EVE battery pack problem

[MariusK]

Hi,

two cells of the 48 pack not reaching more than 3.4 v while charging. I bought them new and only couple months ago. Batteries: MB 31. All are around 3.5 with 0.05v difference. And two around 3.4 v. Cell #1 and cell # 9. Any ideas?

 

[upnorthandpersonal]

Did you balance your cells before assembling? If you have a BMS with decent balancing you can have it do that for you, just give it time.

 

[MattMan119]

Hi,

two cells of the 48 pack not reaching more than 3.4 v while charging. I bought them new and only couple months ago. Batteries: MB 31. All are around 3.5 with 0.05v difference. And two around 3.4 v. Cell #1 and cell # 9. Any ideas?

So a few questions:

Did you balance the cells (charge them all to 100% prior build?)

What BMS do you have?
--Does it have a balance?
-- If so what kind and you will need to get them to balance voltage and give it time to do its work.
-- Most require that you bring them up to a volage and keep in absorption for while it makes adjustments. (can take time depending on imbalance)

 

[MariusK]

Yes, I balanced them. JK BMS with 2 Amps active balancing. I charged 50+ Amps and it raised. Now I find new thing- then charging those two cells (#1 and #9 ) has higher voltage and then discharging- lower than others. More amps - the bigger voltage difference. Are those connections good (connecting #8 and #9 ) ?

 

[MariusK]

So a few questions:

Did you balance the cells (charge them all to 100% prior build?)

What BMS do you have?
--Does it have a balance?
-- If so what kind and you will need to get them to balance voltage and give it time to do its work.
-- Most require that you bring them up to a volage and keep in absorption for while it makes adjustments. (can take time depending on imbalance)

Yes, I balanced them. JK BMS with 2 Amps active balancing. I charged 50+ Amps and it raised. Now I find new thing- then charging those two cells (#1 and #9 ) has higher voltage and then discharging- lower than others. More amps - the bigger voltage difference. Are those connections good (connecting #8 and #9 ) ?

 

[upnorthandpersonal]

I would definitely verify your connections. Hard to tell from a picture...

 

[MariusK]

I would definitely verify your connections. Hard to tell from a picture...

Is it possible that cable is too small size?

 

[upnorthandpersonal]

Is it possible that cable is too small size?

Depends on the current you're drawing.

 

[jorby]

The current in a series circuit is the same everywhere. A poor connection will get warm and cause pack voltage to drop a bit, but won't affect any individual cell in a series string.

 

[jorby]

Yes, I balanced them. JK BMS with 2 Amps active balancing. I charged 50+ Amps and it raised. Now I find new thing- then charging those two cells (#1 and #9 ) has higher voltage and then discharging- lower than others. More amps - the bigger voltage difference. Are those connections good (connecting #8 and #9 ) ?

Sounds like those 2 cells are higher internal resistance than the rest of them.

 

[400bird]

What's with the 2 red wires in the ring terminals at the bottom left cell? And do you have one heavy gauge and one light gauge wire connecting the series at the bottom?

The current in a series circuit is the same everywhere. A poor connection will get warm and cause pack voltage to drop a bit, but won't affect any individual cell in a series string.

A bad connection definitely can cause high/low cell voltage readings and doesn't always cause noticable changes in temperature (short of using a thermal camera). Without seeing a graph of cell voltage to current, it's hard to tell if that's the case here, plus verifying/improving connections is quick.

 

[jorby]

What's with the 2 red wires in the ring terminals at the bottom left cell? And do you have one heavy gauge and one light gauge wire connecting the series at the bottom?

A bad connection definitely can cause high/low cell voltage readings and doesn't always cause noticable changes in temperature (short of using a thermal camera). Without seeing a graph of cell voltage to current, it's hard to tell if that's the case here, plus verifying/improving connections is quick.

You are correct. The readings may be faulty here due to poor connections. Perhaps manual measurement of each cell (at the terminal, not the bar or bolt), while under load, would be a good idea.

I was looking at it from the perspective of output current from the pack (52a) causing poor connections to get warm and cause voltage drop of the pack . Edit: those drops would not affect the actual cell voltages, but would affect bms readings, because of where the readings are taken.

 

[ScrotpusGobbleBottom]

I would add some solder to those crimps. Those are difficult to get perfect.

 

[k490]

You are correct. The readings may be faulty here due to poor connections. Perhaps manual measurement of each cell (at the terminal, not the bar or bolt), while under load, would be a good idea.

I was looking at it from the perspective of output current from the pack (52a) causing poor connections to get warm and cause voltage drop of the pack . Edit: those drops would not affect the actual cell voltages, but would affect bms readings, because of where the readings are taken.

You could verify current is flowing on those two balance leads with a DC current clamp meter. If you have a DC power supply remove those 2 cells and connect them in parallel see if you can get them to charge to 100%. That would rule out the cells are good or bad.

 

[740GLE]

What bus bars are those?

DIY aluminum stock?

 

[HRTKD]

Get rid of the plates that are under the cable lugs on the two bottom cells. Why are those there? I can't see a useful purpose for them. The cable lug should be directly on the cell terminal.

 

[RCinFLA]

The current in a series circuit is the same everywhere. A poor connection will get warm and cause pack voltage to drop a bit, but won't affect any individual cell in a series string.

But it will affect the BMS cell voltage reading.

BMS sense wire is shared between two adjacent cells with bus bar in series on one of the two cells. Poor bus bar (or jumper cable for the #8 to #9 cells) will result in higher voltage reading on the side with the higher resistance bus bar connection in series with that cell when current is passing through series stack of cells.

With a multimeter, under the 50 amp load, check millivolts drop on battery terminal base caused by bus bar its connections.

 

[400bird]

those drops would not affect the actual cell voltages, but would affect bms readings, because of where the readings are taken.

Good clarification! Yes, the issue may be just in the BMS readings, that's what I often see.

 

[jorby]

But it will affect the BMS cell voltage reading.

BMS sense wire is shared between two adjacent cells with bus bar in series on one of the two cells. Poor bus bar (or jumper cable for the #8 to #9 cells) will result in higher voltage reading on the side with the higher resistance bus bar connection in series with that cell when current is passing through series stack of cells.

With a multimeter, under the 50 amp load, check millivolts drop on battery terminal base caused by bus bar its connections.

View attachment 259910

View attachment 259911

YES! that is what iI mean Another possible reason they weld busbars and take reading in the middle on factory packs.