Bottom Balancing Batteries (to avoid using a BMS)

DarkRaven

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Disclaimer:
What you are going to read here is specifically aimed at people new to the business. The method described is a possible solution to run a battery without a BMS in certain situations. It's not suited for all applications so choose wisely. And under no circumstances it's meant to relieve you of your responsibility to monitor the battery from time to time to make sure it still runs within the desired and/or requiredboundaries.
And as this is aimed at people new to the community you might want to make sure you've read and understood the FAQ first:https://secondlifestorage.com/t-Frequently-Asked-Questions-FAQ
I won't cover the details explained in the FAQ here, they are considered are prerequisite. Make sure you don't try to replicatewhat you read about here if you don't understand all the details. The method described here is safe as such, it even aims at specifically operating a battery in a safe manner. But as it is always the case when using lithium batteries there is a potential risk of damaging the cells, resulting in partial or total loss and maybe even a fire under extreme situations. It's your responsibility to know about proper handling of lithium cells and about the dangers if they are mistreated.

One of the more popular questions, especially among the new folks, is "Do I need a BMS?". The reason is usually price, complexity, lack of trust in cheap BMS and so on.And the answer is always the same because it depends. For un-supervised operation 24/7 the answer is yes, for example for a powerwall. So if you're building a powerwall and asking yourself if you need a BMS, then yes, you probably do. You don't need to read any further in this case, you won't find any more details about this later on in the text. But not every battery built ends up in a powerwall. People are building batteries for many different applications that isn't a powerwall. If you're one of them you might find this interesting.

Now, there are several ways to bottom balance a battery and I certainly haven't invented any of them. You can easily find more information on the internet but I thought it might be useful to have something ready locallyto send people towhen thequestion about the necessity of a BMS comes up. One of the answers is always that bottom balancing might be a solution but it's too complex to explain it over and over again. I will go over one possible approach of bottom balancing in this example.

To understand what bottom balancing is and how it might help us we should go over theother balancing method first which is top balancing. Just by comparison of the names it should get clear immediately that they are the opposite of each other. Top balancing is what most balance chargers / RC chargers / hobby chargers / smart chargers do. There is a main charge followed by a balance charge through the balance connector. As soon as the first cell reaches the end of charge voltage, usually 4.2V but it could be any other voltage as well, the main charge stops and the balance charge is used to top off the other cells so that they reach the end of charge voltage as well. The battery is then top balanced, all the cells are at the same voltage at the top of their state of charge. This is perfectly fine and works very well. If you have a new battery with more or less matched cells it will continue to work well for quite a while and maybe you don't even need to balance charge on every cycle.The cells will discharge evenly and will stay close in voltage during use and also on the subsequent charge when the voltages rise again.

But with age and cycles comes the drift. Cell voltages aren't balanced anymore because ageing and deterioration will be different for every cell. Even identical models will start to develop differences at some point. The manufacturer only guarantees operation within the specifications for a certain amount of cycles. After that it becomes an unknown but the cellsarestill very usable. You can continue to top balance but you will find that charging will take significantly longer compared to the charging cycles when the battery was new. This will be especially significant on batteries with high capacities. The balancing currents will be rather small and it might take a while on high capacity cells.

On discharge the cells will then start to delevop gaps between their voltages. This is where you have to be careful because the total voltage of the battery doesn't represent the individual cell voltages anymore. At least not very well. This could lead to damaged cells if you are not careful or have a proper monitoring and/or low voltage cutoff.

Example:
A 4s battery, 14.8V nominal, 16.8V fully charged, 10V fully discharged (could be 11V or 12V as well, depends on the cell specifications, we use 10V for the sake of the argument).
At 10V it's not necessary 2.5V for all four cells. Could be 1.8V, 2.6V, 2.6V and 3.0V. It might look likean extreme case but it is not completely unlikely. Actually most batteries tend towards such a state later on in their life. The reason is that the cells have very little energy in this voltage range. Even small differences could mean significant differences in voltage.
And the more cells you have in series the bigger the effect will be, the total battery voltage won't tell you anything about the state of charge. It might get worse with chemistries that are notoriously difficult to judge by their voltage, like LiFePO.

This is the point where bottom balancing might be a solution. But it's not limited to used cells. It might be beneficial even if you're using new cells. So instead of balancing the cells at the top we are now going to do it at the bottom of their state of charge. It involves some effort but only once. And after that you don't need a BMS or a balance charger to keep your battery within a certain voltage range.

As the name suggests we're turning top balancing around by 180. The cells will now have the same voltage near the end of discharge and different voltages near the end of charge where it doesn't matter.

2nd disclaimer:
The figures I'm presenting here are for display purposes only. They are valid for this case but only for this case.Yours will be different, every case will be different. It's just about the procedures used in this method of balancing.

This is where we are starting:

image_qjmjgm.jpg

You see some rather expensive equipment here, namely the iCharger 4010 and the Fluke 115, but that's not necessarily needed. I will describe what I did and not how exactly I did it. You can fill in the blanks depending on what you have available. It might be easier with some devices while it's more difficult with other, but generally there are always several possible ways available to achieve what's necessary to bottom balance the batteries.

On the picture you see a 6s LiPo battery. The serial connections are accessible on this one which is necessary. As this is aimed specifically at people building their own batteries I assume that you always have access like this. If you don't you have to improvise. As I'm not aware of your problem in particular I'll leave it up to you. For example, everything you see here could be done through a balance connector as well (if you have a premade battery / a battery that you used to top balance in the past). If you're still in the process of building your battery you could match the cells prior to assembly.

The battery has been discharged prior to this attempt ofadjusting the battery for bottom balancing. The lowest cell voltage under load (just 100mA) was 2.95V. After resting we're presented with these voltages:
#1 - 3.542V
#2 - 3.386V
#3 - 3.424V
#4 - 3.245V
#5 - 3.457V
#6 - 3.577V

#4 is the lowest, #6 is the highest and there is a difference of 0.332V between them. The total voltage is 20.631V and we consider this battery discharged. Our aim is now matching all the cells at the desired end of discharge voltage.
To do that we're discharging the individual cells, I choose 3.0V for a start:

image_ngrwbl.jpg


image_tenwrt.jpg


You see I choose 1A discharge current, but I've set the charger to CCCV discharge. It won't stop the discharge when the end of discharge voltage is reached. Instead it will lower the current. The reason is that we don't want the cell to recover over time after the discharge stopped.This is the exact opposite of what you usually do to charge lithium cells. As I said, you don't need an iCharger for that. It offers this as a convenient solution but it's not exclusive to this device. For example a much cheaper IMAX B6 will do a CCCV discharge per default. Or you could just use any suitable load.

What we want is the same low voltage for all cells.
Here are the results of the first discharge with the amount of capacity discharged and the time it took:
#1 - 195mAh, 18m
#2 - 87mAh, 7m
#3 - 108mAh, 12m
#4 - 33mAh, 3m
#5 - 130mAh, 13m
#6 - 243mAh, 22m

As you can see our highest cell #6 took quite a while to reach the target while the lowest cell #4 was pretty quick. But even after the CCCV discharge with very low end current the cells recovered a bit. I've expected this as these are pretty good and almost new cells. We've put them prettyx close together though:
#1 -3.269V
#2 - 3.251V
#3 - 3.241V
#4 - 3.216V
#5 - 3.260V
#6 - 3.263V

I've then settled for 19.5V future end of charge voltage for this battery, 3.25V per cell. So I only had to do some micro adjustments of all cells to 3.25V. This doesn't have to be perfect but obviously the more accurate you work here the better and consistent the end result will be.

After that I charged it back up again:

image_nizaed.jpg


As you can see, no balacing there. The charge current should be appropriately small as you have to monitor the cell voltages manually. I've used only 1A on these 6.4Ah cells so it took a while but it gave me lots of time to write down the voltages of all six cells when the end of charge voltage was reached. As the end of charge voltage I've gone for 4.1V. Bottom balancing isn't done once and then forever, the cells will keep changing. By leaving 0.1V headroom to their maximum end of charge voltage, which you might want to do anyway to prolong cycle life of the cells, you leave room for further drifts so there is more time between bottom balancing adjustments.

While charging you are looking for the moment when the first cells reached your end of charge voltage, in my case the 4.1V I chose. As soon as it happens you write down all the other cell voltages. In my case they were:
#1 - 4.075V
#2 - 4.090V
#3 - 4.090V
#4 - 4.100V
#5 - 4.090V
#6 - 4.072V

Adding these together will give my future end of charge voltage of 24.517V so that cell #4 doesn't go over 4.1V.
I've then finished the charge on a DPS5020 DC-DC converter:

image_iihbjr.jpg


Chargers like the iCharger work on a per cell voltage, you can't just set a target voltage for the battery. I'd have to set 4.086V per cell (6x4.086V =24.516V) to imitate this but I've just used the DPS5020 instead. Any DC DC converter with CCCV is a decent and cheap way of charging lithium cells but there is no automatic cutoff so watch out for that. The DPS5020 isn't super accurate to the last digit so the battery ended up at 24.55V instead of 24.52V but that's fine. We've taken this possibility into account when choosing 4.1V instead of 4.2V end of charge per cell.

And then we are done, we can now run the battery between 19.5V and 24.52V and know the cells will stay within the desired voltage ranges. You now know that at 19.5V the battery is empty, depending on whatever you want to run you can set this as the cutoff voltage. If you have a device with a fixed low voltage cutoff you could adjust the voltages in the process described above so that it fits your device. This will be useful for most inverters, for example.

As I said, this is just one possible approach. You could also use capacity based or time based charging instead after matching the cells, just to give you some ideas.
I hope this helped and you found it useful. If so feel free to leave a like or post your feedback so I can see if this is useful or not. If not leave any questions about bottom balancingyou still have and I (or others) can try to answer them.
 
Very interesting concept. Thanks for sharing. I wonder how well this would work for larger capacities with larger differences between packs.

However the most useful application for this, as far as I can see, would be being able to very accurately measure differences between pack capacities. If you assemble a large pack out of cells tested individually, (especially as many cell testers used can give results of +/- 10%) it would be a quick and easy way to measure the difference in capacity of the finished pack.
 
rev0 said:
I would be interested to see the voltages at top and bottom state of charge over a large number of cycles. In my testing I found that a bottom balanced pack will slowly drift out of balance after about 30-50 cycles: https://secondlifestorage.com/t-Pack-Cycle-Testing?pid=18342#pid18342

Indeed. However your cells were very closely matched. It would be interesting to see how this played out in a larger pack, where secondhand cells are used, and differences are definitely going to be larger.

All said, 60 cycles is a lot. 30 cycles is quite a few. Also that is complete cycles. I wonder if they were only partial cycles if results would change. Would the number of cycles increase, and if so, how much?
 
All valid points. I think DarkRaven needs to do another test with mismatches cells and see how long varying DoD's effects the drift time ;) I'm sure he doesn't have anything else better to do than to watch cells charge/discharge :p He could probably sell tickets
 
Korishan said:
All valid points. I think DarkRaven needs to do another test with mismatches cells and see how long varying DoD's effects the drift time ;) I'm sure he doesn't have anything else better to do than to watch cells charge/discharge :p He could probably sell tickets

You wouldn't have to partially cycle too many times to get a rough idea of what sort of drift you would actually get. Also cycling from say 3.7v to 4.0v should give a good idea, and not take nearly as long as full cycles. Especially if you conducted your experiment with high discharge, low capacity cells. Although there may be variations between chemistries.

I cannot think of a way to test this theory, but I wonder which side of the cycle causes the most drift, the charging, or the discharging.The significance being, what role is IR playing in cell drift? If you charge and discharge slowly, will it 'soften' the effects? rev0's tests were conducted at relatively high currents. 4a Charge, and 2a discharge.
 
You are raising a valid point there. Like top balancing the balance won't stay forever. Sure, if you are top balancing at every charge, then it will, but that's not what this is about. You might have to make adjustments over time to keep your bottom balance. Obviously I don't have cycle results yet but I will keep an eye on the cells.
Also I have one or two other batteries that could use a bottom balance. One is a LiFePO battery with middle age cells and the other a LiIon with used 18650s.
 
DarkRaven said:
You are raising a valid point there. Like top balancing the balance won't stay forever. Sure, if you are top balancing at every charge, then it will, but that's not what this is about. You might have to make adjustments over time to keep your bottom balance. Obviously I don't have cycle results yet but I will keep an eye on the cells.
Also I have one or two other batteries that could use a bottom balance. One is a LiFePO battery with middle age cells and the other a LiIon with used 18650s.

Something else to consider is how many in series. rev0 used 8s. You only have 6s. It is easy to see that less cells in series would leave less room for drift to cause a problem.

Also, it is interesting to note that in rev0's experiment, it appears one cell drifted significantly more than the others. I wish I had an internet connection stable enough to download his results. It would have been interesting if he eliminated this cell and continued to cycle the remainder, or perhaps manually balanced it, and continued.
 
That's correct, I'm sure I've also mentioned it somewhere in the text. But this is a description of the method, it's not about the actual results. I've never expected these cells to get seriously out of balance anyway. But if you have cells that do drift it doesn't really matter how many there are in series. It becomes a problem even on 2s batteries.
 
With every respect to all contributors, i will "contradict" with the following statement, to get the discussion ongoing in details:

Whatever balancing state you use (bottom, top or anything in between) may change the validity of the voltage regerading SOC, but it is no implicite measure agains any sort of drift, which may get the pack out of the balance chosen. Basic reasons for "drifting", which come to my mind first, are Self discharge and inner resistance differences (which causes losses).

Conclusion, when you get drift of the said (or other) reasons during lifetime of the cells, you will need re-balancing sometimes.
 
If it is self discharge then your battery is faulty and you will never be able to do anything against it with any sort of balancing. You need to fix the issue rather then fix the symptoms. Internal resistance, yes, and also different degrees of degradation and remaining capacity.
As we've established, and I've also pointed it out somewhere in the text, you will need to check for balance and do some rebalancing eventually. But that's generally true for all sorts of balancing unless you balance at every cycle.
 
DarkRaven said:
If it is self discharge then your battery is faulty and you will never be able to do anything against it with any sort of balancing. You need to fix the issue rather then fix the symptoms. Internal resistance, yes, and also different degrees of degradation and remaining capacity.
As we've established, and I've also pointed it out somewhere in the text, you will need to check for balance and do some rebalancing eventually. But that's generally true for all sorts of balancing unless you balance at every cycle.

I can do nothing else but totally agree, with the tiniest addendum that self discharge might slowly and tiny "creep in" to your pack during lifetime, and in the beginning you will fight/fix it with balancing without even knowing about it.

And if SD behaves as real world does it will not be concentrated on one pack, but all packs will start more or less (in small quantities). Then balancing is only necessary to fix the differences....
 
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