3-Cell LiFe 18650 Battery Pack Balancing

[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.

LiFe (computer batteries)

These are not LiFe and computers normally do not use LiFe batteries because they have inferior energy density.

[wraper]

https://youtu.be/mbaSAyqhzxc

[Gandalf_Sr]

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.

I realize that the Ultrafire batteries may not be the best and I can go with a 'good' brand later but they are what I have right now and they have been reasonable performance wise- I tested them and found they were around 2,500 mAHr capacity.  They also charge to almost the same voltage in a 3 pack as far as my limited testing has gone.

The LTC4020 is set up to terminate charge at 12.6 Vfloat (adjustable by resistor divider) and goes into a trickle mode when the batteries are close to 90% of full charge.

One way to not over volt them when charging would be to switch resistors over the battery(s) that has reached full voltage - does anyone have suggested circuits for this?

[KL27x]

It is very possible for the cells to be imbalanced. IME, uneven cells are unbalanced. They don't become unbalanced over time. Well, what I mean is, they can potentially get worse over time, but it's not something that you re-balance once, then all is good for awhile. The imbalance on top and/or bottom will reappear after a single cycle.

This is why I don't like the shunt resistor method of balancing. It will occur on every single recharge cycle. And it probably reduces battery life by trying to force the cells to all conform when they really won't. If I individually charge the one low cell out of 3 or 4 to bring it to 4.2, it will lag behind on the very next charge cycle, despite not using up even 1/4 of the cell capacity, right back where it settled, before. It's not something that gets worse and worse on every charge cycle.

Since you're using an adjustable DC converter and removeable cells, the simplest and most effective (IMO) thing to do is just measure the voltage of your cells after a charge and see if there's any difference.

Common sense says that the cell that is lowest is the one that isn't finished charging yet, hence it is the strongest/best. But in practice, I find this is often the cell that is weakest.  Dunno the mechanism of how of why it lags behind, but it doesn't seem to really get worse over time. At least not until a cell actually needs to be replaced.

Any rate, assume you started with all 3 cells fully charged. Then after a cycle or 10, let's say you measure cells as 4.3, 4.3, 4.1.* Tweak your DC converter to be 12.3-12.4V, and this effectively solves the problem. At least the one (problem) at the top end. Whatever your cutout circuitry does, you have to also leave some margin for error on that end, to account for the weakest link. As long as you are not overdrawing the battery on the bottom end, I find the result is typically very very stable. You could also pop out the cells and charge them in single cells chargers, occasionally. Theoretically, you might object this doesn't use the full capacity of the battery, because one cell is not fully charging. I dunno; you'd have to do some tests to see what the practical difference is. I have found it's easier to just use more battery with a larger safety margin than trying to maximize cells by balancing them on every charge.

If you want to mass produce this product, then this may not be a reasonable solution. So forgive the sidetrack.

[wraper]

I tested them and found they were around 2,500 mAHr capacity.  They also charge to almost the same voltage in a 3 pack as far as my limited testing has gone.

Then they should be recycled laptop cells rather than simply fakes.

[KL27x]

^ to see if they are recycled laptop cells, you would test the discharge capacity. As cells age, the biggest change is not the capacity. It's an increase in internal resistance. So they will exhibit more voltage drop/sag under high load and they can't take charge as fast. They won't meet max discharge current as specified.

If not damaged, cells will can work really well, despite being used, as long as they were not abused/damaged. To account for the higher ESR, you just have to derate the max practical discharge, and they might still be good for another 20 years. Li ion give you this weird ability, where instead of throwing away your batteries, you can just end up doubling up on them to get the same effective discharge/charge rates, but actually near doubling your capacity.

One of the reasons why your cell phone goes from 10 hours to 2 hours runtime over the years, is because of this internal resistance. The manufacturer started by maximizing capacity to the hilt. There's a tradeoff between capacity and discharge rate. If you start with a minimal discharge capability for the circuit, then over time, the battery will quickly waste more energy heating itself than powering your circuit. And it will fail to meet the minimum output voltage/amperage sooner. If you started with excess discharge capability, the battery could potentially last many many years longer in a given application. The phone designers are intentionally shooting for a 2-4 year life cycle. There's some engineer who wanted to make it a 9 hr run time that will be 8 hours 10-20 years later, but he had to do what he was asked.

Another way to look at it is when your 4 year old phone dies after 2 hours, the battery is still sitting on 70% of its capacity. It's just not enough output capability to run the phone, anymore.

If the cell holds float voltage of 4.2V for a good long while under no load, chances are it is a good cell, internal resistance/usedness aside.

[Gandalf_Sr]

Thanks guys, interesting that the cells lose their ability to deliver max current rather than capacity.  In my application they typically get nowhere near their max theoretical current draw - my guess is that they are supposed to be able to deliver 1A and my max draw is maybe 0.5A and more typically 0.2A.  The max charge current is limited to 0.5A.  They all hit 4.2V on full charge and then settle at around 4.05V once they've been off charge for a few minutes, they also seem to keep their voltage after long (months) storage times.