4S No-Balance Challenge!

[Substrate]

dih dah dit !

Balance Obsession:

What this reasonable delta of 100mv or less, without worrying too much about it points out a problem that most are totally obsessed about achieving exact voltage parity when balancing. When that can mean squat from a practical standpoint.

Example: You perfectly top-balance 4 of your 280ah cells to within a .000001mv delta. Fine.

Now replace one of those 280ah cells with a 50ah cell. Can you top balance THAT? YES you can. But of course your bank is limited to that 50ah cell in operation, being the weakest link in capacity and it will be worked hard. I personally did this with 3 Winstons, and one smaller GBS cell just to prove a point to myself long ago.

Cells are not perfect in either capacity, nor internal resistance. Ideally they should all be the exact same. Top balance is JUST to make sure that no cell goes over the threshold from a *voltage* standpoint, not a capacity standpoint. Your bms-balancer doesn't actually know anything about it from a capacity standpoint, (in the case of purposely mismatched cells as above), merely voltage. It also says nothing about cell-health.

Take 4 pristine 280ah cells (just because they are popular) that are vintage fresh from the factory. Swap one out for some bloated piece of crap. Can you top-balance that to 0.00001mv delta perfection? Yes you can! Is it practical - no. But a trap here is that there is a sales demographic being targeted for voltage-parity balance obsession with proven questionable practices. Like balancing at 3.4v. Or "balance all the time". Lots of dongles you can hang on. All the while FOOLING the buyer into a false sense of balance, merely because they can obtain voltage-parity balance at low voltages below or just at the start of the upper charge knee with minute deltas.

So yeah, I can live with a 100mv disparity - as long as that is measured while ABOVE 3.4v/cell as long as the system has proved stable with relatively equal cells.

But stuff happens, degradation and aging are different too, so you don't totally abandon it. You look in on it from time to time. You don't give it to your neighbor.

 

[sunshine_eggo]

IMHO, if a top balance allows one to charge to their chosen absorption voltage without BMS intervention, it is adequate - even if it's 0.25V (assuming 3.65V max and 3.40V min)

 

[Substrate]

YES! That would be my go / no-go extreme. Maybe make it 3.45v minimum to ensure one isn't hanging out in that transition area between the potential ability to recharge to full (whether you do or not). It depends on the cell.

Related: An old silver-haired A123 tech warned me about that. It's why if one floats (I don't care what anyone's reasoning for or against it is - gets buried in the noise) is that if you DO, it is best to avoid 13.6v, (3.4v) but make sure it is set lower. The degradation in float hangs on that potential knife-edge which can vary by cell. Go a bit lower. Like 13.5v. (3.375v / cell) Turns off that potential ability.

Likewise for balancing or normal operations, do not hang out at 3.4v. It needs to be higher. The results are variable at this knife-edge value. It's a chemical property of LFP at this voltage. Either the potential gates are open, or closed. If they are open, the cell is willing to be recharged, but never is. It's a real gray area to avoid.

@RCinFLA reminded me of this when in another discussion the word potential was used, but for a different topic. I didn't want to hijack that thread.

 

[Substrate]

I'm not suggesting throwing caution to the wind either. Now that the system has proven stable, go ahead and throw a bms on it that does it the old-school way without any tricks. Now you can give it to your neighbor.

The thread might also relieve some newcomers from a recent phenomena of being able to see balance in their apps. OMG! I have a 70mv delta! Geez, return this thing! Or if the cells are accessible, start buying all sorts of hang-on doo-dads with bad practices that serve only as voltage-parity eye candy.

Honestly, when I saw these bad practices coming out, my first thought was that this was targeted as a life-saving band-aid nanny to just throw any sort of random cells together with speaker wire, where they need that 24/7 life-support just to function.

 

[curiouscarbon]

yesterday i finally found/ordered some plated busbar to hook up my 25Ah navitas/topband cells.

currently operating a 4S pack with 17Ah headway blue jacket cells with JBD BMS. so fun to experiment with smaller packs.



for example, the 17Ah pack reached LVD recently but with three cells at 3.15 and one at 2.80. so i charged it up as a pack and manually connected a peltier as bleed resistor to the three high cells until they all reached 3.45+ together and now all sitting at 3.32 with 4mV delta at idle.

i'll go ahead and disable the passive balancing on the BMS and keep using the pack and see what happene ??️?

 

[Substrate]

Yep. The objective here is to be able to remove all that junk to prove a point.

It also relies on doing it the old-fashioned way at least once, with a programmable bench charger. You know the deal - charge each on up individually, discharge a bit, and then recharge as a paralleled set of cells. And then series wire for use. Whatever your means and methods.

((OR - since your bank is so small, you could use the erroneously-named "Lipo" balance charger, which does LFP just fine. Use the LIFE 4S Balance Charge setting. Needs balance-leads of course. Will never take a cell above 3.6v, because it communicates back to the charger (unlike a bms) to throttle current to keep it that way. Bleeders take over. Take a bit of time. Even the tiny iMax B6 AC v2 balance charger would do this for your headways just fine. Hint: in the 1s mode, no balance leads needed, so one could go that route too to emulate a single-cell charge and balance with one or more paralleled cells. ))

Which is if one has to rely on all that, there is a problem with cell selection, wiring infrastructure, or previous balance-forever damage, necessitating the need for it to be on forever now anyway!

If you aren't able to remove it, then use the other threads pertaining to active balancers, low balance triggers, etc.

Really don't want to noise it up here if we can.

 

[BekimV]

Interesting thread. I recently saw in one of the battleborn videos that if you connect 4 of their 12v batteries in series, they will balance itself without any external balancer. So i presume that a cell that is at 3.65v will self discharge faster that the one that is sitting at 3.45v. That is the only option that would make sense in this scenario.

 

[sunshine_eggo]

Interesting thread. I recently saw in one of the battleborn videos that if you connect 4 of their 12v batteries in series, they will balance itself without any external balancer. So i presume that a cell that is at 3.65v will self discharge faster that the one that is sitting at 3.45v. That is the only option that would make sense in this scenario.

Voltage bleeds off faster, but usable capacity/SoC does not.

have they updated their manuals to reflect this? Last I checked, they require you to periodically break the bank down and rebalance each 12V.

 

[BekimV]

Voltage bleeds off faster, but usable capacity/SoC does not.

have they updated their manuals to reflect this? Last I checked, they require you to periodically break the bank down and rebalance each 12V.

Here is the video:

I re-watched it and what he says doesn't make sense to me. Except if the bms balancer is constantly discharging all the cells when they reach 3.65v to a lower voltage. But i would not want to have that kind of bms in my system..

 

[Substrate]

The issue here is that this thread is about NOT using any sort of balancing bleeders *after* an initial sanity top-balance has been performed on individual cells.

This thread is about the old-school way of doing an initial top-balance yourself, and then using an external LVD of your choice. THEN, you adjust your CV to your liking. (13.5v if you never want to achieve a full charge - done *after* your initial sanity-check balance at higher voltages.)

So, the BB's and others with any sort of built-in bms on it, are not really applicable to the "Danger Zone!" category.

 

[sunshine_eggo]

Here is the video:

I re-watched it and what he says doesn't make sense to me. Except if the bms balancer is constantly discharging all the cells when they reach 3.65v to a lower voltage. But i would not want to have that kind of bms in my system..

Almost all BMS referenced on this site are this type of BMS.

Regardless of what he says in the video, the manual:

https://battlebornbatteries.com/wp-content/uploads/2022/10/BB10012-Manual-Edition.PIM_BB10012Rev019_09062022-1.pdf

recommends on page 9:

How to Maintain the Batteries
Battle Born Batteries require very little maintenance. If your batteries are in series and not being charged by a multi-bank charger it is recommended that you fully charge the batteries individually once a year if the system is used frequently. This will internally balance your batteries to ensure that they will reach their expected life span and allow you to get the full power out of them with each use. If your batteries are in parallel this is not necessary, just make sure the batteries are charged to 14.2V – 14.6V frequently for internal balance. Our BMS has a built-in passive balancing system that will take care of this for you.

 

[BekimV]

Voltage bleeds off faster, but usable capacity/SoC does not.

Voltage=capacity. Even if it is 1%. So if the cell at 3.65v discharges faster to 99%Soc than the cell at 3.45v to 98%Soc that means that the cells will eventually balance itself without a balancer if you charge them up to 14.8v.

 

[Substrate]

Gotta be careful - otherwise lurkers will turn this into urban-legend slapping cells together and charging up!

Needs qualifiers - "As long as the cells are reasonably close in both IR and capacity, accompanied by good wiring infrastructure, (clean bus bars etc), a reasonable delta of 100mv (.1v) amongst the cells is manageable - provided one has already done a sanity-check top balance first, and ensures that no individual cell under-charge, goes higher than 3.6v"

Technically, voltage does not necessarily imply capacity. One can reach full rated capacity between 3.45v and 3.65v - the only difference is the matter of how much time that takes in absorb.

 

[sunshine_eggo]

Voltage=capacity. Even if it is 1%. So if the cell at 3.65v discharges faster to 99%Soc than the cell at 3.45v to 98%Soc that means that the cells will eventually balance itself without a balancer if you charge them up to 14.8v.

Wrong. Voltage is also unusable surface charge. LFP does not have a strong voltage to SoC correlation as they require to be charged to a voltage higher than their operating voltage to accept 100% charge.

 

[BekimV]

Wrong. Voltage is also unusable surface charge. LFP does not have a strong voltage to SoC correlation as they require to be charged to a voltage higher than their operating voltage to accept 100% charge.

I meant to say voltage+amp hours equals capacity. The 100ah cell at 3.65v will give you the 100ah of capacity, but the same cell charged to 3.45v will give you 98ah of capacity. I think you misunderstood me because of my language barrier.

 

[BekimV]

Gotta be careful - otherwise lurkers will turn this into urban-legend slapping cells together and charging up!

Needs qualifiers - "As long as the cells are reasonably close in both IR and capacity, accompanied by good wiring infrastructure, (clean bus bars etc), a reasonable delta of 100mv (.1v) amongst the cells is manageable - provided one has already done a sanity-check top balance first, and ensures that no individual cell under-charge, goes higher than 3.6v"

Technically, voltage does not necessarily imply capacity. One can reach full rated capacity between 3.45v and 3.65v - the only difference is the matter of how much time that takes in absorb.

Tehnically not, but the cell charged to 3.65v will definitely give you more usable capacity than the cell charged to 3.45v.