Electronics > Projects, Designs, and Technical Stuff
3-Cell LiFe 18650 Battery Pack Balancing
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Gandalf_Sr:
I saw a thread about this about a year ago.  I have a design that uses 3 x 18650 3000 mAHr 3.7V Li-ion batteries in series to realize ~12V pack voltage.  The batteries I have are in a holder so I can put in whatever brand I want but I'm using Ultrafire at the moment (see picture).  The charging is controlled by an LTC4020 which includes start up algorithms that trickle charge initially to detect a bad cell but there's no sensing of the individual cell voltages and I am (slightly) concerned that the cells could become imbalanced.

My system includes a microcontroller with ADC and (given that I'm in PCB respin mode already) I could add extra components to measure the 2 inter-cell voltages and easily have the micro calculate if cells were imbalanced.

Questions:
1. Is this even an issue? Won't the cells figure it out for themselves - e.g. dissipate heat when fully charged while the others 'catch up' and aren't LiFe (computer batteries) supposed to be pretty robust?
2. If it is a problem, I could add FETs to switch resistors in over each cell while charging but does anyone have a suggested circuit for this?  I've seen the cheap balancers on alibaba, how do they work?

Thanks for any help.

Ted
mikerj:
The voltage rating of those cells (3.7v nominal) suggests they are lithium cobalt oxide, not lithium iron phosphate (3.2v nominal).  Additionally Chinese cells with "Fire" in the brand name are often aptly named and should be avoided.

Balancing with Lithium Cobalt cells is problem that should be addressed; they won't (safely) sort it out by themselves.  At a minimum you should terminate charging when any single cell reaches 4.2v, this won't provide balancing will will provide safety.
schmitt trigger:
The cheap balancers that I have seen, do it by switching a load resistor in parallel with the cell.

The device shown in your attached link appears to be exactly that.
wraper:
Those batteries are trash. You are lucky is there is 30% of rated capacity.

--- Quote --- LiFe (computer batteries)
--- End quote ---
These are not LiFe and computers normally do not use LiFe batteries because they have inferior energy density.
wraper:
https://youtu.be/mbaSAyqhzxc
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