84V pack not reaching full charge

sailah

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Aug 13, 2010
Messages
52
Hi,

Been away for awhile getting back into this project. Please forgive me if this is a dumb question I did search...

I built an 84V 20S12P pack from Samsung 25R cells. Added BMS. I charged the pack last year during maiden voyage and had a mechanical problem so I put the bike in barn.

When I got bike out today after 9 months, the pack was reading 65V. Put in on charger (2x Meanwell HLG-240H-42, trimmed to 84V) and charging seemed to have stopped at 77.4V, in fact the voltage is drifting slightly measured with my HP multimeter, hooked up continuously during the charging process.

I'm not sure if BMS has kicked in and it's balancing or not, I've never used BMS so I don't know if this is normal.

I unplugged charger leads and read 84.04V with my Fluke so I'm fairly certain the charger is at 84V with no load.
 
sailah said:
Hi,

Been away for awhile getting back into this project. Please forgive me if this is a dumb question I did search...

I built an 84V 20S12P pack from Samsung 25R cells. Added BMS. I charged the pack last year during maiden voyage and had a mechanical problem so I put the bike in barn.

When I got bike out today after 9 months, the pack was reading 65V. Put in on charger (2x Meanwell HLG-240H-42, trimmed to 84V) and charging seemed to have stopped at 77.4V, in fact the voltage is drifting slightly measured with my HP multimeter, hooked up continuously during the charging process.

I'm not sure if BMS has kicked in and it's balancing or not, I've never used BMS so I don't know if this is normal.

I unplugged charger leads and read 84.04V with my Fluke so I'm fairly certain the charger is at 84V with no load.

I'd say your charger setup is no issue the 25r cells are reliable enough and normally its the bms thats playing funny buggers.

If you got a hobbycharger strip the bms off and check the pack in sections to verify it's ok and get a replacement bms, if you don't have a rc charger just take a punt and get a Bluetooth smart one of aliexpress wire it up and hope your not digging the pack apart looking for any obvious or then even worse getting deep and peeling nickel strip off looking for bad cells.
 
Thanks, I have BMS wired up to charging circuit only.

I made up a quick lead to attach the charger XT60 lead directly to pack, bypassing BMS. Now charging voltage is heading back up like before. I'm reading the voltage through the charging lead lol

I guess it's possible BMS is bad, it's literally only been charged once since I built the pack.
 
You really should be checking the individual groups as you bulk charge it, not just the total voltage. A new pack shouldn't self discharge, and if the BMS itself was unbalancing the pack while sitting around, and cutting out because it couldn't keep up with the imbalance now that it's charging, you could be driving several of the groups into overcharge by bulk charging.
Bulk charging is only safe when you know for a fact that all the groups even out at the same time....
 
Well thanks for that!!!

I decided to stop charging and cut open the pack to access the individual balance leads. Started checking from nickel strip to nickel strip. Noticed 1 group was off.

So then I checked between pins on BMS since it's wired so neatly.

1-2---4.17
2-3--4.18
3-4--4.185
4-5--4.212
5-6--4.20
6-7--4.19
7-8--4.184
8-9--4.204
9-10--4.218
10-11--4.214
11-12--4.216
12-13--4.216
13-14--4.212
14-15--4.211
15-16--4.21
16-17-4.205


17-18--> 4.20V
18-19--> 3.34V
19-20--> 4.40V

So it appears I have an issue. Does this mean I need to start pulling apart nickel? I just built this pack and it has 5 min of riding. All hot glued together. I may just have to roll this back in the barn for another 9 months if it requires this much effort.
 
Upon further inspection, after removing all the foam padding and my careful wrapping I found a bad cell. It was blackened around and clearly not right. Also there was a smell.

As I was looking at it, not touching, I saw it spark.

Well that was enough for me. Pack was promptly brought outside and placed in the heavy metal tongue box on my bobcat trailer.

I don't know how much I really want to put into disassembling that since it's spot welded and hot glued together. Seems like a gigantic pain in the ass with chance of :kff:

I may just scrap it and start over.

Too bad too the bike (Quilbx 76R) is a ripper. For the 5 min I rode it lol :roll:
 
Bummer! Better finding it now than later. One possibility is even though the cells could handle it, the ripping session overheated one of the strips into melting thru the wrapper and shorting to the metal can, which would explain the sparking when poking it too.
 
Gutting, least you caught it and no foul other than the pack.

You got to weigh up now is it worth patching up the pack if the surrounding cells seem to be fine under a load test or start from scratch with a expensive loss either way and a pain in the arse that a 5 min ride doesn't cover but onwards and upwards, id try a repair if the signs look good it's by far the cheaper option and you may get years of happy riding.
 
Yeah all the cells were brand new from NKON with no time on them at all.

Benefits of having an infrared camera on hand, I shot the pack and the bad cell is clearly hot. I shot it again after 20 min and it dropped down to 55C so it looks like it won't cause any further harm.

View attachment 1
 
In my limited experience, the hot glue pulls the wrapper off. I'm wondering if there's an established way to extricate the cell from the group?
 
Two ways that I know of,
Heat the cell till glue softens a slide it out.
Methylated spirits releases the glue like magic. Just a few drops will do, and don't get it on the other cells or it will all fall apart.
 
So I decided to take apart pack or at least perform adequate destruction to get to a point where I wasn't willing to go any further.

Bought a metal trash can. Set up pack on my tailgate, dressed in heavy clothes, full face shield, gloves etc.

Applied 99.9% Isopropyl Alcohol to soften up glue and smacked it on the hinge of the nickel sheet. Came right apart. Did this to the 3 bad sections, although 1 was actually fine. After removing sections I tested with DMM and sure enough, the 2 bad sections read 0V. The other group I removed to get to the others read 4.3V, I have that discharging on my bench now but do not plan to reuse.

Bought 36 new Samsung 25R to replace what I removed. Lesson learned, thanks for all the advice glad that didn't go south in my office last night. :bigthumb:

https://www.youtube.com/watch?v=npbJMnrnE-U&feature=youtu.be
 
84v 30ah over like 500 connectins and all that stored energy. I wish to have a 4.2v 20ah cell with the same size foot print with less connections. Good luck.
 
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