msp battery pack damage.

[enaon]

Hello, a friend had an accident with his msp, nothing too serious, but one battery pack got some damage. 

We only realized it after it lost more percentage than it should during a ride, followed by not going to 100 volts after charge, it stayed at 96.6Volts. 

We opened it up, and the damage is easy to see, one out of four mosfets is broken. Also, one pair of cells is at 0volts, all other are at 4.18.

I am writing in case someone with experience in gotway wheels has any clue about the type of the mosfets used, they have no markings. Also, any idea as to why/how one pair could go to 0volts would be nice, I cannot explain it very well

 

 

Edited April 7, 2021 by enaon

[Planemo]

I would just buy a whole new BMS direct from Gotway, they aren't that expensive in the grand scheme of things. Changing a FET would obviously be a lot cheaper but even speaking as someone who would have no problem doing it, I think I would still go for a new board.

[enaon]

yes, It makes sense, two batteries went to 0volts, so they are gone I guess.

 

It is mostly out of curiosity tat I ask about the type of the mosfets, but most important, I would like to understand how a pair could go to zero, it makes little sense to me. 

[enaon]

also, what does experience say about replacing just the one battery pack, after about 9.000Khm on the other one? The internal resistance would have change by now, is that really a problem?

Edited April 7, 2021 by enaon

[Chriull]

51 minutes ago, enaon said:

, I would like to understand how a pair could go to zero, it makes little sense to me. 

This seems to be one of the later degradation stages of li ion cells. (1)

Either caused by some bms failure or just normal aging, maybe "supported" by some misbalancing.

50 minutes ago, enaon said:

also, what does experience say about replacing just the one battery pack, after about 9.000Khm on the other one? The internal resistance would have change by now, is that really a problem?

9000 km are something about 150-200 full charge cycles?

So they have certainly different (worse) characteristics compared to a new pack!

The new pack will take more burden/age faster. The overall possible performance will be of course less than with two new packs, but still better as with two aged packs.

Important you keep still an eye on the final charge voltage to replace the second pack once he "looses" a cell group...

(1) https://forum.electricunicycle.org/topic/22351-what-to-do-battery-not-charging-to-100/

[enaon]

4 hours ago, Chriull said:

The new pack will take more burden/age faster. The overall possible performance will be of course less than with two new packs, but still better as with two aged packs.

thank you, so it will just put a bit more pressure to the new pack, good to know this is a viable option. 

 

4 hours ago, Chriull said:

Either caused by some bms failure or just normal aging, maybe "supported" by some misbalancing.

it is the fact that it went to zero that puzzles me. I guess the bms doesn't charge that pair at all due to the damage, but I could not find anything on the board that looked off, except that mosfet that was broken, but I would expect it to influence more that just a cell pair.

Thank you for your time  

[alcatraz]

Either get a new pack or get a new bms and a few new cells to replace the zero volt ones.

The latter won't produce a "like new" pack but for someone that isn't afraid to tear the pack apart and probe it, it's likely going to be tolerable to live with. 

Selling the wheel will be more difficult.

Edited April 8, 2021 by alcatraz

[Chriull]

18 hours ago, enaon said:

it is the fact that it went to zero that puzzles me. I guess the bms doesn't charge that pair at all due to the damage,

As the cell (groups) are all in series by the nickel strips (without alternative wirings beside balancing resistors+mosfets) either all cells are charged or none. There is no way for a faulty bms to seperate one cell (group) so it does not get charged.

18 hours ago, enaon said:

but I could not find anything on the board that looked off, except that mosfet that was broken, but I would expect it to influence more that just a cell pair.

A faulty bms could permanently discharge a cell (group) via the balancing resistor.

Or by degradation a cell can develop internal short circuits so it cannot keep any charge anymore and stays more or less at 0V.

As unfortionately the weakest cell (group) in such a pack gets the most beating it's the first cell that shows some weakness that will "soon" end with zero V...

18 hours ago, enaon said:

Thank you for your time  

You're welcome!

[enaon]

3 hours ago, Chriull said:

A faulty bms could permanently discharge a cell (group) via the balancing resistor.

 

Nice, that must be it. The issue started after a crash that broke the mosfet on the bms, the pack was going to 100volts before the crash, but maybe some damage is done to the resistors as  well.

So I will look closer at the resistors of this pair, thank you. 

Edited April 8, 2021 by enaon