Best connection method nickel strips <-> cable

Deadrabbit

10 mW
Joined
May 26, 2019
Messages
24
Hi,

i'm thinking about what would the best connection method to connect the cable to the battery nickel strips.

Three methods are in my mind:
1. Soldering. Not so easy on thick cables and nickel strips are not so easy to solder. See example, my iron has to less power for the AWG8 cable.
2. punch a hole into the nickel strip with a normal puncher (see picture) and screw it with crimped cable lugs.
3. clamp the nickel strips with, for example, with a brass bar and screw it again with cable lugs.

What method would you prefer?

Thanks for your ideas.

BW Maximilian
 

Attachments

  • PXL_20221230_164538713.jpg
    PXL_20221230_164538713.jpg
    292 KB · Views: 394
  • PXL_20221230_183211527.jpg
    PXL_20221230_183211527.jpg
    341.4 KB · Views: 393
Deadrabbit said:
Hi,

i'm thinking about what would the best connection method to connect the cable to the battery nickel strips.

Three methods are in my mind:
1. Soldering. Not so easy on thick cables and nickel strips are not so easy to solder. See example, my iron has to less power for the AWG8 cable.
2. punch a hole into the nickel strip with a normal puncher (see picture) and screw it with crimped cable lugs.
3. clamp the nickel strips with, for example, with a brass bar and screw it again with cable lugs.

What method would you prefer?

Thanks for your ideas.

BW Maximilian

I like your first picture, with the soldering plus zip ties to keep the cable from ripping off the nickel.

It's probably too late, but I like to solder cables to nickel first, then spot weld to my battery with cables already attached. This way you can use a ton of heat to get it done. Without worrying about damaging cells
 
Yes, it is for powering my kweld.

For the moment, I can get 900 A out of it. Less than I expected ( 1440 A).
 
I always solder. You need a bigger soldering iron.

If you have enough length of nickel, you can fold it around the wire and sort of crimp it before soldering. You want the minimum thickness of solder in the current path to minimize resistance.
 
fechter said:
If you have enough length of nickel, you can fold it around the wire and sort of crimp it before soldering.

Thanks for that, I'm gonna steal this idea next build
 
harrisonpatm said:
It's probably too late, but I like to solder cables to nickel first, then spot weld to my battery with cables already attached. This way you can use a ton of heat to get it done. Without worrying about damaging cells

^ This.
 
I wonder if the thickness of nickel strips matters.

Let's assume you have a battery pack in a Reention SF2 case, charging a 36V 350W Bafang G070.350.D hub motor.

The battery pack provides 36V 24.5Ah with LG 18650 3500mah cells arranged at 10S 7P.
With pure nickel sheet connections, insulated cell groups & insulated BMS.

Questions:
a) How thick should the pure nickel sheet connections be? Ideal conductivity scenario, forget the cost. Is 0,15mm enough?

b) Can it be argued that if the nickel sheet connections are thick enough, with a sophisticated BMS and good quality cells (not Samsung though), it is not necessary to install a second set of wires for each cell group to be individually measured & balanced for charge?

Can thick nickel sheet connections make up for a second set of wires balancing the charge across the pack?

c) Is a second set of wires for each cell group to be individually measured & balanced for charge indispensable when the pack is at 36V?
My chinese pack builder asserts that modern cells are very consistent and it is not necessary for such a low voltage...
 
I'm not sure I understand all the questions. 0.15 nickel will be plenty for a 7p pack running a 350w motor. This assumes proper nickel layout. It's possible to calculate the resistance of the nickel and do the math on how much voltage drop there will be.

The cells in each 7p group will be balanced by the nickel that connects them. Each group needs a single wire that goes to the BMS so it can measure the voltage of each group.

Some people run packs with no BMS, but that requires the user to keep track of the cell voltages and make sure you never over-discharge or over-charge any group. I always use a BMS.
 
Nearchos said:
b) Can it be argued that if the nickel sheet connections are thick enough, with a sophisticated BMS and good quality cells (not Samsung though), it is not necessary to install a second set of wires for each cell group to be individually measured & balanced for charge?

Can thick nickel sheet connections make up for a second set of wires balancing the charge across the pack?

c) Is a second set of wires for each cell group to be individually measured & balanced for charge indispensable when the pack is at 36V?
My chinese pack builder asserts that modern cells are very consistent and it is not necessary for such a low voltage...

You are asking about a second set of wires to manually check / balance the pack, correct?

If your BMS going to be buried in the pack you can do a second set of wires or a connector to do your thing.

If your BMS is accessible you might be able to use it's connection to unplug then charge / balance.

Realize every extra wire and connection is a potential point of failure / short / etc. Is it worth the risk to add the second set?
 
Nearchos said:
b) Can it be argued that if the nickel sheet connections are thick enough, with a sophisticated BMS and good quality cells (not Samsung though), it is not necessary to install a second set of wires for each cell group to be individually measured & balanced for charge?

Can thick nickel sheet connections make up for a second set of wires balancing the charge across the pack?

c) Is a second set of wires for each cell group to be individually measured & balanced for charge indispensable when the pack is at 36V?

Remember that unbalanced cells are cells that are not just different in voltage (which you can fix by rebalancing them), they are different in capabilities (which you cannot fix, and will only get worse with time, and faster the harder they are used).

If your pack becomes unbalanced at all, much less enough to require something more than what a typical BMS can fix during a typical charge cycle, then any or all of the below are true, and to prevent the need for such balancing you would need to correct it.

--Cells are not well-matched in characteristics

--Cells are being used outside their capabilities (current or capacity or...)

--Cells are being used at the extremes of their capabilities (current or capacity or...)

--Cells are aged or damaged enough to no longer be within specification

Corrections for the above would be, in the same order:

--Replace any cells with mismatched characteristics with cells that are closely matched. You would probably need to buy (many) more cells than you need, and test them all, discarding (or saving for other uses) any cells that do not closely match each other.

--Use more capable cells, or use more of them in parallel, to meet the demands of the system they must supply. This may mean needing cells that can supply higher current with less voltage sag, or have more capacity, or....

--Replace all the cells with new well-matched cells that are easily capable of everything asked of them, so they are not used anywhere near their capabilities.


Note that ensuring cells are not used anywhere near their capabilities will mean they are not pushed hard, they won't sag much in voltage (so the entire system will perform better), and they won't become unbalanced, assuming they are all well-matched in characteristics. But even if htey are only "close' in characteristics, then using them very gently will still help them stay balanced.


If your interconnects (nickel strips, etc) are sufficient for the system load on the battery as a whole, then they won't make any difference to the cell balance. The cells themselves will make that difference.

If you choose to use an insufficient pack for the system needs, or one built from unmatched cells, then building in a maintenance balancing connection won't hurt, and may be useful later on if you find the BMS unable to rebalance the cells.


My chinese pack builder asserts that modern cells are very consistent and it is not necessary for such a low voltage...
GLWT. ;)

While in a properly-controlled process that's likely to be true, even freshly manufactured cells may have slightly different characteristics, especially from different batches.

There will also always be some cells that just don't match up to the others, even if they are

If the pack builder does not test and match each cell to each other cell to have as identical a set of characteristics as possible, then the cells will charge and discharge differently from each other.

Matching parallel groups to each other, so that overall each group is the same total characteristics will help when cells aren't matched, but matching at cell level ensures better performance over time.
 
fechter said:
Some people run packs with no BMS, but that requires the user to keep track of the cell voltages and make sure you never over-discharge or over-charge any group. I always use a BMS.

Thank you very much. If you have any favourite BMS producer to suggest, please do not hesitate.
 
Jrbe said:
You are asking about a second set of wires to manually check / balance the pack, correct?

If your BMS going to be buried in the pack you can do a second set of wires or a connector to do your thing.

If your BMS is accessible you might be able to use it's connection to unplug then charge / balance.

Realize every extra wire and connection is a potential point of failure / short / etc. Is it worth the risk to add the second set?

Thank you, this is a very useful perspective. For a 36V battery pack it might indeed not be worth the risk.

I am also having a Reention DP-9C case at 52V 21AH containing 18650 3500mah LG lithium-ion cells arranged at 13S 6P LG assembled.
This pack feeds a 1000W 48V Bafang M620 / MM G510.1000.C. mid motor.

At 52V I tend to think that a second set of wires is a good idea even though my assembler asserts that below 70V it is not worth it.

Do you have any balancing BMS producer to recommend?
 
I don't have any BMS recommendations (I assume you were asking me.)

You could try to work in the BMS on a side of the pack so you can access the cell group connector, this would be win / win. Use the same connector for the BMS and manual intervention. It should also allow you to swap out the BMS easier if / when it needs to be.
 
amberwolf said:

:-D This is what I also thought when I read it and this is why I am here.

Thanks a lot for your input!!
Effectively, you are telling me to worry less about the nickel strips and more about what the strips bring together, i.e. the cells...
 
Nearchos said:
Effectively, you are telling me to worry less about the nickel strips and more about what the strips bring together, i.e. the cells...
Regarding cell balance, yes. If the cells can all easily handle the load placed on them and are all truly well-matched in all characteristics, they won't become unbalanced in the first place, until they age enough to become different from each other.

As long as the interconnects are properly done and can handle the load placed on them, and are all equal in resistance, and every cell has the same current path, cell loading will be equal as well, so will not unbalance the cells from that.


For instance, I'm using more-than-a-decade-old used-when-I-got-them-several-years-ago EIG C020 NMC EV-grade 20Ah cells in a 14s2p pack. There's no BMS, and they stay within hundredths of a volt with no balancing, because they started out well-matched (from wherever they came from originally), and have not been used at their full 5C (100A/cell) capabilities except for some short bursts for testing a few years ago--usually the pack only see at most a few seconds of <80A, then 10-20A the rest of the time), nor discharged to empty very much, though I have charged them nearly full most of their life with me (recently I dropped charging to only 4v/cell instead of 4.1, just because they're old and that surface charge goes away just going down my street, so makes no difference to usable capacity anymore). I usually keep it topped up every couple of days, using only about 1/4 of it's capacity (I sometimes unexpectedly have to go on longer trips without time to charge first).

I have also had a small 13s4p 18650 LiIon pack (I forget which cells, but there's a thread for it with "Luna" in the title in my threads) that even with light use becomes unbalanced, and heavy use would probably kill it quickly.

A decade or more ago I had an 18650 LiFePO4 pack with very poorly matched cells that became unbalanced just thinking about using it (albeit it also had had failed cells in it I had to remove and couldn't replace, and poor construction, before I even got it).
 
amberwolf said:
Regarding cell balance, yes. If the cells can all easily handle the load placed on them and are all truly well-matched in all characteristics, they won't become unbalanced in the first place, until they age enough to become different from each other.

This is very useful thanks!! Mr. Google also says you are right :)



This is the point that is obscure to me.
I do not assemble the battery pack myself, someone in China does this for me. So he might assure me that he checks the cells for consistency but ...how would I know...

Mr. Google says that Battery pack electric performance consistency tests can be divided in three groups:
a) thermal battery consistency tests
b) electrochemical performance consistency tests
c) mechanical performance tests (essentially vibration and crash tests)

Which leads to two questions:



a) which test is the most important to ask the battery pack builder to run to ensure cell consistency?

Charge test
Discharge test
Internal resistance test
Capacity grading and matching test?

I asked for a list of tests to see what they are doing. Still do not know what to look for as a must-do though...



b) what voltage and ohm/impedance Variance per cell group is desirable for a Reention DP-9C at 52V 21AH containing 18650 3500mah LG lithium-ion cells arranged at 13S 6P LG?

This pack feeds a 1000W 48V Bafang M620 / MM G510.1000.C. mid motor.

Battery pack builder says each cell is tested to ensure their
-voltage is between 3.5V and 3.7V and
-internal resistance (sic) less than 35mΩ.

0,2V voltage variance and no cell more than 35mΩ...
Is this enough for a brand new pack or should I ask for sth more ambitious? I have understood that with time variance will increase and neither the nickel strips nor the BMS will save me. A good starting point will help. So what is a good starting point?...

Thanks for any input.




ps. Btw I have heard two of your music tracks already, not bad at all!!
 
is 5mV maximum voltage difference and no cell above 25 mΩ reasonable for LG cells?
 
chuyskywalker said:
harrisonpatm said:
It's probably too late, but I like to solder cables to nickel first, then spot weld to my battery with cables already attached. This way you can use a ton of heat to get it done. Without worrying about damaging cells

^ This.

This is the method I use and it seems to be far easier than anything else I have done in the past. If you try to solder the wire to the nickel strip after its already been spot welded to the cells its much much harder and the cells soak up all the heat before the solder wicks to the wire, and you can also hurt the cells in the process
 
EDIT: Before reading the (very long) post below the first quote, keep in mind that all "balance" is, is a measure of how well-performing the pack is; how capable each cell(group) is of doing the same job at the same time.

Balance itself at any one instant in time really only tells you the specific state of charge difference between cell(groups), and doesn't tell you anything specific about those cell(groups) beyond that, other than that they are different for some reason that you should either look into if you need the pack to do everything it's capable of, or let the BMS do whatever it can to maintain balance until the pack becomes so unbalanced (or aged) that it is not usable and has to be replaced (which happens to all of them eventually).

There are some subtleties about balance and what balancing can do for performance of an unbalanced pack not covered here, but if it's truly important to keep one balanced, it's more important to start with well-matched cells used nowhere near their extremes of capacity or capability than to have a good balancing system.

If it's all well-matched, you won't even *need* a balancing system (though a monitoring BMS doesn't hurt).

Nearchos said:
I do not assemble the battery pack myself, someone in China does this for me. So he might assure me that he checks the cells for consistency but ...how would I know...
You wouldn't, unless you disassemble the pack and do the tests yourself, which is impractical at best, impossible in most cases.

Or test all your cells yourself first, then send them to a reliable builder to have them assembled into a pack (but not one you don't know for certain you can trust to use *your* cells and not just do whatever...).

But I would feel safe in assuming that the cells are *not* tested at all, or that if they are tested they are not finely-graded, only coarsely, so there can still be significant variation between cells in a pack.

a) which test is the most important to ask the battery pack builder to run to ensure cell consistency?

Charge test
Discharge test
Internal resistance test
Capacity grading and matching test?
Well, the first two test results will only really tell you stuff you can calculate based on the last two.

The spec sheet for a cell will tell you what specific characteristics are tested by the factory, under what conditions, and what range of variation is acceptable for each characteristic under those conditions.

Internal resistance can be tested in several ways, so to know what is acceptable you have to look at the spec sheet to see how they tested it, and what they expect from that test. If it doesn't say, then you would need to check with that manufacturer to see if they have a test-specification of how they test all their cells, to know which method they used. But there are at least two, DC Ri and AC Ri (which can be done at different frequencies, 1kHz is probably common).

DC Ri is what most of the end-user testers you can buy will approximately test for you, but they vary widely in accuracy and repeatability. (if you don't get the same answer from the same cell on the same test every time, the test isn't useful).

There are people on ES that do cell testing, like Pajda, etc., and sites like Lgyte.dk (?), that have info on how they do their testing if you wanted to repeat this kind of testing.


b) what voltage and ohm/impedance Variance per cell group is desirable for a Reention DP-9C at 52V 21AH containing 18650 3500mah LG lithium-ion cells arranged at 13S 6P LG?
Zero variation is desirable. ;)

The more variation, the worse the imbalance will be at the beginning, and the faster it will become worse over time as the pack ages.



Battery pack builder says each cell is tested to ensure their
-voltage is between 3.5V and 3.7V and
-internal resistance (sic) less than 35mΩ.

Neither of those is useful, regarding making a pack of well-matched cells.

For example, if internal resistance is 17 on one cell, and 34 on another, they are both within that limit, but they will react very differently in use, and will not remain balanced, as that resistance is a "symptom" of multiple internal characteristics.

All the voltage measurement tells you is each cells' state of charge (SoC) at the instant that test is done.

It doesn't say anything at all about the cells' characteristics relative to each other, other than that they are all somewhere within the median of their SoC (and not dead, failed shorted, overcharged, etc).


0,2V voltage variance and no cell more than 35mΩ...
Is this enough for a brand new pack or should I ask for sth more ambitious? I have understood that with time variance will increase and neither the nickel strips nor the BMS will save me. A good starting point will help. So what is a good starting point?...
To have a "perfectly matched" set of cells for a pack, you'd need to actually *match* cell characteristics (resistance and capacity at minimum; see cell-testing threads for more characteristics) with as little variation as possible, as close to zero as can be tested for. So say, test for all at (random choice) 20milliohm, plus or minus 0.01milliohm--then all cells would be the *same* resistance and behave the same (regarding this characteristic) during usage and remain balanced.

Neither of the things being tested for above will do any of that.

Say you have a hundred cells all of which are less than 35milliohm, but which vary by say, up to a dozen milliohm from each other; they are not even matched, much less well-matched.

Within a milliohm might be better matched, but keep this in mind: The percent of variation is still high. Say you have those same hundred cells, and all are at 30-31 milliohm. 1 milliohm out of 30 is about 3%. So they can still be up to 3% different from each other, which is ok, but will still be different in capacity, etc., and continue to drift in balance (actual charge state of each cell/group) on every charge/discharge cycle.

It will be easier for the BMS to keep a pack with this much difference balanced, but it still means the lowest capacity group (if the Ri is indicative also of capacity in this case) determines the entire pack capacity, since it can't discharge any further than the lowest capacity group without damaging that one. If the capacity difference is also 3%, then if you have 21Ah in each group, you could (roughly) have as much as 630mAh difference between groups (there are ways to minimize this in building a pack with multiple parallel cells by matching total group capacities with each other, but over time variation will still increase).



It may not be critical to your application or pack lifespan, etc., for how well-matched the cells have to be, but if you do want them to remain balanced, to be able to use maximum capacity of each cell, etc., the better matched they are the more usable the pack will be, for a longer time.



Nearchos said:
is 5mV maximum voltage difference and no cell above 25 mΩ reasonable for LG cells?
Voltage difference beween cells at assembly makes no difference at all to cell matching.

Every cell *at* 25milliohm would make a matched-cell pack, but cells just having to be between 0 and 25 makes no difference to a matched-cell pack, and won't result in a pack staying balanced over time.


For an assembled pack, 0.01 (1mv) maximum voltage difference would be balanced for SoC between groups. Doesn't say anything about matching, though, or how balance will be at any other instant in time for the pack, unless they remain within that range during the entire charge/discharge cycle empty to full and full to empty under no load to maximum load. (That *would* indicate a well-matched set of cells in a pack).





ps. Btw I have heard two of your music tracks already, not bad at all!!
Thanks--the most recent I've spent significant time/effort on is

The Tomorrow Option
https://amberwolf.bandcamp.com/track/the-tomorrow-option

The other recent-ish ones spent a lot of time on are
Convocation of Lies
https://amberwolf.bandcamp.com/track/convocation-of-lies-feat-lauscho-remix

and
Just Give Me a Voice
https://amberwolf.bandcamp.com/track/just-give-me-a-voice
 
Back
Top