Completely dead string?

Batteries, Chargers, and Battery Management Systems.
Post Reply
mark.curry3   100 W

100 W
Posts: 103
Joined: Nov 20 2014 2:52pm
Location: Tampa, fl

Completely dead string?

Post by mark.curry3 » Jul 08 2018 3:07pm

batt1.jpg
batt1.jpg (128.6 KiB) Viewed 632 times
I have a 20s 14p 72v nominal (84v) battery that came in a sealed case that somehow got water inside. I took it apart and cleaned it up and re-welded the loose tabs but one entire string reads 0.0 on my meter. Is it possible that all 14 cells died? is there another cause, maybe a single cell I should look at? the other cells seem fine and the entire pack shows 77.6v with most strings at 4.1 and a few at 4.0v, Is there a way to test the cells other than trying to separate them(they are spot welded)?

Jon NCal   1 kW

1 kW
Posts: 369
Joined: Jun 15 2015 3:59pm
Location: Mendocino, California

Re: Completely dead string?

Post by Jon NCal » Jul 08 2018 3:42pm

All the cells in that one dead group are paralleled, so one bad cell would drain all the cells in that group. All the cells in that group are 0 volts, and most likely ruined.

Note: In battery terminology, "string" means serial string, not parallel group.

User avatar
fechter   100 GW

100 GW
Posts: 14731
Joined: Dec 31 2006 3:23pm
Location: California Bay Area, USA

Re: Completely dead string?

Post by fechter » Jul 08 2018 6:56pm

Most likely the BMS failed on that channel and drained it. The cells may be OK if you charge that group with a very low current (like under 100mA) until it gets up to around 3v, then you can charge at a higher rate. I have had good results reviving zero volt cells but different chemistries behave differently. If the low cells were charged at normal current, they will be damaged. You probably need a new BMS too.
"One test is worth a thousand opinions"

EbikeAus   100 W

100 W
Posts: 267
Joined: Feb 04 2018 5:58am

Re: Completely dead string?

Post by EbikeAus » Jul 08 2018 8:04pm

It looks like there was a bit of moisture in there. Was it just from condensation?
Some of those busbars look a little corroded
Choose LiFe 8)

May all your batteries be fully charged and perfectly balanced :P

User avatar
amberwolf   100 GW

100 GW
Posts: 29844
Joined: Aug 17 2009 6:43am
Location: Phoenix, AZ, USA, Earth, Sol, Local Bubble, Orion Arm, Milky Way, Local Group
Contact:

Re: Completely dead string?

Post by amberwolf » Jul 09 2018 3:06am

In my Vpower 18650 pack (and others) I had cells that failed internally shorted and drained the entire group. A binary search (look up my repair threads) will help you find the bad one so you can disconnect it (and replace it if you like, or live with teh lower capacity and current capability if not).

If it's a BMS channel doing the draining, then leaving hte BMS balance wires disconnected from the pack will not drain the cells anymore (if you either repalce the group with a good one, or charge up the existing group).

If the group still drains with the BMS disconnected, then the BMS is probably not at fault (not solely, anyhow), and it's probably a cell or cells in the group doing it.

mark.curry3   100 W

100 W
Posts: 103
Joined: Nov 20 2014 2:52pm
Location: Tampa, fl

Re: Completely dead string?

Post by mark.curry3 » Jul 10 2018 4:51pm

Thanks for the input. Guess i will need to cut them out and replace.

mark.curry3   100 W

100 W
Posts: 103
Joined: Nov 20 2014 2:52pm
Location: Tampa, fl

Re: Completely dead string?

Post by mark.curry3 » Jul 10 2018 5:15pm

Wouldn't the pack cut off if the bms wires were disconnected? And how do i find your repair threads for a 'binary search'

User avatar
amberwolf   100 GW

100 GW
Posts: 29844
Joined: Aug 17 2009 6:43am
Location: Phoenix, AZ, USA, Earth, Sol, Local Bubble, Orion Arm, Milky Way, Local Group
Contact:

Re: Completely dead string?

Post by amberwolf » Jul 11 2018 12:17am

Most cheap BMSs don't bother with checking for open cells; they just shutdown if a cell drops below the trigger voltage. But *no* votlage doesn't do anything. :/ That's part of why they can cause problems--a cell group can be disconnected entirely from the BMS, yet the BMS doesn't know that, so it has no idea that group is very low (or very high) compared to other groups, so it doesn't shut off charging or discharging when that group is wildly out of balance to the rest. Can kill cells, or even start a fire. :(


threads with the phrase, in case the battery troubleshooting ones are helpful:

search.php?keywords=binary+search&terms ... mit=Search

Just mine:
search.php?keywords=binary+search+amber ... mit=Search

User avatar
fechter   100 GW

100 GW
Posts: 14731
Joined: Dec 31 2006 3:23pm
Location: California Bay Area, USA

Re: Completely dead string?

Post by fechter » Jul 11 2018 8:23am

mark.curry3 wrote:
Jul 10 2018 5:15pm
Wouldn't the pack cut off if the bms wires were disconnected? And how do i find your repair threads for a 'binary search'
Yes, if you disconnect all the wires it will turn off for sure. If one cell group it low, the charge port should still be on.

You really need to try individually charging the low group and see how it behaves after charging. If one of the cells is shorted, the voltage will never come up. You can disconnect the BMS while charging individual cells.

I use a piece of solid strand wire to just plug into the BMS connector for single cell charging/discharging. You can measure the individual cell voltages easily this way too.
Single Cell Charging Hookup 2.jpg
Single Cell Charging Hookup 2.jpg (88.98 KiB) Viewed 553 times
"One test is worth a thousand opinions"

Post Reply