bottom balancing 32650 LiFePO4 question

I am starting to bottom balance my cells to set up a 4s20p pack.
I have a iCharger 206B and have the discharge settings at 2.7V and 5A(1C).
As soon as the voltage hits 2.7 the discharge stops but the batter voltage starts creeping back up to around 3.1V
Is this normal and is it considered bottomed out enough?
I've run the same cell twice and it always bounces back up.(I let itrest 24 hours after the first discharge)

Thanks

That's the bounce back or the recovery voltage. All batteries will do that.

Under load they will drop a certain amount. When the load is removed, it bounces back up a bit. So yes, your charger is doing what it's supposed to do.

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And actually, if a cell "didn't" bounce back, it's a bad cell and you don't want to use it for anything. It means it's about dead dead.

bottom balancing is usually done in 2 stages.

Stage 1: high Discharge Usually around 2*Capacity Down to 2.5V Let it rest for a couple hours
Stage 2: Low Current Discharge .5 *Capacity down to 2.65V and Again let rest it should recover to 2.8ish.

Once all the cells are done check the voltages after resting for a few hours and Discharge any High cells to try and get withing 0.05v or less per cell difference (total highest to lowest)

Recovery voltage is the true voltage to base yourself on the SOC of the cells. As an example, while many ICs cut off voltage at 2.5V, that is at load. At 100% of SOC, the recovery voltage is 3V. So that 2.5V is actually pretty realistic under load.

At 5A this is normal. If you don't want the cell to do that you have to choose a smaller discharge current. And you probably don't for bottom balancing. This is related to the usable amount of capacity under load. Well, not only related, it is the same thing really.

Any given cell has its capacity rated for a specific discharge current. This is usually a rather small current. Because the voltage will drop under load AND with sinking state of charge the cell will reach the cutoff before it is actually empty because the two factors influence the cell voltage. You have to rule out one of them to use the full capacity. Since voltage drop with sinking state of charge is an inherent property of the cell, and as such you have no influence on that, you have to rule out voltage drop under load. This is done by choosing a small discharge current. Unless you have a cell that is VERY load resistant, which means it barely drops its voltage under (heavy) load.

Bad cells can still have a very good capacity. If we take a 18650 as an example it may have 2000mAh. But at 1A discharge is has almost 0mAh capacity because it isn't very resistant to loads at all due to age and degradation.

Korishan said:

And actually, if a cell "didn't" bounce back, it's a bad cell and you don't want to use it for anything. It means it's about dead dead.

Click to expand...

Actually, no. This is the ideal dream scenario because it means you have used all the capacity. The better the cell the less bouncing around happens

Yeap less bouncing = Better cell and in generally lower IR.

So is it worth doing two discharges, one at 5A and another at 2.75A?
This pack won't be used for any high draw scenarios, just to run lights and such on a camper and out building, all 12V.

If you want to categorize them in terms of discharge current yes that could be the case.

Yes, no, maybe! It depends on what you want to do and how accurate it should be

If you want to properly bottom balance the cells you probably want to use an even lower current for the 2nd discharge (or a 3rd discharge) to closely match the cells. It also depends on the cells if they play along or not. You have to do some fine tuning at the end anyway, the major discharges are only needed to empty the cells. It doesn't matter if it is 5A and 2.75A or any other combination of numbers. You want to get them to the exact same idle voltage if possible and this voltage should be as low as the cell allows or you need for your application.

Another question is why do you want to bottom balance them? Do you want them to be bottom balanced because the voltage is too high when the cells are top balanced or is there another reason?

daromer said:

Yeap less bouncing = Better cell and in generally lower IR.

Click to expand...

Hrmm, ok. So, it's kinda like holding a heavy object. You want to be able to hold that object in the same spot for as long as possible. Not allow it to drop. If it drops, you're actually weaker than if you had held it in place.
If your arm lowers over a period of time to be almost pointing straight at the ground, let go of the weight, and you can bounce your arm back to parallel with the ground, this is weak.
If your arm only lowers a little bit (over the same course of time), and let go of the weight and your arm only moves a few inches/mm's, back to normal state, then you are stronger.

Is that a better understanding of the bounce back?

Dunno...

Have nothing that can help understanding it with comparing. If you have high resistance you have high voltage drop. If a cell have a high IR it will meassure low voltage on high current. If it cannot deliver the current or move the current to the ends it will look empty and when you remove the load it will equalize internally and the voltage raises back up again.

It would be like holding gely or something thick in a bowl. If you try to get it out it will not be even inside and you cannot meassure the height inside since its uneven. But over time the gel or whatever you have will settle again and you can meassure

Take 2 bowls. 1 with jelly and one with water... The one with water you can get the water out faster than the one with Jelly. The one with water with settle faster in terms of being able to meassure with mm or cm how much water is in side than jelly...

My thinking is to discharge each cell individually to 2.7V and then assemble and charge parallel packs to 3.6V. I'll set my minimum voltage cutoff at 2.7 volts for the 4s20p pack. From what I understand this will be a bottom balanced pack and I will not need to run a battery management system, only monitor voltage and protect the pack from over-discharge with occasionally measure each series pack for weakness.
If I'm my plan/thinking is off please let me know.

Just because you bottom balance that doesnt relief you from using a BMS. You still need to monitor the top voltage. If they are just slightly uneven in capacity and you plan to charge to 3.6... Lets say 3 of the cells isnt full. They will then be at 3.3v and lets say 1 is full before all others...

then you got 0.9V to hand somewhere... then you potentially have 1 cell at 4.5V.....

So just because you bottom balance you dont get anything back in terms of trying to charge them up to the top. For LiFePack that have a very flat voltage to SOC curve they wont peak up untill you actually get them full so if you dont have something to monitor them by then you will get cells overcharging.
Its generally better to top balance packs if you even consider not having BMS or monitoring of each cells since then you theoretically dont overcharge them atleast. Yes at bottom end you might over-discharge instead but where do you plan to keep your cells? At top or bottom? Most of us want to be able to keep em charged

I think you should rather build a 7S battery, top balance it and use a 12V DC-DC converter to power your stuff.

The approach with bottom balancing isn't very effective. You have to put in the effort to discharge the cells to the same low voltage level. Then you build the parallel packs and put them in series. And then you charge the battery and watch for the capacity or time until the first pack reaches the desired state of charge. The parallel packs will probably have different voltages then, but they will discharge down to the bottom voltage where they will be even again. That means you don't have to BALANCE them everytime you charge them, but you still have to monitor the voltage so you don't overdischarge them.

Got it. I just got sidetracked on the concept of bottom balancing as a way to avoid a bms of some kind.
Thanks for explaining this to me. I'll plan on balance charging with the 206B and then hook up some type monitoring system to watch each parallel pack and cut off the load if the voltage drops to 2.7V on any pack.
Any recommendations?

Batrium? Nah I havent tested any other I can recommend im afraid. I have 3 incomming in my mailbox though for testings...

TinyBMS looks interesting, but I haven't used it myself. Also, the 206 can't charge 7S batteries. If you go that way you need another charger, like the 208.

Yes, Batrium would work but I don't know if it would worth the setup for a small portable pack. I'd rather get the right equipment first though and not have to worry about replacing it later.

I got the 206 kind of as a dedicated charger for this project and when I get started on a larger pack later I'll get something else.(kind of like backups)

Got my nickel strips in today so I'll be working on the pack this weekend!

Bottom balancing is a strategy for preventing an individual cell or modulein a series string from going too far over the knee into the steep part of the discharge curve, where going too low (below 2V) can quickly drivethe cell or module into reversal, which is irreversible damage. If you want to bottom balance, you have to find the State of Charge at which the first cell reaches your cutoff voltage and subsequently set your charger to the voltage of the entire pack at that point and hold it at that voltage (Constant Voltage and Float). 3.4 or 3.45Vis a good place to stop, at which you can't overecharge any cell in the series. In a Series Circuit, the current is the same through each component in the series. In Constant Voltage--and Float is Constant Voltage, too--the current will taper to what the highest cell will accept at that voltage. With Bottom Balancing, an arbitrary EMPTY has been established. Each cell or module in series is at the same state of charge(voltage)AND capacityat that point. In charging, each cell or module in series gets the same amount of current and thus has the same capacity throughout the cycle, though not necessarily the same State of Charge (voltage). The weakest cell will reach the top (cutoff voltage) first. As that cell saturates, the current will be limited by what that cell will accept at that charging voltage. The voltage of the stronger cells in the series will continue to rise. If any reaches the same voltage, they'll saturate at the same rate. Some ccells in the series might not get all the way up there, but it's okay, because they'll all have the same capacity.

Limiting the charging voltage to 3.4-3.45Vprevents damage after saturation due to heat, which begins after saturation at about 3.6V. This stratEgypt makes sense for solar, where you want to use available solar power after your battery bank is topped off, so that you can save the stored energy for when the sun isn't shining. You'll still need to monitor the cells, because shit happens, but after the initial bottom balancing the cells should stay in balance for a long time.