'll be safe. The worst case is you ruin a cell.

If one cell is so weak that it depletes way faster than the other cell, it should be discarded anyway. So, no loss if you discharge it too far.

Katherine Alicia said:

4(ish) the only possible way they could have any form of LVP would be to Add the lowest discharge voltage of a cell to 4.2v and call that the cutoff point, so for instance if we say that 3v is the lowest we`ll allow a cell to discharge, and there`s 2 cells then that`s 7.2 volts, then set the cutoff point at 7.2v, Anything above that and you`re good. that way you ensure that even in the worst case scenario (one full and one flat) the light won`t fire up and cause damage, but you still have from 7.2v to 8.4v to work with, and since these cells spend most of their dischage cycle at 3.7v you`re still good to go at 7.4v

that`s the only way they could do it really and still be affordable and not have to use separate monitored compartments for them.

for point 3. you really need to look at the manufactures data sheet to get an accurate answer, and it`s a different answer for each brand too!

Click to expand...

Looking at the flashlight specifications it says 5.6v-8.4v. If I deduct 4.2v from 5.6v it can theoretically let a battery down to 1.4v am I correct?. So I guess I shouldn't be pushing the run times. Charge at "half power" when the battery indicator is on orange or red.

If you don't mind me asking where did you get the figure of 4.2v? Im trying to learn.


for point 3. specs say its 2.75v. Ill stick to 3v

WalkIntoTheLight said:

That assumes the user is so dumb that he's stuck a fully charged cell and an empty cell together. That's why I said you have to assume the user is somewhat responsible. They shouldn't be using lithium-ion cells, if they're not responsible.

Click to expand...

Therein lies the problem, you Can`t assume this, and you can assume that some are as dumb as a sack of hammers! moreover it will always be "some one elses fault" when they do this dumb assery and get hurt. sadly those are the Facts
This is ignoring the chance of genuine accidents happening and mistakes made (and they do).

Beyond that I`m in total agreement with you, and I doubt there`s many (if any) here that would be that stupid, but they are out there and there`s lots of them!

orbital said:

+

A prerequisite to becoming a CPF member is having a multi-meter & knowing how to use it.

Click to expand...

I should get on this right away, thanks

advcat said:

Looking at the flashlight specifications it says 5.6v-8.4v......

Click to expand...

+

Your cutoff voltage is 5.6V,, divide this by your 2 cells & it's 2.8V per cell.
In a real world test/review, it was reported at 2.85V per cell.
Just have your cells fully charged when you put them in.

When depleted; recharge them right away
and take them off the charger once fully charged.... that's it.


--------

for the volt meter/multi meter, just figure out to read DC voltage first, the other stuff later on = cheers