If you are measuring the voltage at the cells terminals they won't overcharge at 3.64 volts with a .15 amp current if you take them to 3.65. You might see an amp or two difference if that. I think your top balance is fine.
battery ballooned
- Thread starter jvbutter01
- Start date
It occurs to me that I may have been overlooking a factor when thinking through top balancing with a low amp charger. I think I was overlooking/undervaluing the effect of a very low c-rate.
Whereas I was focused on the difficulty of measuring 'end of charge' with a <20A charger and EVE cells. And was focused on the possibility of a low current charger taking a longer time near the top of charge. I was overlooking the possibility that with a lower C-rate gradual charge, at a given voltage the pack may be closer to full and require a shorter CV period compared with charger at a higher C-rate where the CV period may be more necessary. Does this make sense? I think some of you may have hinted at this or tried to point it out here and elsewhere, but I didn't comprehend it at the time (and actually I'm not really sure if its true, or if I'm missing the mark here). Thoughts?
Whereas I was focused on the difficulty of measuring 'end of charge' with a <20A charger and EVE cells. And was focused on the possibility of a low current charger taking a longer time near the top of charge. I was overlooking the possibility that with a lower C-rate gradual charge, at a given voltage the pack may be closer to full and require a shorter CV period compared with charger at a higher C-rate where the CV period may be more necessary. Does this make sense? I think some of you may have hinted at this or tried to point it out here and elsewhere, but I didn't comprehend it at the time (and actually I'm not really sure if its true, or if I'm missing the mark here). Thoughts?
Solarfun4jim
Solar seduced :-)
I'm a total novice at the top balancing(using a longwei bench top PS), so can't comment with any confidence, but if you charge at 3.4v( @<0.05CA) till the amps go to zero, is the pack not already full at that point? If you then set it to 3.5 and do the same again, is that not in fact overcharging it? I have the feeling (but can't confirm by experiment) that at charge rates below 0.05C you are seeing the Amps tapering off right from the start of the charge. At least thats what appeared to happen with me. The amps started at 5.8A and was reduced to 0.15A by the time i hit 3.64....there was no 'constant current' in this first phase(which i admit was a bit disconcerting). I had been expecting the amps to stay at 5.8 right up till i hit 3.65v and then taper off, but that didnt happen.
Only reason i aimed for 3.65v, was that i wanted a higher 'calibration value' for the pack, so that subsequently, hitting 3.6 would allow me some room for the balancer to get time to work.
Not sure if this makes sense?
Last edited:
No.
No. When the voltage at the cell terminals reaches 3.65 and the power supply reads zero amps then the cell is fully charged.
My charger did not go into CV mode until the cells were 3.60 volts. It was supposed to go into CV mode at 3.65 volts because that's what I had it set to do. However it did go into absorption mode. It's these low voltages that are a pain to deal with and the waiting period requires patience. Thankfully this should be a one time thing.
Yes that does make sense. I hope my cells behave and the passive balancer will work for them. If they start to get out of tune, then I might be looking at active balancers. However most keep the cells voltages within the knees. I am curious to find out how well my cells behave kept within the knees.
Solarfun4jim
Solar seduced :-)
Thanks for the reply gazoo....only reason i thought the pack might be full at 3.4v if left for amps to tail off, was because of this chart below...
I have seen that chart and I think it's a good guide for capacity. However at 3.4 volts the cell is not fully charged and one will get more capacity if charging the cell all the way to 3.65 volts. This is why it's important to charge to 3.65 volts when top balancing to allow the cells to balance as close to each other as possible.
I am going to attach a graph that has been floating around in the threads so you can see for yourself. I have viewed many graphs and they are all pretty much the same.
I am going to attach a graph that has been floating around in the threads so you can see for yourself. I have viewed many graphs and they are all pretty much the same.
Attachments
Do keep in mind that that graph also shows why you can be at nearly 100% state of charge at 3.4V - if you charge at fractions of C. From the graph, you can see 0.05C being at a much higher state of charge than 0.2C at the same 3.4V. For a 280Ah battery, 0.05C comes to 14A. People need to keep this in mind when they are using 10A max power supplies and the like that you are way below that.
ArthurEld
Solar Wizard
Wouldn't it be smarter to top balance near the high that you are actually using?
If your system charges the cells to 3.6, I would think it is better to top balance at 3.6 rather than 3.65.
And if your system doesn't do an absorb then it would be best to balance at 3.6 with no absorb.
If your system charges the cells to 3.6, I would think it is better to top balance at 3.6 rather than 3.65.
And if your system doesn't do an absorb then it would be best to balance at 3.6 with no absorb.
One point to bring into the conversation.
The point (well at least A point) of taking the cells to 3.6 or 3.65, with a top balance is not to make sure the cells are full but to push the cells into the upper knee where small differences in SOC correspond to measurable differences in voltage.
The point (well at least A point) of taking the cells to 3.6 or 3.65, with a top balance is not to make sure the cells are full but to push the cells into the upper knee where small differences in SOC correspond to measurable differences in voltage.
Nordkyn said:
I'm not positive but I believe that the CV/Absorb phase is an integral part of the top balance procedure. Can anyone confirm?
ArthurEld
Solar Wizard
From what I have read for lifepo4 it is best to not do the absorb or do a very short absorb like 5 minutes.
Sorry I don't have a quote on hand but I'll look for it.
edit: I found this info saying no absorb is better. Someone linked to a more concise article but I can't seem to find it.
"There is a lot to be said for simply setting the absorb Voltage to 14.4V or 14.6V, and then just stop charging once the battery reaches that Voltage! In short, zero (or a short) absorb time. At that point your battery will be around 90% full. LiFePO4 batteries will be happier in the long run when they do not sit at 100% SOC for too long, so this practice will extend battery life. If you absolutely have to have 100% SOC in your battery then absorb will do that! Officially this is reached when the charge current drops to 5% – 10% of the Ah rating of the battery, so 5 – 10 Amp for a 100Ah battery. If you cannot stop absorb based on current, then set absorb time to about 2 hours and call it a day."
Last edited:
Solarfun4jim
Solar seduced :-)
I think perhaps the issue is more with using a 10A bench top PSU. When i started my top balance the max amps was under 6A, or 0.02C amps on a 280AH cell. I think if you could use anything with the ability to drive more than 14A(0.05C), then it would perform as expected, but below this, not so much in my limited experience.
For those of us that have played with electric RC hobbies, I think it helps to think of the big LiFePO4 cells as oversized hobby batteries rather that using lead acid batteries as a baseline reference point for charging, storage etc.
I am working on top balancing 16 of these cells at the moment. I’m building them as 2, 24v packs which will initially be wired in parallel driving a 24v, 2000w inverter. At some point in the future, if I feel the need to go to a 3000w inverter, I’ll wire them in series for 48v. When I started, I decided to parallel one set of cells and charge them in parallel. The other set, I wired in series to compare the process both ways.
My resources for charging the batteries include a the cheap Tuffion 30v, 10a power supply that Will semi-recommends as at least being durable
, an ISDT T8 DC smart charger, and another generic 36V, 27A fixed power supply that I can use to drive the T8 to near full capacity when charging in series. The T8 supports a max current of 30a and up to 8s batteries, with 2.2a balancing current. It also does a nice job of stopping the charge automatically once the set voltage is reached. I would describe it as providing adjustable constant current power, up until just before the target voltage is reached, at which point, it drops the charge rate to avoid overshooting the target voltage.
I’ve been charging the paralleled and series packs in blocks of a few hours at a time since I’m not entirely comfortable leaving the setup unattended when charging. I’ve been charging the packs at 27a to stay 10% below the max rated capability of the T8.
As can be expected, the series pack has made much faster progress since it pushes about 750W into the pack when charging. When I was using the T8 to charge the series pack, I’d use the Tuffion to keep feeding the parellel pack about 9.3 amps.
Today, I got the series pack to the point that the highest SOC cells started to climb quickly, so I stopped charging them as a pack, and used the T8, set to 14a to peak each cell at 3.60v. I then re-seriesed the pack and allowed the T8 to repeak the whole pack to 3.60v, giving it time to top balance the pack using its balancing capability. It took about 40 mins for it to reach a state that it considered balanced and shut off the charging. I then told it to repeak at 3.65, which only took it a few minutes to do. I’m not sure how well balanced I think it really is, but I’m going to capacity test, cycle it a few times and see what it looks like. If the parallel top balanced cells behave more uniformly as a series pack, then I’ll break this one down, wire it up in parallel and rebalance it that way. Right now I am using the series pack as a power supply for the T8 to keep 27a pushing into the paralleled pack, so I get some capacity testing and finish charging the other pack at the same time
I am working on top balancing 16 of these cells at the moment. I’m building them as 2, 24v packs which will initially be wired in parallel driving a 24v, 2000w inverter. At some point in the future, if I feel the need to go to a 3000w inverter, I’ll wire them in series for 48v. When I started, I decided to parallel one set of cells and charge them in parallel. The other set, I wired in series to compare the process both ways.
My resources for charging the batteries include a the cheap Tuffion 30v, 10a power supply that Will semi-recommends as at least being durable
, an ISDT T8 DC smart charger, and another generic 36V, 27A fixed power supply that I can use to drive the T8 to near full capacity when charging in series. The T8 supports a max current of 30a and up to 8s batteries, with 2.2a balancing current. It also does a nice job of stopping the charge automatically once the set voltage is reached. I would describe it as providing adjustable constant current power, up until just before the target voltage is reached, at which point, it drops the charge rate to avoid overshooting the target voltage. I’ve been charging the paralleled and series packs in blocks of a few hours at a time since I’m not entirely comfortable leaving the setup unattended when charging. I’ve been charging the packs at 27a to stay 10% below the max rated capability of the T8.
As can be expected, the series pack has made much faster progress since it pushes about 750W into the pack when charging. When I was using the T8 to charge the series pack, I’d use the Tuffion to keep feeding the parellel pack about 9.3 amps.
Today, I got the series pack to the point that the highest SOC cells started to climb quickly, so I stopped charging them as a pack, and used the T8, set to 14a to peak each cell at 3.60v. I then re-seriesed the pack and allowed the T8 to repeak the whole pack to 3.60v, giving it time to top balance the pack using its balancing capability. It took about 40 mins for it to reach a state that it considered balanced and shut off the charging. I then told it to repeak at 3.65, which only took it a few minutes to do. I’m not sure how well balanced I think it really is, but I’m going to capacity test, cycle it a few times and see what it looks like. If the parallel top balanced cells behave more uniformly as a series pack, then I’ll break this one down, wire it up in parallel and rebalance it that way. Right now I am using the series pack as a power supply for the T8 to keep 27a pushing into the paralleled pack, so I get some capacity testing and finish charging the other pack at the same time
Attachments
D
Deleted member 9967
Guest
I know the article you are speaking of. However if it is the same one. Look closer and you will find that they are comparing lead acid chargers and lifepo batteries and saying that the lead acid chargers will "float" charge and THAT is the type [or manner or kind or whatever] of float charge that is dangerous for lifepo batteries.
The minor type of float recommended here by many [not all] is extremely low current [amps] and the same voltage nonstop. Where as lead acid chargers float at higher amps and even much higher voltages at times.
I hope this clears it up for you.
D
Deleted member 9967
Guest
I looked at the picture and for a couple of seconds I thought. A wall attachment? Now there is confidence in ones work.
Even so, it still looks very neat and well done.
ArthurEld
Solar Wizard
I don't think there is anything to gain by doing absorb. And there is a possibility of harm.I know the article you are speaking of. However if it is the same one. Look closer and you will find that they are comparing lead acid chargers and lifepo batteries and saying that the lead acid chargers will "float" charge and THAT is the type [or manner or kind or whatever] of float charge that is dangerous for lifepo batteries.
The minor type of float recommended here by many [not all] is extremely low current [amps] and the same voltage nonstop. Where as lead acid chargers float at higher amps and even much higher voltages at times.
I hope this clears it up for you.
I tried absorb a couple times and learned what I need to know. I won't be doing absorb anymore.
D
Deleted member 9967
Guest
Not sure why you are telling me this. But ok.
ArthurEld
Solar Wizard
It's a forum, I'm sharing my experience. But my .02 isn't worth much.
And I don't care if other people want to absorb.
D
Deleted member 9967
Guest
I know this is a forum. I don't drink or do drugs so I figured that part out a long time ago.
I was just wondering why you picked me to tell that too is all.
ArthurEld
Solar Wizard
I thought I was answering your reply to me. But I do drugs and drink.
D
Deleted member 9967
Guest
No I was clarifying things for this guy.
cwstnsko said:
For those of us that have played with electric RC hobbies, I think it helps to think of the big LiFePO4 cells as oversized hobby batteries rather that using lead acid batteries as a baseline reference point for charging, storage etc.
And then you said what you said which made no sense to me is all.
And you do drugs and drink. Got it and thanks for the heads up.
cwstnsko said:
For those of us that have played with electric RC hobbies, I think it helps to think of the big LiFePO4 cells as oversized hobby batteries rather that using lead acid batteries as a baseline reference point for charging, storage etc.
And then you said what you said which made no sense to me is all.
And you do drugs and drink. Got it and thanks for the heads up.
ArthurEld
Solar Wizard
No problem
It's a one time thing. After the top balance is finished one should never have to do it again. Heck, I plan to try to keep my cells at a 70% SOC after I have tested them for capacity. This might take a few cycles.
So you said you learned what you need to know. I am curious what did you learn? I think that's what SherylinRM was asking too. If you don't want to share what you learned that's fine but it could be helpful.
I looked at the picture and for a couple of seconds I thought. A wall attachment? Now there is confidence in ones work.
Even so, it still looks very neat and well done.
Yes, I guess without some sense of the angle I took the picture from, it is not clear that these are sitting on a table
. I was just trying to get a picture that included most of the stuff that I'm working with, but not much else. So far according to my AiLi meter, I've pulled about 40 ah out of the series pack, and according to the T8, I've put about 230 ah into the parallel pack since I started powering the T8 with the pack. By my math, that makes the T8 very flexible/versatile, but not very efficient (a little over 70% ?) Based on my experience so far with the T8, I find it very useful, but If I were buying today, I might spend a bit more for the icharger X8, to get a similar feature set and the 3 decimal display of cell voltages. The 2 decimal voltages on the T8 have me feeling a bit underinformed at times.
ArthurEld
Solar Wizard
I'm fairly sure SherylinRM wasn't interested in what I was saying. But I'm kind of an old prick so who knows.It's a one time thing. After the top balance is finished one should never have to do it again. Heck, I plan to try to keep my cells at a 70% SOC after I have tested them for capacity. This might take a few cycles.
So you said you learned what you need to know. I am curious what did you learn? I think that's what SherylinRM was asking too. If you don't want to share what you learned that's fine but it could be helpful.
I just learned that the amp hours above 3.6V aren't worth worrying about. At least for me.
I want to run all of my cells through the same test. I have decided not to charge my cells above 3.6V and I won't do any absorb.
I don't need to test the unusable part at the bottom either. Since we don't bottom balance, I'll test to 2.8V.
I want to test the usable part. The BMS can worry about the part I won't be using.
Similar threads
