only cells in particular position is discharging. Why?

ketoneenergies

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Oct 22, 2018
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i made 2 battery packs(16s8p,48ah) with 128(32650,lifepo4,6ah) cells each. The bms i am using can do 50A continuous and 100A peak. when i applied load of 500watts, bms is cutting off the battery. same for both the packs. i am attaching the picture of bms software of each pack. it is seen that on both the packs ,current is drawn from cell no:3,6,9,12,15. bms is cutting off the battery because these cells are going below 2.5v. anyone knows why? is it because of the design? or anything else.
 

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ajw22

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Could be a number of things, hard to say exactly without more details.
Have you fully top balanced the batteries before starting to use them? Ie. Fully charged all 32 packs to the same voltage?
Have you tested all cells for capacity before building the packs? Perhaps you had a couple of low capacity of even dead cells in there?
Perhaps some cells are not properly connected?
Perhaps the sensing/balance leads do not have a good connection.

Pictures might help.
 

ketoneenergies

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Could be a number of things, hard to say exactly without more details.
Have you fully top balanced the batteries before starting to use them? Ie. Fully charged all 32 packs to the same voltage?
Have you tested all cells for capacity before building the packs? Perhaps you had a couple of low capacity of even dead cells in there?
Perhaps some cells are not properly connected?
Perhaps the sensing/balance leads do not have a good connection.

Pictures might help.
All cells are new and are balanced before packing.
 

ajw22

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It's interesting that exactly every 3rd pack has the issue. What might be the common denominator? I think pictures of the packs would really help.
Another possibility is that the connection to/from those packs are way too thin?
 

ketoneenergies

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It's interesting that exactly every 3rd pack has the issue. What might be the common denominator? I think pictures of the packs would really help.
Another possibility is that the connection to/from those packs are way too thin?
 

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Redpacket

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The strips may be too thin for the current. At 500W, 48V system, that's approx 10A
Depending on voltage drops and the BMS connection points, you might be seeing effects of current flow on the strip resistance.
The way those strips are laid out it are you sure you have even current flow through the cells under load?

Can you post what voltages the BMS shows when the pack has no load?
 

ajw22

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Oh, yes, the picture says it all. The nickel strips are only rated to 6 Amps or so, and the way they are used creates several choke points (red circle) where all the current has to flow through. Current flowing nicely in some places and not others increases/decreases voltage depending on where you measure. You will likely have areas getting dangerously hot during use - Do not use the battery until you have _massively_ reinforced those areas.

It's really late, I don't work with nickel strips, so get a second opinion.
But from quick analysis, you need to add extra strip in the green locations, and add a _thick_ cable where the yellow lines are. Similar modifications needed on the reverse side.

1610199347237.png
 

Ebikeforlife

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From the Pictures of your battery pack, when you are discharging, can you feel where the series connectors are, are they getting warm?

Although building a pack that way will work, its not advised to have 1 single connection to each series string...

I know this is probably going to be a pain in the backside for you but i would advise rebuilding that pack...

Take a look at my ebike batteries thread here https://secondlifestorage.com/index...e-battery-porn-a-few-builds.10357/#post-71262

Although 500w is not a lot, it is too much to be put through a single nickel strip connecting the series connections.

The way you have built it looks like you are putting the BMS above String 14, is this correct?

Also is space a premium becuase of the way it is made?
 

ketoneenergies

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Oh, yes, the picture says it all. The nickel strips are only rated to 6 Amps or so, and the way they are used creates several choke points (red circle) where all the current has to flow through. Current flowing nicely in some places and not others increases/decreases voltage depending on where you measure. You will likely have areas getting dangerously hot during use - Do not use the battery until you have _massively_ reinforced those areas.

It's really late, I don't work with nickel strips, so get a second opinion.
But from quick analysis, you need to add extra strip in the green locations, and add a _thick_ cable where the yellow lines are. Similar modifications needed on the reverse side.

View attachment 23262
Nickel strip has 2 layers. And 3 layered in the red chock points.
 

Ebikeforlife

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i made 2 battery packs(16s8p,48ah) with 128(32650,lifepo4,6ah) cells each. The bms i am using can do 50A continuous and 100A peak. when i applied load of 500watts, bms is cutting off the battery. same for both the packs. i am attaching the picture of bms software of each pack. it is seen that on both the packs ,current is drawn from cell no:3,6,9,12,15. bms is cutting off the battery because these cells are going below 2.5v. anyone knows why? is it because of the design? or anything else.
The easiest way to redo this without a total dismantle would be to Disconnect the single series connections that are grey.... Connect series Red... and then use 3 x 14AWG pref 12AWG at the bottom... Blue... if you connect all series like the top set of red you will get good current flow.

Also if planning on pulling 50A and using 8mm x 0.15mm nickel, then i would double up on the bottom series connections
 

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ketoneenergies

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The strips may be too thin for the current. At 500W, 48V system, that's approx 10A
Depending on voltage drops and the BMS connection points, you might be seeing effects of current flow on the strip resistance.
The way those strips are laid out it are you sure you have even current flow through the cells under load?

Can you post what voltages the BMS shows when the pack has no load?
 

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Ebikeforlife

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This took a while to find... But here is what eventually happens when a string is connected to a single nickel strip...

This was a reputable brand name battery that came in for repair...
 

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ketoneenergies

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Thank you guys after your suggestion i rewired the pack, now its working properly thanks a lot.
 

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