Gross balancing time

[Jonndeka]

Hi dear friends,

I'm trying to calculate the balance current from the following formula:

Balance current [A] = Pack size [Ah] / gross balancing time [hours]

Ref: http://liionbms.com/php/wp_balance_current.php

But I don't know the gross balancing time. Is it in the datasheet? How can I find it?

Pack size is 192 Ah

 

[lnanek]

Let's say you have a 14S pack and your pack has 13 p-groups at 4.1V and one p-group at 4.2V. With a balance current of 100mA per p-group, you'd only be able to charge the other cells as fast as the BMS can burn off that highest cell at 100mA (assuming your charger can do more than 14 * 100mA = 1.4A charge current anyway).

192Ah is 13,714mAh per p-group. 4.1V is 89% charged, so 1,509mAh left to charge. 1,509mAh divided by 100mA equals 15 hours to balance.

Note this only applies to a BMS with a balance implementation where it has one resistor per p-group that can dissipate 100mA each. Some BMS may give you the amount it can dissipate total, which then has to be divided by the number of p-groups - so even longer to balance. Other balancing systems like flying capacitors and inductors work differently with balance current dropping as voltage difference decreases.

 

[batteryGOLD]

192Ah is 13,714mAh per p-group. 4.1V is 89% charged, so 1,509mAh left to charge. 1,509mAh divided by 100mA equals 15 hours to balance.

Nice math!
but if battery is 192Ah and supposing use 3000mAh cells , it means 192Ah/3 equals 64P battery
soo (top@4,1V) means 10% of 3000mAh equals 300mAh left, but U have 64P-elements battery. This means 64Px300mAh equals 19200mAh aprox ~20Ah (anyway 10% of 192Ah is aprox 20Ah)
soo if U charge 20Ah with just only 100mAh gess?? it takes some math 20k/100 is 200hours balance >> 15hours (your math)
correct me if I'm wrong!
200 hours balancing for delta 0,1V difference at S-elements!! a year has 365days, so 200hours is just 8,3days or 2,23% of a year time balancing..

Some people say that leaving a charger connected many days to a debalced battery it would equalize it.. Is this real or a conspiracy?
Most top bms cut charge occurs when cell reaches 4,23V
Soo most chargers turn off when bms cut top charge at 4,23V at any P-element (maybe daly bms will keep it charging until 5V..dont know )
This means no balance function from charger to battery if bms cuts.

I have a theory.. maybe not works
example you have a 42V 10S batttery, soo use x3 diodes at positive of charger, it will reduce top charge to 80% (4,0V per cell 40V total), but it does not allow to bms cut. soo leave that 40V source to 42V 10S battery many days to see if it balances(for a small debalance maybe ok delta 0,1Volts ,but if big debalce level bms will cut and you have to go inside battery to fix tha P/P's-group problem)

Anyone could test this to a debalanced battery?? Is educative :thumb:

using Arduino/ESP32 coding to develop something to monitor battery S-elements voltage?..

 

[lnanek]

Ah, you are right, I shouldn't have divided capacity by p-group count. That would only provide that capacity at single cell voltage, not pack voltage.

Still, if the 100mA balance current is per p-group, that means we can charge the 13 other p-groups at 100mA each = 1,300mA total charge current while top balancing the 14th p-group already at 4.2V.

192Ah * 10% left to charge = 19,200mAh left to charge. / 1,300mA charge rate would be 14 hours, wouldn't it?

FWIW 192Ah is huge by ebike standards. I can go 20 miles at full throttle on my 48V ebike with a 13Ah battery. .1V is a rather large difference too, if the pack is new and made of cells with all similar measurements.

 

[Jonndeka]

Thanks for your good explanation. But I still don't know what's the meaning of "gross balancing time"?
It means how much time it is needed for balancing between cells? Is it charging time?

 

[eMark]

But I still don't know what's the meaning of "gross balancing time"?
It means how much time it is needed for balancing between cells? Is it charging time?

Yes, when the charging time is gross ... excessively long 16-24 hrs and longer.

Gross Balancing ... https://www.google.com/search?q=gross+balancing+time+of+lithium+ion+battery&rlz=1CAPPDO_enUS802US802&oq=gross+balancing++of+lithium&aqs=chrome.2.69i57j33i160l2.37877j0j7&sourceid=chrome&ie=UTF-8

It takes almost one week for a BMS with a 1A balancing current to balance a 100 Ah pack that has some cells fully charged, and some cells totally empty. A balanced current of 10 mA cannot balance a 1000 Ah pack within the lifetime of its owner.

The above definition (IMO) is an overboard futile example of Mission Impossible of 'Gross' (flagrant/blatant sick battery) Balancing. Hopefully even a noob would never try to balance a lithium battery if he already knew some p-groups are fully charged and some p-groups are totally empty.

A more realistic definition of "Gross Balancing" would be when the voltage variances between say the 14 LFP parallel groups is more than say 0.15V = 150 millivolts (IMO) ...

An example of a LFP "Gross Balancing" challenge (at least a 0.150V p-group variance) would be when say 12 of the LFP parallel groups are at 3.40V resting and 2 of the LFP parallel groups are at 3.25V resting. Basically a sick battery with some or all of the cells in 2 of the LFP parallel groups needing to be replaced. Possible cause could be a defective BMS that went unnoticed for several charge/discharge cycles.

That's why it's always important to periodically monitor the 14 parallel group voltages for excessive (gross) variance. Also why some prefer being their own best BMS :thumb:

These module are not new and now they are working. Type of cell is LFP. I just want to know how much voltage difference should be between these modules if each module is 51V in nominal capacity? Yes the configuration is 14S6P.

LFP nominal cell voltage is 3.20V or 44.80V nominal for a 14S6P battery (51.1V fully charged).

 

[lnanek]

It's the time it takes for all p-groups in the battery to have the same voltage. It's not in the datasheet typically. Balance current is in the datasheet.

It would be impossible to put in a datasheet as a single number anyway. The page you link has a graph, for example. They give several example gross balancing times for different starting voltages too. E.g.:

In the above graph you can see that:

A BMS with a 1 A balancing current will take almost 1 week to balance a 100 Ah pack that has some cells fully charged, and some cells totally empty

So for that example balance current is 1A. Gross balancing time is 1 week. Pack capacity is 100Ah. Starting voltage of some p-groups is 4.2V if using LiIon chemistry. Starting voltage of other p-groups is 3.4V. End voltage of all p-groups is the same voltage throughout the battery for every p-group which is the definition of balanced.

Some tools like the Tenergy Cell Meter I have ( https://www.amazon.com/dp/B0178P8H9U ) will do a discharge balance. So it would discharge all the p-groups to match the lowest at 3.4V. That's considered balanced.

Cheap BMS will typically only top balance during charging. So it would discharge the p-groups at 4.2V already while it charges up the p-groups starting at 3.4V. So ending voltage in that case is all p-groups at 4.2V. That's also considered balanced.

 

[eMark]

Some tools like the Tenergy Cell Meter I have ( https://www.amazon.com/dp/B0178P8H9U ) will do a discharge balance. So it would discharge all the p-groups to match the lowest at 3.4V. That's considered balanced.

That RC Tenergy Cell Monitor/Balancer is typical of a 'cheap' RC accessory (not all that accurate). Also that iDST BG-8S Cell Monitor/Balancer can go astray (accuracy) when the JST-HX connector (2S-8S) is not plugged in correctly (off half-a-space). When the imbalance is 'gross' whether with that RC Tenergy or iDST BG-8S they aren't all that trustworthy and close to worthless. Besides they both are only good up to 8S (not 14S).

Cheap BMS will typically only top balance during charging. So it would discharge the p-groups at 4.2V already while it charges up the p-groups starting at 3.4V. So ending voltage in that case is all p-groups at 4.2V. That's also considered balanced.

A "cheap BMS" (with overcharge protection) always used to be one that only uses resistance discharge balancing and only at the top end. A so-called "Cheap BMS" with overcharge protection is incapable of active equalized balancing during charging to equalize the p-groups as needed - Gross Imbalance.

Now with the recent use of "cheap" equalizing BMS Balance Boards it would be helpful if you could recommend a "Cheap 14s BMS" that uses active equalized balancing of the 14 p-groups via both active discharge and charge equalized balancing ...

A so-called SMART BMS equalizes p-groups voltages by diverting some of the charging current from higher voltage p-groups to lower voltage p-groups – active balancing.

Were you thinking of something like this Heltec 2A BMS ($118) ... https://heltec-bms.com/product/1670/ ... OR ... this "cheap" $21.60 stand alone 14S 1.2A Li-ion LiFePO4 Battery Active Equalizing BMS ... https://srikobatteries.com/product/14s-1-2a-li-ion-lifepo4-battery-active-equalizer-bms/?gclid=Cj0KCQiAorKfBhC0ARIsAHDzslsepZ1pGnZGzC9rXOOoiluXw3TzilrF8evINS7bGNhURIfC0UDVqFkaAq2mEALw_wcB . Using it just when the LFP battery is at rest (either at bottom after partial discharge and/or at top after charging?

This is assuming there is at least a "cheap BMS" aleady in the battery pack to prevent overcharge, etc. Besides another set of 14S balance leads for using either of the above Heltec BBs when the 14S LFP battery is at rest (either after a cycle partial discharge or after a full charge (at rest which is usually 1-2 hours after a discharge or full charge).

Apologize for taking nearly an hour for correcrions. Hopefully helpful and now makes sense.