Voltage dropped after charging ? New battery pack.

[alexis57]

Hi !
I made my own battery pack 13s4p new panasonic 18650PF bought in the nkon shop.
My charger is a 4A CC CV : http://fr.aliexpress.com/item/48V-3A-E-Bike-Lithium-battery-charger-54-6V-3Amp-13S-Lipo-battery-charger-High-Quality/935973593.html
and I use this BMS http://fr.aliexpress.com/item/Free-Shipping-48V-40A-BMS-13S-48V-54-6V-li-ion-battery-BMS-PCM-40A-continuous/32283657890.html

I charged to 54.6V (4.2v) and as soon as the charger cut off, I unplug every wires.

But in 30min, the battery pack voltage decreased from 54.6 to 54.3...

I'll see tomorrow morning but I think it'll decrease more.

Is it ok ? Maybe the BMS is consuming current...
I monitored the current with arduino and bluetooth and the current well decreased to 400mA just before cut off.

It's my first battery pack then it's maybe normal.

Thank you.

 

[riba2233]

That's 100% normal. You should see how much does lifepo4 drops after being charged

No reason to worry.

Charger is reaching proper end of charge, AKA red led turning green?

 

[Gregory]

So cells were all around the same 4.18V? I wouldn't worry about it so long as they are all balanced.

They claim 20uA quiescent current which is nothing.

Dropping 0.02V per cell is fine and may just be down in the noise/accuracy limits of your multimeter or temp changes.
Give it a few cycles and check individual cell balances again.

 

[alexis57]

After the night 54.2V and seems being stable now.
Each cells are at 4.17V. Very well balanced.
The battery pack life is only 2 cycles haha

Yes red light turning green and no current. (I monitored with voltage and current)

Thank you for having reassured me !

 

[dnmun]

your BMS should allow a balancing current at full charge so i am surprised you said there is no current flow when the light turns green on your charger.

 

[riba2233]

your BMS should allow a balancing current at full charge so i am surprised you said there is no current flow when the light turns green on your charger.

That's because batteries are in perfect balance, no need for balancing.



That's just how chemistry works, it's not about current draw or anything like that.

 

[dogman dan]

2 cycles.

I'm guessing your battery is not completely balanced, and then when you unplug the bms is still draining the highest charged cells. So of course it drops some. Leave it plugged in overnight to completely balance, and see what it does when you un plug then.

Later on, a year from now perhaps, you should still see something similar as fully charged cells cannot hold 4.2v anymore. They will drop to 4.18 or whatever on their own. Good cells won't do this right away, but really cheap cells can.

 

[Ykick]

Small % Voltage loss following charge termination is normal. Cells that continue self-discharge over a few days should be considered "leakers" and defective.

 

[999zip999]

Before building battery you should charge large parallel strings of cells and all cells to same voltage. Let seat them check before assembly. Don't rely on a tiny bms to balance random cells even if new and all the same. Build a balance pack and have the bms keep it there

 

[dnmun]

your BMS should allow a balancing current at full charge so i am surprised you said there is no current flow when the light turns green on your charger.

That's because batteries are in perfect balance, no need for balancing.



That's just how chemistry works, it's not about current draw or anything like that.

even if the battery is in perfect shape and fully charged, the charger should be able to continue to push a balancing current at the final voltage. otherwise there is no way to balance the pack to full charge.

 

[riba2233]

Before building battery you should charge large parallel strings of cells and all cells to same voltage. Let seat them check before assembly. Don't rely on a tiny bms to balance random cells even if new and all the same. Build a balance pack and have the bms keep it there

No need to do that with new panasonic 18650 cells. They are balanced from the factory. That's also true for other major manufacturers, like Sanyo, Sony, LG, Samsung...

 

[999zip999]

No need ? That's B.S.
You should know what the cell voltage is at the very minimum. Even in the best made cells there is a failure rate no matter how lucky you feel. A voltmeter is better than luck. Still all cells should be charged in big parallel string ( s ) to a set voltage.

 

[riba2233]

No need ? That's B.S.
You should know what the cell voltage is at the very minimum. Even in the best made cells there is a failure rate no matter how lucky you feel. A voltmeter is better than luck. Still all cells should be charged in big parallel string ( s ) to a set voltage.

How many pack have you built with new high quality 18650 cells? I guess 0 because if you did, you would now that I'm telling the true.

 

[riba2233]

your BMS should allow a balancing current at full charge so i am surprised you said there is no current flow when the light turns green on your charger.

That's because batteries are in perfect balance, no need for balancing.



That's just how chemistry works, it's not about current draw or anything like that.

even if the battery is in perfect shape and fully charged, the charger should be able to continue to push a balancing current at the final voltage. otherwise there is no way to balance the pack to full charge.

I didn't even see this gem...

So, you have balancing set exactly at over 4.2 V. And you have your charger set at exactly 4.2 V per cell. If all cells are in perfect balance, and they are full, so their voltage is same as chargers, what would cause balancing to start, since none of cells are over 4.2 V, but exactly at 4.2V??

Balancing only happens when one or few cells are (slightly) more charged due to (small) goes higher then 4.2 V. In that case the total voltage at the end of the charge across all cells will be 4.2 times the number of cells in series, but some cells will be over 4.2 V (they will be balancing) and some cells will be below 4.2 V. Now, as the balancing takes place, cells that were over 4.2 V are slowly getting discharged or charged slower, thus letting cells that were below 4.2 V to reach 4.2 V. Eventually high cells will drop to 4.2, and low cells will climb up to 4.2 V, all cells will have same voltage, and there will be no more need (or way) for balancing.



Only way your sentence would be true, is if chargers final voltage was set to voltage slightly higher than 4.2 V times number of cells in series. But that is not standard case.

 

[alexis57]

Hi,
For replying to everybody :
The cells are Panasonic 18650PF, totally new.
They were ALL at 3.6V.
And my battery pack was very well balanced from the first charge !

Anyway, thank you, as someone said, it's the chemistry, not because of my BMS or charger.

 

[dogman dan]

Also, bear in mind, that .3v is a pretty small % of discharge from 54.6v. We're discussing how many angels can dance on a pinhead.

Cracks me up, folks that fret over .03v or so on a 4.2v cell. This is about the same ratio.

Anyway, I still think the charge stopped with the total was 54.6v, but then the bms discharged half the cells that were at 4.22v or so when the charger stopped.

Like it's supposed to. Leaving the charger plugged in longer would result in a final charge of 54.6v, AND the bms dischargers finished. If the charger keeps a tiny trickle flowing when the voltage drops.

If not, you have to push the pack to 54.8 or so, to get all cells full, after which it will pull down the higher cells to end at 54.6v.

Bear in mind, though you are supposed to stop at 4.2v, cells don't just explode or get ruined if you charge to 4.21 v.

 

[dnmun]

yes, the charger should be pushing the balancing current at the final voltage. if it does not then you have to adjust the charger so that it does or you will never balance the battery at full charge.

 

[riba2233]

yes, the charger should be pushing the balancing current at the final voltage. if it does not then you have to adjust the charger so that it does or you will never balance the battery at full charge.

Here we go again... *facepalm* Read my post and TRY to understand.

 

[dogman dan]

The way I understand it, if the bms stops the flow when one cell gets above 4.2v, then other cells can, and usually will be lower than 4.2v when the bms stops the flow. I've watched this happen enough times to think that is indeed what happens.

So if two or three cells are at 4.1v, and charging stops because one cell is at 4.3v, and Viola, the pack is charged at 54.2v. If the charger is then unplugged, or shuts itself completely off, nothing on earth is ever going to make those two or three cells reach 4.2v.

Then, you might get some additional self discharge, if any cells are worn and can't hold more than 4.18v or whatever.

One way to force a quicker balancing, is to set the charger to 54.8v, so at least the charger keeps pushing just a few watts for a bit longer without shutting itself off, or going to constant voltage trickle. But you still need to leave it plugged in awhile, for the bms to cycle on and off a few times and bring up the low cells.

 

[riba2233]

The way I understand it, if the bms stops the flow when one cell gets above 4.2v, then other cells can, and usually will be lower than 4.2v when the bms stops the flow. I've watched this happen enough times to think that is indeed what happens.

So if two or three cells are at 4.1v, and charging stops because one cell is at 4.3v, and Viola, the pack is charged at 54.2v. If the charger is then unplugged, or shuts itself completely off, nothing on earth is ever going to make those two or three cells reach 4.2v.

This might be the case with chargers that are smart enough to stop charging once current gets below some level, but 99% of chargers we use don't do that and they will continue to charge after high cells gets bleed down enough for BMS to turn on charging again. Also, hvc (turning charger off) doesn't happen as soon as one of cells starts to bleed, but on higher voltage, those are two separate voltage levels. This gives cells some time to balance before charger is shut off, in case that the imbalance isn't that serious.

 

[dnmun]

i don't know why you decided you could insult me but i think i know more than just a little about how a battery charger and BMS work.

it is sad that people here can just misinform people because they can bully others in print on these pages.

no lipo BMS will allow a cell to reach 4.30V. none.

a lipo BMS will shut off charging when any cell reaches 4.25V and will not restart charging until the cell voltage drops below the reset voltage which is usually 4.05V.

if you do not adjust your charger so that it is producing the balancing current at the point the battery reaches full charge then it can never fully balance at full charge.

 

[dogman dan]

Correct, very few chargers actually stop completely when they reach pack voltage. That's what makes naked lipo charging with them such a risk. That's why when I do that, the charger is tweaked to stop pushing CC below 4.2v.

To clarify what I meant though, If one or two cells that are pretty good out of balance, a VERY common event on a brand new battery or any battery that was pushed hard last discharge, then the charger will turn green light when some of the cells are significantly below full charge.

THEN Alexis 57 is unplugging the charger right away!

That does shut off the charge, and it will never balance completely that way.

Then the bms discharges his high cell or cells, and he sees a voltage drop. The same thing could be happening if he has a charger that shuts off completely, but likely he does not have that.

Betcha when he does get his pack better balanced, he stops seeing that tiny, almost insignificant voltage drop for a while. Then later, with many cycles on the pack, he might start to see it happen again for the reasons others explained.

But right now, he just needs to leave it plugged in longer IMO. Or do what I call batter break in, ride around the block, recharge, ride around the block, recharge, till it starts to balance faster. About 5 cycles of that tends to get it done a bit faster for me. Do that, then do a balance charge, and the balance charge takes less time.

 

[alexis57]

I used a CC CV charger.
I monitored with lab voltmeter and ampermeter.
The battery pack reach 54.6V or maybe 54.58V and stop when the current is as low as 400mA (for whole pack, then 100mA for each cell).
For the balancing, the maximum of difference between cells is 5mV. (0.005V)
Why should I increase the voltage of the charger to get more than 4.2V ?

I think it's better for the battery life to get between 4.1 and 4.2V.
If it's only for balancing, I really don't care since the battery pack is already perfect balanced.

I can let it plugged but it's useless since no current is provided by the charger.

 

[dogman dan]

No, it's not useless if you want a completely balanced pack.

But yeah, chances are you don't need that much balance each ride. In fact, your original voltage drop is still what I would call plenty balanced enough in my opinion. So you can unplug when it goes green if you like.

But leave it overnight from time to time. Let that tiny trickle continue to balance the pack with the bms helping, and see if it holds the full voltage when you unplug then. I think it will, till the pack gets some cycles on it.

For sure, we already know the pack is pretty good, no real stinker cells in it. If you want it to balance faster, then it can help to turn up the charger some. I have mine set to 54.7v, which drops to the bms regulated 54.6v soon after I unplug. I can feel it that the bms warms up more when set .1v more, and I believe that the pack is more balanced each cycle since I tweaked the charger up a tiny bit.
My pack (allcell with LG cells) is now a year old, so overnight it will fade to 54.5v now.

Bear in mind though, I doubt my cheap voltmeter is all that accurate, not to .01v for sure.

 

[dnmun]

I used a CC CV charger.
I monitored with lab voltmeter and ampermeter.
The battery pack reach 54.6V or maybe 54.58V and stop when the current is as low as 400mA (for whole pack, then 100mA for each cell).
For the balancing, the maximum of difference between cells is 5mV. (0.005V)
Why should I increase the voltage of the charger to get more than 4.2V ?

I think it's better for the battery life to get between 4.1 and 4.2V.
If it's only for balancing, I really don't care since the battery pack is already perfect balanced.

I can let it plugged but it's useless since no current is provided by the charger.

can you post up a picture of the charger so we know what to avoid? if your charger is turning off the output and never turns back on that is called a latching output and should not be used with a regular shunt balancing BMS. your battery seller should know that already.