Choose fast but safe charger for battery packs

Hello EV friends,
i'm about to build a 14s4p battery pack with samsung 21700 40t cells and a BMS that handles a charging current of 30A.
Now i'm having trouble to figure out what Charger works best with my battery and how to know and calculate for further projects with different specs. I want to charge my packs as fast as possible BUT still in the range for ideal cycle life (in case there is a correlation). The Datasheet of the cells says:

Standard charge: CCCV, 2A, 4.20 V, 200mA cut-off
Rated charge: CCCV, 6A, 4.20 V, 100mA cut-off
This " Standard charge" means charging the cell CCCV with charge current 0.5C
(2,000mA), constant voltage 4.2V and 200mA cut-off in CV mode at 23℃ for capacity.
.Rated charge means charging the cell CCCV with charge current 6A and 100mA
cut-off at 23℃

Click to expand...

As i didn't even know what CCCV means i started my research there and quite understand that now but still don't know how the specs do tell me what charger to buy. According to the rated charge of 6A i could charge max. 24A (6Ax4p). But is that also reasonable for the cycle life ? And although i don't know what is meant by the 100mA /200mA cut-off, i guess it somehow describes the curves of the CCCV process ? But how do i know if for example a 20A charger reduces the current the way the cell needs it ? Do i think to complicated ?
If a charger says "58,8V/20A Lithium-ion Charger" how do i know if it's what i need ?
And btw... i really want to avoid buying a expensive adjustable charger so i'd rather go with a slower charger (but still, how low do i have to go to make sure its safe ?)

Sorry for my confusion and thanks a lot for your help.

Best regards Sparfuchs

CC constant current up to the constant voltage phase (58.8V)
CV constant voltage (58.8V) from there the current is reduced.
100mA Cut off, if the current in the constant voltage phase is lower than 100mA , the charging shall stop.

The charger itself does not reduce the current, it is pure Physics that reduces the current as the battery voltage raises.

Safer would be to stop charging at 4.1- 4.15V per cell and then you can charge your pack with 10-24A, and if the pack is not in a good balance the BMS stops charging when one cell reaches 4.17- 4.2V .

dominik h said:

CC constant current up to the constant voltage phase (58.8V)
CV constant voltage (58.8V) from there the current is reduced.
100mA Cut off, if the current in the constant voltage phase is lower than 100mA , the charging shall stop.

The charger itself does not reduce the current, it is pure Physics that reduces the current as the battery voltage raises.

Safer would be to stop charging at 4.1- 4.15V per cell and then you can charge your pack with 10-24A, and if the pack is not in a good balance the BMS stops charging when one cell reaches 4.17- 4.2V .

Click to expand...

Thanks a lot for your reply dominik h,
so the "curves" of CC and CV do not have to be controlled by the charger but doesn't the cut off (100ma/200ma or whatever) have to be ("shall stop") ? And how do i know if the charger does ? I mean the only information most chargers give is voltage and amps like "Output: 58,8V/20A". Is this enough information to make a decision ? I still don't really understand what i have to look for on aliexpress, amazon ect.

The cut off has to be in the charger.
My chinese 10A charger stops charging at 1A.
My two meanwell 6A chargers integrated in the scooter do not stop charging as these are LED power supplies. I set the CV voltage to 4.17V per cell. In your case 58.4V.
My 40A charger is set to 4.15V per cell.
My 85A charger is set to 4.13V per cell.
My new 120A charger (a little more than 1C) will be set to 4.12V per cell.
If I want to have my battery completely full I would change the voltage of my slowest charger to 4.2V per cell and plug it in parallel to the fast charger.
So at the end of charging the BMS has enough time to balance the cells.
And between 4.12V and 4.2V is less then 5% capacity off battery. But at 4.22-4.25V the cells will be damaged.

Those chinese chargers always charge to 4.2V per cell which can reduce the usable cycles of the battery when the BMS can not balance the cells fast enough.

Expensive adjustable chargers can be programmed what they do at the end of charging. For example reduce the CC value at a certain voltage (4.12-4.15V) and then charge slower up to 4.2V.

I think for a new battery the 20A charger should be ok. If the battery is a few years old it could be to much for the cells and the BMS.

Here's something to ponder (respective of safety & cycle life) ...

Why is it that a 10S5P 12.5Ah (w/2500mAh cell capacity) battery comes with only a 2A charger? Is it primarily because it's less expensive than a providing a 4A charger? Or why a 52V 20Ah or 24Ah battery comes with only a 3A or 4A charger? Maybe for the good of the battery during its senior twilight cycle life

What about this 72V 50Ah Power-Plant that comes with only a 5A charger ...



https://www.amazon.com/Lithium-Battery-Li-ion-Electric-Bicycle/dp/B09QQCXX8T/ref=sr_1_13?crid=3H6P5XXVQR6RZ&keywords=72v%2Bebike%2Bbattery&qid=1650806564&s=sporting-goods&sprefix=72%2Csporting%2C137&sr=1-13&th=1

Is it mainly for safety and less expensive than a 10A charger OR maybe because of those Chinese "POLYMER CELLS". Don't lithium polymer cells have both a higher charge and discharge rate than lithium ion cells?

Is the following ad for this battery true ...

【High Qulity】Top A grade POLYMER CELLS are more stable and less prone to explosion under severe impact, safer than the other cell.

dominik h said:

The cut off has to be in the charger.
My chinese 10A charger stops charging at 1A.
My two meanwell 6A chargers integrated in the scooter do not stop charging as these are LED power supplies. I set the CV voltage to 4.17V per cell. In your case 58.4V.
My 40A charger is set to 4.15V per cell.
My 85A charger is set to 4.13V per cell.
My new 120A charger (a little more than 1C) will be set to 4.12V per cell.
If I want to have my battery completely full I would change the voltage of my slowest charger to 4.2V per cell and plug it in parallel to the fast charger.
So at the end of charging the BMS has enough time to balance the cells.
And between 4.12V and 4.2V is less then 5% capacity off battery. But at 4.22-4.25V the cells will be damaged.

Those chinese chargers always charge to 4.2V per cell which can reduce the usable cycles of the battery when the BMS can not balance the cells fast enough.

Expensive adjustable chargers can be programmed what they do at the end of charging. For example reduce the CC value at a certain voltage (4.12-4.15V) and then charge slower up to 4.2V.

I think for a new battery the 20A charger should be ok. If the battery is a few years old it could be to much for the cells and the BMS.

Click to expand...

So you don't know when or if a cheap charger cuts off current before you buy it right ? But can you at least somehow check for example with a multimeter after you bought it to know ?

As mentioned i'll not buy a adjustable charger. So is there at least a current value to be safe ? As a good lifespan is much more important for me than fast charging i'd also go down to a 2A charger if its needed for ideal cycle life (without adjustable charger). But i don't wanna charge unnecessarily slow if i could use a faster charger like 5/8/10A ? without any disadvantages.

So is there a way to calculate a value according to the charge information of the datasheet, that is safe with cheap chargers when you don't know there cut off details ?

Sparfuchs said:

Hello EV friends,
i'm about to build a 14s4p battery pack with samsung 21700 40t cells and a BMS that handles a charging current of 30A.

Now i'm having trouble to figure out what Charger works best with my battery and how to know and calculate for further projects with different specs. I want to charge my packs as fast as possible BUT still in the range for ideal cycle life (in case there is a correlation). The Datasheet of the cells says:

Standard charge: CCCV, 2A, 4.20 V, 200mA cut-off
Rated charge: CCCV, 6A, 4.20 V, 100mA cut-off

This " Standard charge" means charging the cell CCCV with charge current 0.5C
(2,000mA), constant voltage 4.2V and 200mA cut-off in CV mode at 23℃ for capacity.
Rated charge means charging the cell CCCV with charge current 6A and 100mA
cut-off at 23℃

Click to expand...

Accordingly with the "Standard Charge" for just 1 Samsung 40T cell (4,000mAh) being 2 amps (0.5C) ... then with 4 cells in parallel (16Ah) a "Standard Charge" would be an 8 amp standard charge. The "Rated Charge" for just one 40T cell being 6 amps (1.5C) ... then with 4 cells in parallel the "Rated Charge" would be a 24 amp rated charge.

So using a FAST 24 amp "Rated Charge" with your 40T 14s4p would not transgress the Samsung 40T Datasheet ... True or False ?

eMark said:

Accordingly with the "Standard Charge" for just 1 Samsung 40T cell (4,000mAh) being 2 amps (0.5C) ... then with 4 cells in parallel (16Ah) a "Standard Charge" would be an 8 amp standard charge. The "Rated Charge" for just one 40T cell being 6 amps (1.5C) ... then with 4 cells in parallel the "Rated Charge" would be a 24 amp rated charge.

So using a FAST 24 amp "Rated Charge" with your 40T 14s4p would not transgress the Samsung 40T Datasheet ... True or False ?

Click to expand...

If it would be that easy i wouldn't be concerned about my cells. But there is also the value of cut of at 100ma/200ma we can't just ignore and the fact that "rated" might not be the same than "best for lifespan" and that the heat in a battery pack will act different than in a single cell.
True or false ? As far as i got "dominik h" right it might be both, true and false because it doesn't only depend on the amps but also on the cut off it it harms your battery.
As long as i don't know more about alternatives to a expansive but safe and adjustable charger, i'll just use a cheap 58.8v/5A charger and hope that it won't harm my cells.

5A is safe. 10A will also be safe. Heat should Not be a problem if you stay way under 1C.
The cut off is not a real problem as long es the charger does not go higher than the 58.8V
And normaly there is a BMS which should cut off as things are getting wrong.

dominik h said:

5A is safe. 10A will also be safe. Heat should Not be a problem if you stay way under 1C.
The cut off is not a real problem as long es the charger does not go higher than the 58.8V
And normaly there is a BMS which should cut off as things are getting wrong.

Click to expand...

Your helpful information made me overthink and i found out that many or even most of the cheap not adjustable chargers have a potentiometer inside that allows you to adjust and lower the voltage. So if i'll find a 15A or 20A charger with a potentiometer inside to reduced the voltage to 58,1V it would give me a cheap, and although safe solution that on top also enhances the usable cycles.

Am i right so far or is the thought to good to be true ?

And does anybody know a charger that would do what i want ?

Yes, you are right.
My 10A chargers have no potentiometers.
But if you find one with a potentiometer this would work very well.

Could anyone explain why the charging cut-off (200mA @ 2A, 100mA @ 8A) is higher at lower current and lower at higher current ?
I would have thought that if you charge with more current, the charger has to cut of earlier to protect the cells from overcharging but i guess it has something to do with the balancing time ?

And does this cut-off value also change in a battery pack like all the other values ? Because if you calculate the capacity/discharge/ charge you take the value times the parallel count. So capacity: 4000mAh x 4 = 16000mAh, Discharge: 35A x 4 = 140A, Charge: 2A/8A x 4 = 8A/24A. But does that also work like that for the cut-off ? Like 200mA/100mA x 4 = 800mA/400mA ?