# Why are Li Ion batts labeled at 3.7v, when they charge to 4.2v?



## HighlanderNorth (Oct 7, 2012)

A few questions here related to voltage....

I just checked the voltage of a cheaper 18650 that I really would rather not be using at all, but these 2 are the only 18650's I have that are short enough to fit into my Lava Tube e-cig without completely crushing the springs all the way down! So I am stuck using them for now. So I make sure of their voltage before and after charging them, and just now the one I charged the other day reads 4.16v. I checked the other one right after charging it a few days ago and it read like 4.29v. **I dont know whats up with the discrepancy there, or if this battery has lost a little voltage in 2 days, or whatever, maybe someone can shine some light on that(get it, shine some light, and its a flashlight forum? Ha Ha)

Anyway, when I check my other good quality Panasonic batteries after charging them they are at around 4.2v as well, which is what I expected after reading this site for over a year. So when I run the cheaper 2200mah 18650's down to the point where the Lava Tube's indicator/LCD light starts blinking and it wont heat the atomizer anymore, I check the voltage and its around 3.59v. **Does that sound right considering this is a variable voltage device that I have dialed up to 4.8 volts? Is it supposed to stop working at 3.59v?

**Back to the thread subject, why is it that these batteries that charge to 4.2 volts are called 3.7 volt batteries? If they will cause a device to stop working at only 3.59v, then it seems that if they were only 3.7v, that wouldnt be enough! That probably doesnt happen to most other devices though, **so what is the usual low voltage of an 18650 when it is cut off by the PCB or by the device's own protection circuits?


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## moozooh (Oct 8, 2012)

Batteries don't give off constant voltage so they're usually rated at the average voltage across their discharge curve from 100% to full discharge. For most Li-ions that's from 4.2 to 2.7 volts, and the discharge curve enters the kind of a plateau between 3.7 and 3.6 volts, so that ends up as the main voltage rating. Some batteries that can be charged above 4.2 v usually state their maximum, such as 4.35 v.

Similarly, AA batteries actually start off above 1.5 v, and NiMH above 1.2, but that's where their average ends up, or something like that. In fact I think it's just a kind of an industry standard every manufacturer adheres to rather than meaningful figures. Under high load NiMH will typically give off a higher voltage than an alkaline.


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## shadowjk (Oct 8, 2012)

The 3.59 might be a bit high, but if it's a particulary weak battery, and the atomizer has high demands, the voltage under load might have been considerably lower than that. Checking with the known good panasonic cells would be a way to find out.


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## Joe Talmadge (Oct 8, 2012)

HighlanderNorth said:


> I checked the other one right after charging it a few days ago and it read like 4.29v.



Well, that's alarming.


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## AnAppleSnail (Oct 8, 2012)

Your charger seems to be trying to cook cells. Proceed with caution! And replace it if it does that often. Li-Ions shouldn't go above 4.2v.


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## HKJ (Oct 8, 2012)

HighlanderNorth said:


> Anyway, when I check my other good quality Panasonic batteries after charging them they are at around 4.2v as well, which is what I expected after reading this site for over a year. So when I run the cheaper 2200mah 18650's down to the point where the Lava Tube's indicator/LCD light starts blinking and it wont heat the atomizer anymore, I check the voltage and its around 3.59v. **Does that sound right considering this is a variable voltage device that I have dialed up to 4.8 volts? Is it supposed to stop working at 3.59v?



3.6 volt is about right for most unloaded empty LiIon batteries, when you put a load on the voltage will drop to a much lower value.




HighlanderNorth said:


> **Back to the thread subject, why is it that these batteries that charge to 4.2 volts are called 3.7 volt batteries? If they will cause a device to stop working at only 3.59v, then it seems that if they were only 3.7v, that wouldnt be enough! That probably doesnt happen to most other devices though, **so what is the usual low voltage of an 18650 when it is cut off by the PCB or by the device's own protection circuits?



3.6 or 3.7 volt does look very reasonable when looking at a battery under load.

Look at the voltage for the cyan line, it is a roughly 1C load:


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## Joe Talmadge (Oct 8, 2012)

AnAppleSnail said:


> Your charger seems to be trying to cook cells. Proceed with caution! And replace it if it does that often. Li-Ions shouldn't go above 4.2v.



Slight addition, max charge is 4.2V plus or minus .05V -- so 4.25V or lower, with the correct algorithm, is fine. 4.29V is out of spec, I'd re-check my meter, and if it's not broken, take a hard look at the charger and cell


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## jasonck08 (Oct 8, 2012)

3.6/3.7v is the nominal or average voltage under 0.2C load.

Same with Ni-mh. They are labeled as 1.2v, but are about 1.45v off the charger.


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## HighlanderNorth (Oct 8, 2012)

Joe Talmadge said:


> Well, that's alarming.




While I was posting this last night, I had the other cell on the charger, and it took about 2 more hours to charge, when it finished, I checked it, and it was at 4.16v, just like the first one. So as long as they finish up around 4.16v that'll be fine.


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## HighlanderNorth (Oct 8, 2012)

Joe Talmadge said:


> Slight addition, max charge is 4.2V plus or minus .05V -- so 4.25V or lower, with the correct algorithm, is fine. 4.29V is out of spec, I'd re-check my meter, and if it's not broken, take a hard look at the charger and cell




I assume that if these 2 lower priced, lower quality cells were protected, the circuit protection would cut in and prevent it from charging up above 4.25v or so, correct? Again, I just recharged that same cell last night and it finished after I posted this thread, and it charged to 4.16v just like the other battery did. So things seem to be back to nromal, but I would rather not be using these cheap batteries, and maybe I'll just buy the IMR Panasonic 2250mah 18650 or 2. Its truly 65mm, so it should fit into the e-cig, and fitment issues are the whole reason I'm not using my Panasonic protected 18650's.


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## LightCrazy (Oct 9, 2012)

*Question about 18650 voltage*

I bought a Fenix 18650 battery here in the US from one of our sponsors. The battery was marked 3.7 volts. The battery arrived with 3.77 volts and charged in my WP2 II charger to 4.15 volts. I just got another Fenix battery off eBay from China. This Fenix battery says 3.6 volts, also arrived with 3.7 something volts and charged up to the same 4.15 volts. Any idea why one package and battery is labelled 3.7 and the other 3.6?


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## hellokitty[hk] (Oct 9, 2012)

*Re: Question about 18650 voltage*

No real reason don't worry about it.


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## lwknight (Oct 11, 2012)

*Re: Question about 18650 voltage*

Maybe they are all 3.65 ave and some round up and some round down. Its just a label anyway.
Cells vary from brand to brand and with age and even within their same brand specs.


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