battery charging some up to 3.99 some at 3.3

[cinergi]

Two things:

1) When there are no cells in parallel, IR doesn't matter. IR cannot be the cause of "runner" cells. Previous statements in this thread to that effect are false. The only thing that can cause a runner in such a case is difference in capacity.

2) With respect to resting voltages, are we comparing apples to apples? There's resting open-circuit voltage and resting "connected" voltage. And by open circuit, I mean literally not connected to anything, not even a BMS. 3.55 is what I've seen for open-circuit resting voltage. 3.35-ish is what I've seen for connected resting voltage (can take hours to get there, depending of course on your capacity)

 

[snoobler]

I have a lot of respect for @Steve_S . I would be surprised that he would say cells in parallel have different voltages. Perhaps you are referring to a discussion about resting voltages?

Expanding Battery Bank w different Ah sets?

As I work through my design, I am wondering if I can expand the portable system with different Ah cells. Specifically, I will start with 8x 100 Ah LiFePO4 4S, 2P for 24V. Once I get that going, I would like to then add a second batch of batteries to expand the storage capacity -- most likely...

diysolarforum.com

Thinking about using two different, but same chemistry, battery banks

Hello again guys. I have 8 x 3.2V used sinopoly batteries 90AH each, and just got a brand new set of 8 x 3.2V 120 AH batteries, both Lifepo4. I am thinking of using them both under the same mppt charge controller, and at the same time, connected to the same inverter. I am thinking of 1st...

diysolarforum.com

The above two posts contain wording easily interpreted as such. At a minimum, it's confusing.

 

[snoobler]

Two things:

1) When there are no cells in parallel, IR doesn't matter. IR cannot be the cause of "runner" cells. Previous statements in this thread to that effect are false. The only thing that can cause a runner in such a case is difference in capacity.

2) With respect to resting voltages, are we comparing apples to apples? There's resting open-circuit voltage and resting "connected" voltage. And by open circuit, I mean literally not connected to anything, not even a BMS. 3.55 is what I've seen for open-circuit resting voltage. 3.35-ish is what I've seen for connected resting voltage (can take hours to get there, depending of course on your capacity)

1) not entirely true. Significant IR discrepancies mean more of the current flow is converted to heater rather than stored or delivered capacity. They lose more capacity during a discharge and they gain less during charge.

2) The chart I posted above was a single cell's open circuit voltage. I have 80-ish CALB cells that behave that way.

This is one of those goofy Miady LFP 6Ah pack from Amazon. Will tested the 16Ah version on Youtube:



13.75/4 = 3.44V after 15 minutes.

 

[Ampster]

The above two posts contain wording easily interpreted as such. At a minimum, it's confusing.

I understood the first post to say if you put 4 batteries of different voltages in parallel they will average out to a value. As we have learned in parallel top balancing it may take weeks for enough current to flow for their voltages to equalize depending on their SOC. Of course there will be a surge when they are placed together and the common buss bar will measure that average voltage. I don't think the wording could be easily interpreted as such.
I don't see any statement in the second post about the voltages of cells in parallel. That post was talking about packs in parallel.

 

[cinergi]

1) not entirely true. Significant IR discrepancies mean more of the current flow is converted to heater rather than stored or delivered capacity. They lose more capacity during a discharge and they gain less during charge.

Hrm ... now we're getting into chemistry and physics. Regardless of differences in IR, the same # of electrons will flow through the cathode/electrolyte/anode ... it seems to me that IR will only impede movement of electrons, not cause them to be lost (in other words, the same # of electrons will move from one side to the other); the implication of lower capacity due to heating implies losing electrons or not transferring them in the first place, no?
(and I know it's not literally electrons moving around, it's carried by Li and stored in C6 yadda yadda)

 

[Ampster]

2) With respect to resting voltages, are we comparing apples to apples? There's resting open-circuit voltage and resting "connected" voltage. And by open circuit, I mean literally not connected to anything, not even a BMS. 3.55 is what I've seen for open-circuit resting voltage. 3.35-ish is what I've seen for connected resting voltage (can take hours to get there, depending of course on your capacity)

My previous experience was with Winstons and Thunderskys and they rested at 3.32. My recent experience with these LF280 cells is that they have rested at 3.35 but I have one that was resting at 3.44 after days of rest. It was in a pack with 7 others. I eventually pulled it down with a power resistor to 3.34 volts. I have just taken it out of the pack and am charging it to 3.65 and will log the voltage after I disconnect it.

 

[cinergi]

8 of my top-balanced EVE 280's are resting at 3.567-3.588. They were top balanced (to 3.650) about 2 weeks ago. Interesting ...

 

[jvbutter01]

I wouldn’t hold my breath, remember this is the same person who claims cells in parallel can have different voltages..

ummm hey there... yes they do... it all depends on how long they are in parallel ... i had some 3.34 and 3.35.... If you let them sit long enough then sure they will balance out... could be days, or weeks or months depending on how many you have.

 

[Ampster]

ummm hey there... yes they do... it all depends on how long they are in parallel ... i had some 3.34 and 3.35.... If you let them sit long enough then sure they will balance out... could be days, or weeks or months depending on how many you have.

I guess it all depends on where you are standing. Or whether one means the common buss bar of cells in parallel or the voltage of the underlying cells just after you take off the buss bars.
In another discussion about a 4P16S pack the comment was made that the common buss bar that connected the 4 positives to the 4 negatives would have enough resistance that the 4 cells in parallel would be at slightly different voltages under load because of the resistance of that portion of the buss bar.

 

[jvbutter01]

I guess it all depends on where you are standing. Or whether one means the common buss bar of cells in parallel or the voltage of the underlying cells just after you take off the buss bars.
In another discussion about a 4P16S pack the comment was made that the common buss bar that connected the 4 positives to the 4 negatives would have enough resistance that the 4 cells in parallel would be at slightly different voltages under load because of the resistance of that portion of the buss bar.

yes true... just saying they can be during charge from my experience... mine did level out after a while. some of you are far more experienced than I am, just sharing my experience.

 

[Ampster]

Earlier in this thread we had a discussion about the settling voltage of some of these cells. I have experienced some abberations. One cell has refused to settle. Here is a picture of it 18 hours after being charged to 3.65 volts and finishing 18 hours earlier. It is now on a discharge test to see what kind of capacity it has.
Here is where I discussed this earlier in this thread.

My recent experience with these LF280 cells is that they have rested at 3.35 but I have one that was resting at 3.44 after days of rest. It was in a pack with 7 others.

 

[jvbutter01]

A good parallel top balance will probably help to achieve this.

Keep us posted on your results and good luck!

Ok, back on topic. so I did top them all up the other day. After paralleling them, I did get them all up to 4.3X v. Hooked up to the inverter with heat gun and started the discharge. If i keep doing discharge tests, I really need to get a measurement device/meter with supplemental PS. using my Victron712 is not best choice, as when the BMS shuts off, so does my power to the Vic712. I use the batt as a power supply and ground through the shunt.

either way, bms shut off at 11.78 due to single cell hitting 2.5v. I'm ok with that for now. back in the big charger (45a trailer charger) all cells seem to be charging up some what even, or close. Yes the party is not over, still have a ways to go. Do you think they will continue to charge up close to equal? I have not watched the charging this close. Currently i am checking ever 2hr or so.

Cell-1 ~ 3.376
Cell-2 ~ 3.343
Cell-3 ~ 3.354
Cell-4 ~ 3.351

or will my runner take off at some point. I estimate I still have 3hr more to fully charge them. my 280ah 4s bank have taken 43a for the last 3hr.

 

[Ampster]

or will my runner take off at some point.

If cell 1 was the one that hit low voltage first it looks like it is in its way to being a runner because of lower capacity. Othewise it could be a cell imbalance as noted earlier.

 

[jvbutter01]

yea @Ampster , not sure which hit the LVD. Remember the part about battery shutting off so I lost my Vict712 data. From my previous data, Cell was a runner, but it was on the high side. I don't remember it hitting Low voltage previously. I don't have notes about that. its 2hr later and voltages still closer than previous tests ( like last week testing)....

Cell-1 ~ 3.414
Cell-2 ~ 3.378
Cell-3 ~ 3.381
Cell-4 ~ 3.385

I don't have enough experience in charging cells, so I don't really know, but seems 0.05 they are all close at this point. Not exactly tight, but as new cells will they or should they stay within 0.02 of each other?

 

[Ampster]

but as new cells will they or should they stay within 0.02 of each other?

They will diverge after 3.4 volts as you can see. You already have 36 millivolts (.036) between 1 and 2. That is where the imbalance will show up. There usually is not much capacity after 3.4 volts so you have several choices. One, you can cut off charging at the pack voltage just before that cell hits 3.6. Over time if you balancer cuts in at 3.4 volts then it may slowly bring that cell into line. Two, you can reduce charge current so it spends more time having the BMS bleed off capacity. Three, you can use a resistor to bleed off capacity which might be faster. Four, you can disassemble your pack and try to parallel top balance again.
NOTE: I did not read the thread completely to see if you already top balanced earlier. Based on @MisterSandals comment, if you already top balanced and you indeed did it to 4.3 volts I am now not sure what to suggest.

 

[MisterSandals]

After paralleling them, I did get them all up to 4.3X v.

Yow, 4.3v?

 

[Solarfun4jim]

What would happen if the three good cells were at say 3.40v and the 'runner' was added at say 3.35v. Since that is much lower in the flat part of the curve, would it take longer to become a runner when getting towards the knee at the top? Would need to run with balancing off. (lost capacity at the bottom end no doubt, but not everyone likes to discharge right to the bottom....just a though experiment)

 

[jvbutter01]

Yow, 4.3v?

They will diverge after 3.4 volts as you can see. You already have 36 millivolts (.036) between 1 and 2. That is where the imbalance will show up. There usually is not much capacity after 3.4 volts so you have several choices. One, you can cut off charging at the pack voltage just before that cell hits 3.6. Over time if you balancer cuts in at 3.4 volts then it may slowly bring that cell into line. Two, you can reduce charge current so it spends more time having the BMS bleed off capacity. Three, you can use a resistor to bleed off capacity which might be faster. Four, you can disassemble your pack and try to parallel top balance again.
NOTE: I did not read the thread completely to see if you already top balanced earlier. Based on @MisterSandals comment, if you already top balanced and you indeed did it to 4.3 volts I am now not sure what to suggest.

sorry misprint.. 3.43v...the latest charge up was after parallel connection and slow charger on them....
cell1 ~ 3.43
cell2 ~ 3.43
cell3 ~ 3.42
cell4 ~ 3.40

 

[Ampster]

he latest charge up was after parallel connection and slow charger on them....
cell1 ~ 3.43
cell2 ~ 3.43
cell3 ~ 3.42
cell4 ~ 3.40

Is that an update from post #54 or an earlier charge cycle?

 

[Ampster]

What would happen if the three good cells were at say 3.40v and the 'runner' was added at say 3.35v.

It would depend on what the actual capacity was of that cell. If it was a lot less than the others it could still be a runner and catch up with the the others. The only way that would not happen is if they were all balanced at the top and as you said the loss of capacity would show up at the bottom.