Voltage dropped after charging ? New battery pack.

[dogman dan]

Is his meter even accurate to .05v? I'm certain my cheap ones are not.

Of course, even a less accurate volt meter can still detect the drop. Just saying, who knows if his pack really charges to 54.6 or 55v? Maybe it drops because the bms is draining the slightly overcharged cells.

Awful lot of worrying about a tiny voltage drop after the charge IMO. Now, if it drops 5v after a week of storage or less, THAT you'd worry about.

 

[alexis57]

Yes yes the multimeter is accurate, agilent 34410A.
But you're right, I wanted to be too accurate ^^
Thank you !

 

[riba2233]

agilent 34410A.

damn 8)

 

[dnmun]

your meter is of if it balances at 4.17V on your meter. all of the lipo BMS balance to 4.20V so you are charging to that level if it is balancing the cells.

You don't get it, it's voltage at rest... It drops, it doesn't stay at 4.2 V after charging.

why would someone measure voltage after they take it off the charger? not fair when these weird conditions are used. i was under the impression he was measuring the cells while it was balancing since the cells were all the same voltage as he said.

 

[alexis57]

agilent 34410A.

damn 8)

Of course, not mine haha but I'm working in an electronic laboratory.

Do you think I could add a 1p to my battery pack ?
I didn't succeed to sell my left 52cells then instead of let them in their box, I'd like to improve the pack.
My battery pack has maybe only 5-6 cycles. I'd like to convert 13s4p to a 13s5p or even 13s6p.

 

[riba2233]

Why not? Just add it, no problem at all!

 

[dnmun]

you could even build a separate pack of 2P and connect it in parallel to the 4P to get your 6P. or add 3P to make it 7P. all you need to do is connect them in parallel through the sense wires with some small sense wires and then use a large current collector on the terminals to connect the added pack to the main pack. so even put it in a different location and run the sense wire bundle and main terminal connectors over to the main pack.

 

[alexis57]

Ok haha, mmmh it will be more complicated to make a new small pack because of the bms and as you said small wires.
I will solder directly to the main battery pack.
Unfortunately, I cannot use 7p because of the size of my bag... but I can easily go to 6p :lol:

 

[dnmun]

you do not need a separate BMS.

you do not need anything larger than 26AWG for the sense wires because the only current they see is the balancing current which is only a few mA.

you need to use the same wire size for the main current collectors on the terminals of the added pack.

by relocating the added capacity you can find space on the bike to add it.

 

[alexis57]

Ok I maybe misunderstood what you said before.
No relocating, my battery pack is in the bag behind and it wont be anywhere else. The bag is big enough to bring the battery and the charger (when I don't come back).
Anywhere, I will solder them soon.

 

[alexis57]

Hi !

Still a problem (maybe).
Now with my battery pack (10 cycles maybe) the charger go to 54.6V (4.2V) and then the BMS cut off the power (cause one parallel cells went to 4.2v).
In 2sec, the battery pack voltage decreased to 53.8V and in 5min, 53.6V.
I lost 1V in 5min :O

Did I get very bad cells ? My fault running a BBS02 25A with 4p only ?
I'm gonna solder one or two more serie cells (13s4p => 13s5p or 13s6p) in order to increase the range and the stress cells.

Could you tell me how I could see if it's bad and which ones are bad ?

Thanks !

 

[dnmun]

without proper voltage measurements while charging there is no way to comment with an informed opinion.

i am sure there are plenty of people who will tell you what is wrong without any useful measurements but i cannot.

 

[alexis57]

Ok, I'll try to make the measurement of each cells during the charge and I'll post here.