Top AND bottom balancing?

[stig44]

Thanks for accepting me as a member! My first thread:
I have a bank with 16 CALB 200Ah LFP. Some running away when charging, some running away when discharging.
If I first do top balance and second do bottom balance, will the cells be like new again? Or will all be balanced to 2,5V but still some run away when charging? Is it the latest balance that counts if a cell do not have 100% capacity?
Suppose this is a chemistry issue?

 

[FilterGuy]

A set of cells can only be top ballanced or bottom balanced. Not both.

- If you top Ballance and then bottom balance, they will be bottom-balanced and they will NOT be top-balanced
- If you bottom balance and then top balance, they will be top-balanced and NOT be bottom-balanced.

For home solar applications, you want to have the cells top-balanced.
You want to set the BMS balance to only start above 3.4 or even 3.5V. Below that voltage, the charge curve is so flat that a balancer is likely to make the cells less balanced.
You want to set the accumulation time on your charger for at least an hour. THis will give the balancer time to do it's thing.

 

[stig44]

Thanks a lot! Valuable information!

 

[Warpspeed]

The cell capacities will never be exactly the same, so as already said, its not possible to balance both ends.
The weakest cell will reach full charge first, and crap out first during a deep discharge.

Generally we try to fully recharge each day, and hopefully very rarely reach full discharge, a top balance makes more sense.

 

[webbbn]

I am currently trying to top-balance a large 16S LiFePo4 pack. It's former life was in some sort of EV, so it's not something that I can separate out the cells to do a proper, parallel top-balance. Since it's had some use, I don't expect to get it perfect either.

What I did so far is put it on a charger and charge the top cells to around 3.5V, I then let the cells balance a bit (JK BMS 2A balance), and have since been repeating the charge / balance process to try to get the voltage on the laggards up. There's currently over 0.3V difference between the front and back of the pack, so there are definitely some balance issues.

Where I'm at now is with 4-5 cells hitting near max voltage at 3.65, and 3-4 cells struggling at around 3.35V. The rest are in between somewhere. The pack voltage in around 55.5V (if I remember correctly).

I see, what appears to me to be conflicting information on LiFePo4 batteries, in that everyone says that voltage is unreliable as a measure of SOC, yet top-balancing to a specific voltage is important. There's a chart posted in the stickies that shows my pack voltage being 99.5% charged. Is top-balancing only going to get me 0.5% or so more capacity? Is there a way to know if the lower-voltage cells are currently holding less charge, or are just naturally lower-voltage at full charge?

In any case, I want to do what I can to get a top-balance, but don't know quite how to proceed. I would prefer to continue charging as I am, since it will help me keep a balance in the future, but how is it best to proceed? When I charge, the top cells are going to go over voltage, so it seems like all I can to is charge until they get to 3.65 and then balance, which is essentially what I'm doing.

Would/could it help if I discharge the pack and start over again? It might be easier to draw power out of individual cells, than to individually charge them. If the top-performers are still higher after discharge, would it be beneficial to pull some energy out of them? I should also be able to charge individual cells if necessary to bring them up faster. I just need to be careful I don't short out the pack by tapping into the middle of it.

 

[Warpspeed]

The story is, as the battery charges, its amps x hours that puts the charge into the battery.
The voltage changes very little over the "flat" part of the voltage charging curve.
Its the flat part of the curve that contains most of the amp hour capacity.

Once you get up to about 3.4v the voltage curve starts to gradually bend upwards.
At around 3.45v its starting to rise quite noticeably.
The higher the voltage, the faster the rate of voltage rise becomes during charging.
The voltage then just takes off, and up around 3.6v and higher it just goes ballistic.

Now if you think about all this, stored charge is amps x time (amp hours).
If it goes from say 3.6v to 3.65v in only a few minutes, the extra stored charge over that few extra minutes is going to be absolutely minimal.
In fact going over 3.65v will definitely shorten the life of the battery, and its just not worth the risk.

The point at which each cell goes into the sudden dramatic voltage rise mode will be different, depending on the state of charge compared to all the other cells. Some may still be well down in the flat part of the curve, and the cells with a higher state of charge will be the ones that start to go into serious voltage rise first.

Top balancing means we need to drain off some of the charge from the highest voltage cells, and add charge to the lowest voltage cells.
The higher the voltage we top balance at, the larger the voltage difference we will see between cells.
But trying to do this at a VERY high voltage such as at 3.65v can increase the risk of cell damage if a cell goes well over 3.65v by accident which will happen very quickly.

Anyhow, charge up your cells to something much less than the absolute safe maximum, say 3.45v or 3.5v average cell voltage, then discharge any cells that are a bit high with a one ohm resistor to bring them back.
Charge individually any cells that are below the average at a low rate, perhaps two or three amps to bring them up.

This will need to be done after a normal night time discharge followed by a full normal solar charge, where charging current has fallen to zero.
If its a cloudy day and you cannot reach full charge, skip balancing until the next day.

Its very strongly recommended you only correct one cell at a time (one cell per day) then just let the whole battery go through another normal 24 hours of operation before correcting the next worst cell.
Gradually the voltages will all become much closer at the end of charging, and with a lot of patience can end up being within +/- 5mV or +/-10mV.

Doing it one cell at a time allows all the other cells to adjust. For instance if you charge up one cell to be 160mV higher, all the other fifteen cells must each be an average of 10mV lower at the same total battery voltage. Tweaking one cell effects all the others.
If you do it one cell at a time, you will definitely make steady progress.

If you try to correct several cells at the same time, you will have chaos, and end up chasing your tail, and it all becomes a frustrating waste of time.....

 

[FilterGuy]

But trying to do this at a VERY high voltage such as at 3.65v can increase the risk of cell damage if a cell goes well over 3.65v by accident which will happen very quickly.

If the power supply is set to 3.65v before connecting them to the cells, as each cell reaches that voltage, it will taper off the current but there is little danger of the voltage going higher unless someone tries to adjust the voltage after it is set.

Its very strongly recommended you only correct one cell at a time (one cell per day) then just let the whole battery go through another normal 24 hours of operation before correcting the next worst cell.
Gradually the voltages will all become much closer at the end of charging, and with a lot of patience can end up being within +/- 5mV or +/-10mV.

I do not understand this recommendation. If you use the battery between topping off each cell, the cell that just got topped is no longer at a known top point. Yes, if you keep doing it you will probably eventually get them close, but never as close as if you did them all at once.

If you try to correct several cells at the same time, you will have chaos, and end up chasing your tail, and it all becomes a frustrating waste of time.....

That has not been my experience. The key is having patience with the process.

 

[mikefitz]

What I did so far is put it on a charger and charge the top cells to around 3.5V, I then let the cells balance a bit (JK BMS 2A balance),

Over time things should improve, since it was an EV pack it most likely was a bottom balanced pack, so inbalance at full voltage is to be expected. If you can hold the charge voltage constant with the high cells around 3.50 for several hours at a time, the 2 amp balance should have a useful effect.

 

[Warpspeed]

If the power supply is set to 3.65v before connecting them to the cells, as each cell reaches that voltage, it will taper off the current but there is little danger of the voltage going higher unless someone tries to adjust the voltage after it is set.

We are not charging one cell with a 3.65v voltage regulated supply.
We are charging sixteen cells that are connected in series to a solar charger.
One cell can very easily exceed 3.65v if you are not careful.

 

[webbbn]

A bit of an update. I didn't have much time to deal with it for the last few days, so it mostly sat balancing, and balanced out very close, but at under 3.4V / cell. This evening I started charging it slowly at about 10-15A, and the BMS has been able to mostly keep up. Most of the cells are now at or very close to 3.6V, with 6 cells currently just under 3.4V.

The total voltage is at 56V, which is what I have the charge voltage set at, so it's just trickle charging and balancing. I might boost the charge voltage a bit more, but the plan is to mostly just let it balance through tomorrow until I get more time over the weekend to work on it.

It seems a bit odd that the batteries have divided into two fairly distinct groups, with no obvious reason. The wire resistances are all fairly close, and the positions of the laggards don't show any pattern that I can see.

The low voltage cells are: 6, 9, 10, 11, 14, 16. The 6th cell is at the end of a group, and 16 is at the end of the pack, but the rest are not distinguishing.

 

[Skypower]

A bit of an update. I didn't have much time to deal with it for the last few days, so it mostly sat balancing, and balanced out very close, but at under 3.4V / cell. This evening I started charging it slowly at about 10-15A, and the BMS has been able to mostly keep up. Most of the cells are now at or very close to 3.6V, with 6 cells currently just under 3.4V.

The total voltage is at 56V, which is what I have the charge voltage set at, so it's just trickle charging and balancing. I might boost the charge voltage a bit more, but the plan is to mostly just let it balance through tomorrow until I get more time over the weekend to work on it.

It seems a bit odd that the batteries have divided into two fairly distinct groups, with no obvious reason. The wire resistances are all fairly close, and the positions of the laggards don't show any pattern that I can see.

The low voltage cells are: 6, 9, 10, 11, 14, 16. The 6th cell is at the end of a group, and 16 is at the end of the pack, but the rest are not distinguishing.

You can manually assist boosting the low cells but it will still take hours (4-6) with a bench top power supply. RULE #1 You can NOT leave it for a moment (connected) if you do this! In order to get the power supply to output its highest amp output, you will probably have to set the voltage higher on the supply than what’s safe if the cell actually was left to charge unattended. The reason why you have to watch is because when the cell does run, it runs very very quickly. If you have to walk away or talk to someone, disconnect! Charge the low cell up so it’s voltage is slightly higher than the average, then move to the next low cell and repeat. Cell Bop-a-mole. If you are using the bms app to monitor the cell progress, always be sure you are looking at live data, not frozen! I’ve seen both the JK app and the Xiaoxiang apps do this. Data is always changing to be live while charging. You may want to use larger leads and better alligator clips than those that came with the power supply because they are usually too small and waste time.

 

[webbbn]

I'm okay with slowly letting it balance for now, and I keep a close eye on it even if it's charging the full pack.

I recently bought an RD6024 with the intent of using for this purpose, since it should be able to charge individual cells as well as a full pack at a not too bad rate. It didn't come with leads, and I've been wondering what I should use for leads. The jacks that came with it look pretty good, I have, or can make some decent leads from some banana plugs that I have that were designed for high-power speaker connections, but it can output over 1kw, which still seems like alot.

 

[Skypower]

I'm okay with slowly letting it balance for now, and I keep a close eye on it even if it's charging the full pack.

I recently bought an RD6024 with the intent of using for this purpose, since it should be able to charge individual cells as well as a full pack at a not too bad rate. It didn't come with leads, and I've been wondering what I should use for leads. The jacks that came with it look pretty good, I have, or can make some decent leads from some banana plugs that I have that were designed for high-power speaker connections, but it can output over 1kw, which still seems like alot.

That pack you got was probably retired because the bms’s balancer couldn’t deal with it. It was probably mistreated by not allowing enough time for balance above balance voltage. I’m betting that it will recover but it going to take your kind of attention. The JK is an excellent bms and has good balancing power. Keep in mind it works one cell at a time, either depleting or adding as needed. Once you’ve got things balanced, all it’ll take to keep things happy is to get the batteries to absorb/full a least a few times a week for about half hour / 45 minutes. My settings: float 54.2 absorb 55.2 start balance 3.41(JK’s 2amp) Note; when commissioning a group of cells I get them balance at 56V then drop it back to 55.2 thereafter. Also note; to get the JK to learn what full state of charge is, you simply just lower the OVP and OVPR(lower value) temporarily so the highest cell arriving at absorb will trigger them. For example with a 55.2 absorb, temporary OVP 3.46 and OVPR 3.45. Once you see “Battery fully charged” notification on the app you can set the OVP & OVPR to higher values so that they don’t unnecessarily trigger. You may have to do this procedure more than once to make it learn.

 

[webbbn]

I've finally started top balancing individual cells, and I think it's going to have a significant affect. I did a couple of capacity tests of the entire pack, and it came in around 10kWh (max new capacity should be around 14kWh), charging the top cells to 3.6V and fully discharging, with one of the laggards, of course, stopping the discharge early.

I spent some of yesterday evening and all morning charging the worst performing cell at 8-9A. I didn't track the time exactly (although I could probably figure it out), but I'm sure I put 50Ah+ into that cell, which is rated new at 270Ah. I charged another one similarly, and it took all afternoon. I stopped at just over 3.4V, which is where the other cells are currently at, and when the voltage was starting to rise fairly quickly.

I now have 3 remaining low cells that are all in a row. Is there any problem with charging those in parallel (at least initially) at, say 11V? Seems like it aught to be okay, and it should significantly speed up the process.

 

[Warpspeed]

Excellent !
You will probably find that the cells you have brought up will drop back slightly after a few normal daily charge/discharge cycles, but they will be a lot better. Its just a case of slowly nibbling away at it bit by bit, and eventually it will not only be top balanced, but should then stay that way for a very long time.

 

[webbbn]

The RD6024 is pumping 20A at 11V into those three cells! I'm pretty impressed with that power supply.

 

[webbbn]

Just did another discharge test after finishing top balancing to 57V. I charged my EV at around 80A (DC-side), and was able to get 230Ah out of the battery. I stopped charging when the voltage started to near the 48V bottom limit that I have set on the inverter. All cells are still over 3V, with around 70mV of deviation. I'll let the battery rest for a bit before I charge it back up to see where it ends up at the top.

I'm very happy with that!

Now I have to do the same for another set, which I'm not looking forward to, since it means unstacking 100 lbs+ boxes!

 

[Cuemaker]

280ah EVE 3.2v cells, 16s-48v, 250a Daly BMS, active balance set at 3.45v, cells top balanced to 3.6v 2 months ago.
Everything works great while system voltage is above 49v, below that, cells start getting way out of balance.
System is setup for off-grid with shore power backup.
At 6am batteries SOC=34%, volts=46, cells out of balance high to low .90mv differential.
How to I fix the problem?

 

[SeaGal]

Everything works great while system voltage is above 49v, below that, cells start getting way out of balance.
System is setup for off-grid with shore power backup.
At 6am batteries SOC=34%, volts=46, cells out of balance high to low .90mv differential.
How to I fix the problem?

TADTS

46V is only 2.875V per cell. No way is that 34% SOC. A spread of 90mV can be expected in the knees, especially at the bottom for cells that have been top balanced - the weakest cell will drop voltage quickly when that low.

 

[Cuemaker]

TADTS

46V is only 2.875V per cell. No way is that 34% SOC. A spread of 90mV can be expected in the knees, especially at the bottom for cells that have been top balanced - the weakest cell will drop voltage quickly when that low.

The 34% SOC is what the Daly BMS software was telling me.
Going change some settings, top balance all the cells & put in back on-line.