Eagletac 18650 3400mAh Cell

ArmoredFiend

Newly Enlightened
Joined
Jun 3, 2009
Messages
151
Location
Penang Island, Malaysia
Guys,
Jst bought an EagleTac 18650 3400mAh cell couple days ago and it's my 1st time using a 18650.

When i just receive it, i check it with a digital MM and saw the voltage is at 3.7..so i charge it with an Ultra Fire WF-139. After a couple hours, the light turn from red to green and i off it immediately. Let it cool down for a couple minutes before chcking with MM. It shows 4.15.

I have it charge again couple hours ago after using it over the weekend. And this round, the charging light turn green at 4.16.

But i remember full charge will be at 4.20...

So is WF-139 stop charging when it's about to be full, meaning it is impossible to get 4.20..

Or is it due to the protection circuit in Eagletac cutting off charges when it detect that it's about to be full?

Or i have a not so good 18650 from Eagletac?

Thanks in advance.
 

RetroTechie

Flashlight Enthusiast
Joined
Oct 11, 2013
Messages
1,007
Location
Hengelo, NL
Dunno how a WF-139 handles batteries, but if you charge a battery with a voltage of exactly 4.200000V, even until the battery is saturated, then when you pull it off that charge voltage and measure the battery's own output, it will be below 4.2V.

Internal resistance affects this (during charge), also a battery's voltage will always drop a bit right after pulling it from a charger. With some batteries that's more pronounced. For example I have a LiFePO4​ cell that's charged up to ~3.6V. Buy the time I notice the charger indicates "full", that cell's resting voltage already dropped to 3.3-3.4V or so (which is normal behavior for LiFePO4​ cells, iirc).

But i remember full charge will be at 4.20...
That's just a nominal value, meaning most Li-ions will be at 95, 98, 99% or so capacity by the time they have that as resting voltage. Whatever few % you could put in after that, comes at the cost of reduced cell life.

Then there's the accuracy of your DMM: if it has a base accuracy of 1%, then a displayed value of 4.16 might actually be 4.20V. Or 4.13V. Or 4.17V. Well you get the point... :)

In short: for this purpose, 4.16V = 4.20V = fully charged.
 

hiuintahs

Flashlight Enthusiast
Joined
Sep 12, 2006
Messages
1,840
Location
Utah
Your battery is fine. But not all chargers are created equal.

4.20v is the nominal fully charged value. But in order to get a battery to rest at 4.20v after charging process is over requires the charger to sit at the constant voltage mode of 4.20v (or higher) until the charge current drops to a very low level.

Every charger is different but I think the standard is for termination to occur once charge current drops to 1/10th of the original constant current charge. For a 1 amp charge that is 100mA. For a 250ma charge that would be 25mA, etc.

As RetroTechie indicated, the charger has to raise the voltage higher than the battery in order to force charge into the battery. A higher current requires a bigger delta voltage between battery and charger. And so if the charger is designed to never get above 4.20v, then in order to get a full charge into the battery would require the charger to sit at this voltage level until charge current dropped to a very low level and that only happens with chargers that offer multi level charge options.

The final resting point of the battery is determined by the charger and specifically
1) the constant voltage (CV) value designed into the charger.
2) along with the current termination point (C/10)

 

ArmoredFiend

Newly Enlightened
Joined
Jun 3, 2009
Messages
151
Location
Penang Island, Malaysia
Thanks heaps guys! You folks definitely know alot about batteries...flashlight aside. Glad to know i am still in the right forum..

Guess 4.16V is good enough then..

And again, thanks for the knowledge and assurance! Deeply appreciated!
 
Top