Weak cells vs runners

[Sverige]

I see the term “runners” being used to describe some underperforming cells, but but other times “weak cells” and it’s clear from the context that they’re two different things. So what is a runner and how is different from a weak cell?

 

[Solarfun4jim]

For me, they are both laymans terms for cells that diverge from the pack, when nearing the knees. It will be interesting to see other opinions.

 

[Steve_S]

I use the term RUNNER as a general term to describe cells that will diverge from the others either during Charge or Discharge. These can cause a BMS to trip on LVD (Low Volt Disconnect) or HVD (High Volt Disconnect) at the cell level as these will cross the threshold before the other cells. These are most pronounced above 3.400V or below 3.100V typically.

 

[Sverige]

I use the term RUNNER as a general term to describe cells that will diverge from the others either during Charge or Discharge. These can cause a BMS to trip on LVD (Low Volt Disconnect) or HVD (High Volt Disconnect) at the cell level as these will cross the threshold before the other cells. These are most pronounced above 3.400V or below 3.100V typically.

Ok thanks. So that could be down to low capacity, or high IR? So in that case do you differentiate “weak cells” as something different, or is it like @Solarfun4jim says, two different terms for the same thing?

 

[Steve_S]

Ok thanks. So that could be down to low capacity, or high IR? So in that case do you differentiate “weak cells” as something different, or is it like @Solarfun4jim says, two different terms for the same thing?

I'm no chemist or battery expert by any means (just a keen hobbyist) and this is one of "those" areas. I believe it is a combination of things that are contributors and not a single factor by itself. Internal Resistance is certainly a key point and that varies depending on the voltage, charge/discharge factor, temps.

These are commodity cells for the most part. On the "new" cells, they can be various grades and classes. Some are better than others to put it simply. They are not fully matched properly. EV's for example, all use High Quality Grade-A Fully Matched Cells in their packs, regardless of the chemistry. These will perform uniformly throughout their full operating ranges. Including loads in/out, temps etc. Hence why EV packs are so damned costly. ESS stuff does not have to be that "specific" fortunately, simply because they never see 5C+ Charge or Discharge nor the pack voltages (EV's 300V to 1000V + for Bus & Trucks).

I've just received another batch of 4x 280's. I'm going to experiment a bit with these 1st, after they thaw out nicely. (sitting in cold warehouses, trucks - it is -14C outside right now,) I have a Yaorea 1035+ IR Tester and a TekPower TP1540E Bench PS which will let me do charge & discharge per cell tests to measure the IR at different points, then a few other tests depending on the outcome. It also depends on what my running 280 packs because I'm finishing my Thrashing of them this week. May end up swapping a cell or two around. I have 1 cell per 280 pack that Run High, fast after they hit 3.460 but there is possibly another factor that I am investigating.

 

[Solarfun4jim]

"I've just received another batch of 4x 280's. I'm going to experiment a bit with these 1st, after they thaw out nicely. (sitting in cold warehouses, trucks - it is -14C outside right now,) I have a Yaorea 1035+ IR Tester and a TekPower TP1540E Bench PS which will let me do charge & discharge per cell tests to measure the IR at different points, then a few other tests depending on the outcome. It also depends on what my running 280 packs because I'm finishing my Thrashing of them this week. May end up swapping a cell or two around. I have 1 cell per 280 pack that Run High, fast after they hit 3.460 but there is possibly another factor that I am investigating."

Really looking forward to any data you might post on this.
Might it be, that you will always have a 'runner', but simply that, the better they are matched, the higher into the knees you can get, before it 'runs'???
You are producing a fantasitic amout of info on these. So thanks for that.

 

[Ampster]

I use the terms interchageably. A runner better describes what the cell does when it his the knees. My BMS describes a weak cell as a cell which has hit the high or low voltage set point. In either case I attribute the result to either imbalance or less capacity. Techically an imbalance might not be a cell with less capacity.
One way to look at it would be all weak cells could be runners but not all runners are weak cells. Also a cell with equal capacity to the others might only run at one end depending which way the imbalance is scewed.

 

[Steve_S]

Very well put Ampster !

 

[Gazoo]

If staying within the knees then there are no runners....

 

[Ampster]

If staying within the knees then there are no runners..

In that case it is only a former runner, poised to break out of the pack if I get greedy and try to push to pack harder.

 

[snoobler]

If staying within the knees then there are no runners....

If I weren't a mod now, I'd try to twist this into innuendo...

 

[platypus]

After recent testing of my compromised pack your eloquent descriptions of weak knees and runners are spot on and gazoo's observation " If staying within the knees then there are no runners...." confirms my analysis and plan of action.
I am sure this terminology will enter the lexicon.
Something left over from my previous experiences with lead acid chemistry (sulphation) is should i continue to slightly and conservatively push the knees hoping to extend the range , so far i accept cell voltage delta .200v , is that reasonable or should i constrain it more, can anyone reccomend a differential range of cell voltages?
Fyi 4s 135ah lifepo4 solar with typical discharge < 15ah
regards newbie

 

[Gazoo]

.2 volts when the cells are full? That's not so good. Did you parallel top balance. My delta when fully charged is around .100 and when fully discharged it's .500. I have a weak cell. I think the consensus is to keep the cells between 3 and 3.4 volts. Going outside that range does not aquire much more capacity.

 

[time2roll]

I believe a runner could be just out of balance compared to the others.
For example you will probably have a runner if you have skipped the top balance process.

 

[Steve_S]

135AH bulk cell, I would allow no more than 130mv differential. 1mv per AH capacity is not too far.
Top Balancing and levelling of cells helps somewhat but cannot compensate for unmatched cells.
Not a "fix" but a workable "patch" is to use an Active Balancer, I have one on all of my packs. The bulk cell ones, the EV Grade cells packs and the Matched & Batched as well. I have that mix in my bank.

Watch this and think on it.

 

[platypus]

Thanks for your replies , twice i paralleled the cells and bought them up in voltage and held them there for quite some time but felt uneasy as the charger i used is an old transformer type with analogue reading , i monitored with dmm but was not confident.
I have ordered wanptekDPS605U DC power supply with power display 60V 5A it has 4 digit display so looking forward to it arriving soon.
In the meantime i will continue to rely on the feedback from the Daly smart B/T BMS and manage the delta readings as best i can yes gazoo between 3 and 3.4 volts seems a safe bet.
steve i will most certainly check that vid .