I need a charger for my DIY battery pack

[Tryptych]

Is anyone here a wizard when it comes to building lithium battery packs?

For one of my DIY PEV projects I have a 52v 14s4p battery made with Molicel P42A cells (21700) and a bms.

Can anyone link me an appropriate charger for that battery pack? or, can you tell me if this 4amp 14S charger I found is appropriate? 

...or, is 4amps too much for this pack?

Any tips in the right direction would be appreciated, thanks in advance!!

 

 

 

Edited February 10, 2022 by Tryptych

[Chriull]

52 minutes ago, Tryptych said:

Is anyone here a wizard when it comes to building lithium battery packs?

For one of my DIY PEV projects I have a 52v 14s4p battery made with Molicel P42A cells (21700) and a bms.

Can anyone link me an appropriate charger for that battery pack? or, can you tell me if this 4amp 14S charger I found is appropriate? 

The link does not work - points to aliexpress login page...

52 minutes ago, Tryptych said:

...or, is 4amps too much for this pack?

At the link of the cells you provided is written that 4.2A is the standard charge current for these cells - so _just considering the cells_ these 4 of them parallel could take 4x4.2=16.8A.

But no idea how much current your connectors, wiring and BMS can take.

[Tryptych]

Thanks Chriull, I have updated the link: https://www.aliexpress.com/item/32814642478.html?spm=a2g0s.12269583.0.0.1c0ac7cd3zc9dd

 

[alcatraz]

58.8v is the right end voltage for Li-ion 14s yes.

Does it have the right plug?

The cells can easily be charged even faster than 4A but how good is that bms? If all the over-charge protection works properly then there shouldn't be an issue. Some cells will arrive at 4.2v before the pack reaches 58.8v and it's the bms' responsability to cut off the charger. With slow charging the voltage sag will be a lot lower and thus it'll put less pressure on the bms.

You could try and then monitor the voltages when it approaches 100%. When the light switches from red/green on the charger the voltages should all not have exceeded 4.25v.

[alcatraz]

Sometimes the new cells are balanced so you won't really know the odd behavior until they've aged a bit. The bms will be put to work later in the packs life cycle. That's when running a fast charger could potentially be unwise.

[Chriull]

19 hours ago, alcatraz said:

The cells can easily be charged even faster than 4A but how good is that bms? If all the over-charge protection works properly then there shouldn't be an issue. Some cells will arrive at 4.2v before the pack reaches 58.8v and it's the bms' responsability to cut off the charger. With slow charging the voltage sag will be a lot lower and thus it'll put less pressure on the bms.

I'd see no real difference between slower or normal charging - the higher the charging current the earlier the CV stage will be reached and the CV stage will take longer times. Withe the ?exponentially? decreasing current during CV stage the cells have quite soon the same "final treatment" no matter with which current charging happened. Imho.

19 hours ago, alcatraz said:

You could try and then monitor the voltages when it approaches 100%. When the light switches from red/green on the charger the voltages should all not have exceeded 4.25v.

This could be easily checked with some current measurement. Once a cell reached overviltage and the bms cut off no current is flowing abymore. In every other normal case the charge current will go on and settle at the self discharge current of the cells and the current through the activated bleeding resistors.

[alcatraz]

5 hours ago, Chriull said:

I'd see no real difference between slower or normal charging - the higher the charging current the earlier the CV stage will be reached and the CV stage will take longer times. Withe the ?exponentially? decreasing current during CV stage the cells have quite soon the same "final treatment" no matter with which current charging happened. Imho.

This could be easily checked with some current measurement. Once a cell reached overviltage and the bms cut off no current is flowing abymore. In every other normal case the charge current will go on and settle at the self discharge current of the cells and the current through the activated bleeding resistors.

That's interesting. Does a high amp charger really follow the same current curve as a low amp charger? (When the current goes below the rated max for the slow charger?)

When arriving to that point, the higher current on the fast charger has resulted in more imbalances accumulated. Maybe that changes the charge characteristic for the remaining percentage.

Anyway, I find it hard to express the general idea. I'm trying to point out that there is a difference, somehow. 

I've seen some weird behavior by my original inmotion bms. I used to charge it 100% (left it for several hours) and then probed it. For years I used to see nothing but good stuff like 4.19, 4.20. Then one time I saw most of the cell groups at 4.25v, and I hadn't changed anything. 4.25v is on the limit but iy surprised me how it could just be solid 4.19 for years and then 4.25v one day. I believe I was using a non stock charger but I checked the voltage and ensured it was the same as the original. It even came with a potentiometer to dial it in. Maybe it trickle charged the cells or something, which I hear is bad. Here's the thing though. If I use that very same charger today, I'm 99% sure it's going to stop at 4.19v. Seemed like a one time fluke thing. A ghost in the machine. 

[alcatraz]

So while you want to give the bms time to sort the pack out, it doesn't come without risk. Personally I'm just happy to be able to probe my packs. Imagine being in the blind and just mixing packs in parallell and fast chargers. Who knows what could actually be going on.

[Chriull]

6 hours ago, alcatraz said:

That's interesting. Does a high amp charger really follow the same current curve as a low amp charger? (When the current goes below the rated max for the slow charger?)

Yes. Charging shall be very efficient - above 99% afair from most readings.

Most modern high capacity li ion cells used for EUCs have roughly about some C/3 (21700) or C/2(18650) specified as max charging current. So with sone 25-45mOhm internal resistance that's just some neglectable 1.5²A x 35mOhm=80mW power dissipation.

The charging curve (voltage is "very well" defined, but (very?) temperature dependend) so there is no real divergence to be expected.

So differences could arrive from different cell temperatures, which i would not expect from all cells beeing burdened equally and all beeing in one compartement. Maybe with some designs with a cell beeing not too far above the mosfets some differences (mostly in ?degradation/aging?) could occur over time during riding?

The very real (over)burdening happens during discharge - peaks well over each manufacturer specification. Especially at lower charge states were internal resistance rises and needed currents for the same output power rise, too. Once imbalances started to appear single cells will be at voltages well below the average 3.3V/3.15V minimum average voltage and start chemical processes degrading them even more.

7 hours ago, alcatraz said:

When arriving to that point, the higher current on the fast charger has resulted in more imbalances accumulated.

So i'd worry about discharging and not about some (imho) minor differences whilst charging.

As similar, but much much less serious the longer trickle charging after the green led many do for balancing stresses the batteries unequally. Without any real balancing advantages. The decreased charge current by the bleeding resistors and the discharging of such slightly overcharged cells after unplugging the charger should be as effective and sufficient.

I'm looking very much forward to (dis)charge logs on cell basis from the new wheels, which hopefully will be implented in our apps! Do get some real empirical data.