What is the theory behind the distaste for a 3-4x18650 power source in series?
I think the main issue with multi cell setup is the danger that can come if the cells are not matched. matching as in individual cell voltage charge and cell discharge. the more cells the more the matching becomes important. I have the trustfire X8 which takes 1 or 2 18650's. i just make sure that the cells i use match in voltage. ive never really checked them for discharge rates. theres been a few posts of batteries exploding but more than likely it might have been user error.
It has to do with the 1st cell in a 3-4 cell setup, in which the through amperage for the 1st cell is boosted by any cell "behind" it in series. Higher the draw more likely that 1st cell will exceed 4.25V and burn/explode, and your last cell will draw to below 2.75V and die.
From Flashlight Wiki:
Lithium Ion (Li-ion) - 3.7V/cell
Important: ***These are not the same as disposable lithium batteries sold by Energizer (Energizer e2 or Ultimate Lithium), Saft (who make it more confusing by selling 3.6V lithium cells that are not rechargeable), and other companies since those can not be recharged and will explode if you try it (example).***
Though there are other lithium ion chemistries (listed below), but usually when people talk about lithium ion batteries they are talking about lithium cobalt, LiCoO2, after the chemistry of the battery anode. They are available in a wide variety of sizes including CR123A, AA, and AAA. However, because of the higher voltage, Li-ion cells can only be swapped for NiMH or alkalines if the device manufacturer recommends it. Be careful! A fully charged li-ion battery has 3 times the voltage of a NiMH cell. Some flashlights that use two CR123A batteries can take a single 18650 battery (though this battery is wider than CR123A cells) which offers substantially more capacity than the two smaller cells together.
Some brands of 10440, 14500, and 16340 cells are longer than their AAA, AA, and CR123 counterparts, so make sure you are getting cells that are not too long and that your light can deal with the size variance by reading the forums and reviews of the batteries and/or lights. Just because a cell is sold as an 18650 does not mean it is exactly 65 mm long: some are 68 mm long.
Li-ion cells must be monitored more closely than other chemistries. If they are charged too high (above 4.2 volts) or discharged too low, they can overheat or cause a fire. Some good advice is that if you don't have a volt meter and aren't willing to monitor the cells during use and won't be able to watch the cells the entire time they are in a charger, then you shouldn't be using them. See this thread for safe use of li-ion batteries.
Cells are categorized as being unprotected or protected. Protected cells include circuitry that will essentially turn the battery off if the voltage goes too high, too low, the drain is too high (the current drawn from a li-ion cell should not exceed twice the nominal milliamp-hour capacity in milliamps - so a 2000mAh cell should not be asked to produce more than 4000mA (4 amps)), or the polarity is reversed (not all protected cells protect for every one of those cases, and the voltage cutoffs vary as well). For this reason many people recommend protected cells. However, the low-voltage protection often kicks in only when the battery voltage goes below 3 volts, but 3.6 volts seems to be the point where most people recommend recharging the battery. The protection is there to prevent a fire, not to tell you when your battery should be recharged. See this CPF Thread for info about protection circuits.
Li-ion cells have fairly low self-discharge, but in order to prolong the life of the battery, should be stored partially discharged.
Outstanding explaination. I guess I've never sat and thought about it before.
Is this the correct logic? So let's say you have a 3x18650 source in series, producing 1.5A tailcap current.
The cell (#3) at the tail is giving .5A to the middle cell (#2).
The middle cell (#2) is receiving that .5A and is giving the front cell (#1) 1A
And the front cell (#1) is receiving that 1A and giving the driver 1.5A
If there were some issue with the #1 cell and it wasn't discharging properly (i.e. #2 and #3 were actually charging it) there could be a potential problem?
Here is an entire forum that deals with actual bad stuff that happened to flashaholics when their Li-on batteries went *poof*
I was speaking more along the lines of an SST-90 being driven at 9 Amps on a 4 cell series setup would be a pipe bomb waiting to happen. J)
/edit
AND somone being dumb and or ignorant enough not to use IMRs.
[quote=monanoke] It has to do with the 1st cell in a 3-4 cell setup, in which the through amperage for the 1st cell is boosted by any cell "behind" it in series. Higher the draw more likely that 1st cell will exceed 4.25V and burn/explode, and your last cell will draw to below 2.75V and die. [/quote]
[quote=ZW99GT] Is this the correct logic? So let's say you have a 3x18650 source in series, producing 1.5A tailcap current.
The cell (#3) at the tail is giving .5A to the middle cell (#2).
The middle cell (#2) is receiving that .5A and is giving the front cell (#1) 1A
And the front cell (#1) is receiving that 1A and giving the driver 1.5A
If there were some issue with the #1 cell and it wasn't discharging properly (i.e. #2 and #3 were actually charging it) there could be a potential problem?
[/quote]
Completely Disagree with both
To put it simply. In a Series circuit, Voltage is summative, Current is the same among all components in a series circuit.
(In a Parallel circuit, the voltage is the same but the current is summative)
The voltage is the summation of the individual voltages of the cells. If we take a two cell setup, and assume one is 4.2v, the other is 2.5v. The total voltage seen by the driver is 4.2 + 2.5 = 6.7v.
Each of the cells do not "see" or "feel" the voltage of any other cells. There is no such thing as charging or boosting the first cell. The only way you can charge/boost a cell is to put that cell in a series circuit backwards. (another story)
The current that flows through a series circuit is equal through ALL components that are connected in series in this circuit. The wires from the battery have (lets say 2A) flowing through it. The wire going back to the battery has 2A going through it. AND both batteries #1 and #2 will EACH have 2A flowing through each. You dont ADD current flow in a SERIES circuit.
How is this bad for unbalanced cells....?
Current flow on the other hand in the context of our flashlights is controlled by many factors (driver, circuit, batteries, led. etc....) with most of it determined by the driver. For the sake of this discussion about battery power, lets just talk about the circuit the battery is connected to as a resistive load
With a resistive load. Current flow is determined by the (properties of the driver) in this case resistance, vs the voltage of the battery. It goes by Ohms law - the formula V=IxR where V = voltage, I = Current, R = resistance
This can be re-arranged mathematically to I = V/R Current is the voltage divided by the resistance.
Practically this means for a given (resistor), Current is increased by increasing voltage.
Now lets put this into the context of a battery.
A fully charged battery at 4.2V - will allow a large current to flow though a resistor of 1 Ohms. (I = V/R = 4.2/1 = 4.2Amps)
If the battery was to be depleted over time, lets say it gets to 2.5V, the current flow will be reduced (I = V/R = 2.5/1 = 2.5Amps) - equating to about 1/4-1/3 the power output.
We see this as a dimming light, and we change the battery. This is the low battery warning we get. Or even better, lets assume the driver wont turn on after the power supply drops be
low 2.5V.
IF we have 2 batteries in series
2x fully charged batteries provide (4.2Vx2 = 8.4V). If both cells were then discharged to 2.5v each, the total voltage is 5.0V. SO accordingly the power drawn reduces significantly, and our driver turns off at 5V (2.5V x2)
IF we had imbalanced cells. One at 4.2V, the other at 2.5V, then the total voltage is (4.2 + 2.5 = 6.7v).
Now - remembering that the current drawn is a function of the VOLTAGE, then at 6.7V, theres still a relatively high cumulative voltage. So the current that flows will still be reasonably high. Also - the driver will be seeing 6.7V, which is still above 5.0V - so the driver wont cut off power.
We know that a rechargeable lithium cell at 2.5V is dangerously low, however in this setup - current flow is the same across all cells in series, so the #2 cell will continue to provide power until such time that the total voltage seen by the driver drops below 5.0V. By this time the cell would have been well and truly discharged below 2.5V.
So even with a driver that turns off at low voltage, a series setup can potentially mask the voltage status of individual cells.
Other factors to keep in mind regarding Series battery setups
- Batteries of different charge level can create this battery imbalance issue as described above.
- Batteries of Different capacities also create the above problem - both cells can start at full charge, but if one has lower capacity, it will reach a lower voltage before the other one - creating the same imbalance situation
- Batteries of Different health/age - As Above - battery capacity is a function of battery health/age
- Number of series cells - the problem is compounded by having more cells.
Solutions to this problem could include the following (more of these are better, but for torches, only the first one is practical)
- Individual cell cut-offs - protected cells
- Monitoring of individual cells - as used in most multicell devices (laptops)
- Careful battery management
- - Monitor how much capacity your cells have
- - At least check battery cell voltages after you have used them - When I set my cells to charge, I check individual cell voltages for signs of a battery that has been discharged more than the others.
My 3S torch!
Measuring each cells voltage. (still pretty high though. but theres already a slight difference.) The iCharger also has the ability to be user calibrated - which I have done to +- 0.01V. Look for this feature when buying a charger - its quite useful/important.
Measuring Internal resistance - and cell #3 also shows a slightly increased internal resistance...
However in the whole scheme of things, these cells have a very high internal resistance anyway! These are cells that I use for my RC stuff.
Thanks for explaining this better okwchin. Who makes the 3s? thx
Hmm, the article above recommends monitoring voltage while 'in use'. How would I go about doing that? I presume it doesn't mean while I'm actually going for a walk at night or hiking through the woods.
The potential for fires etc, extra monitoring, is scaring me off this chemistry. Perhaps i'll play safe for now and stick to nimh.
Yeah , I felt the same way at first . Then I went ahead and got an 18650 light and a protected cell . After a few years now , no problems . So now I have two 18650 lights and one more on the way . I rarely use my lights for more than a hour at a time and I check the voltage on my batteries after I use them as well as after I charge them .
So far , so good .
The 3S torch was made by someone/company? in taiwan I think it was, but the guy has a SERIOUSLY BAD sales history, taking payments, making promises and then never delivering goods. I would not recommend buying from this guy, although his torches were Very unique and had some very very good design elements! UI is like none other! - Neofab Legion II SST-50 is the one above if you want to read further. Beam is TK35 like, very general purpose, but with more throw and a narrower spill. Not Budget either.
Regarding safety with 3S torches, I'm personally not really concerned. As long as my cells are in reasonable shape, and I dont deep discharge them, and keep and eye on their performance, I think its manageable. The individual cell protection is the biggest factor for the safety of multi cell setups. The only other way is to use cells in parallel - however parallel setups mean that a dead cell will bring all the other cells down too! (like when you pull batteries from laptops - its the once cell that dies that pull its partner down too).
Im of the opinion that a 3S setup that gives 1.5 hours battery life is safer and better for batteries than a single 18650 pushing the whole 3A itself, having a 40 minute battery life (thats 2.5 times the discharge rate) I am comfortable with the concept of managing batteries though..
As I think others have mentioned, its more likely to be negligence and unsafe practices such as mechanically unsound torches and poor maintenance creating potential for shorting/overheating/overdischarging that is the problem.
Yes sir, that is what I think too. Having a 1-cell high-capacity type and draining the heck out of it, like a P60 host with Ultrafire drop in on high for 30 mins tailstand, hence raising the battery temperature to well over 60 deg C, that'd be more dangerous. A lot really depends on the user, by using good cells and good practices you really cut the risk down a lot.
That is the reason why I love this forum!
Thanks okwchin for taking the time for such a detailed article, I love being corrected!
Thank you :bigsmile:
+1 monanoke - This forum has saved me from myself many different ways.
And, I agree with okwchin; that set-up has got to be safer than my 980L drawing 4.50> amps from a single 18650. (the more I read, the more nuts that light seems - 150° F/66° C after 15 minutes on high!)
Foy
Thanks for all the replies guys, I am glad I have an understanding of how this all works now. I regularly check the voltages and current draws of my cells, and always try use the same cells together if possible. I personally was never worried about it, but wanted to hear the concerns of everyone.
great explanation okwchin
Good to hear that what ive put down isnt overly confusing! haha.
I've currently also got 2 torches that report battery voltage, the Legion (pictures above) provides a 6 step battery level indicator (cumulative voltage), and a LF5XT, which I find very nice for knowing if I should or should not be taking a torch for a half hour walk. More often than not I end up hitting the battery protection circuit cutoff in my normal torches, which is not as good for the cells. I've got another on the way, which also has voltage readouts to 2 significant figures. I totally don't expect budget lights to have voltage readouts any time soon, But its something I would really like to see in more torches.
The only other way is to use cells in parallel - however parallel setups mean that a dead cell will bring all the other cells down too! (like when you pull batteries from laptops - its the once cell that dies that pull its partner down too).
Not really. The weak battery will just keep depleting until it's dead, and other two will take up the slack, but I suspect the cells tend to self-balance with similar cells since the better ones will maintain the voltage across the weaker ones and the battery internal resistance tends to go up as they wear. Plus it's safer in an unmonitored light because it will not reverse charge in a complete circuit which is what often causes lithiums to explode.
I totally don't expect budget lights to have voltage readouts any time soon
I don't think it's necessary due to their price. A voltage meter costs basically nothing (maybe $1-2 including display). It's probably more because only the kind of people who buy expensive lights care about that kind of thing. That and it's not really convenient to mount, but it's not cost per se.
Voltage meters as in the software type. Flashing the output. Its just code + a few more components, however its a bit of software development time, and thats not within budget of most torches.