What thickness nickel plate needed for 135Ah 4S8P cylindrical battery pack?

[iClick]

Looking to spot weld nickel plate to connect these batteries as illustrated. This will be my 1st time spot welding (I will DIY build a welder)

I'm seeking advice on what thickness nickel plate to purchase given the high amperage of this pack (135Ah) also, I'm assuming the thicker the plate, a more powerful or longer time the spot weld would be needed. The pack appears to have been laser welded(?) in the beginning.

The batteries appear to be 33135

Some photos to help illustrate:


This is how the pack was originally configured.


Close up image of terminals. Once they are cleaned up (sanded?) Can I spot weld onto these?

 

[OffGridForGood]

I have made a number of E-bike batteries - the nickle plate thickness is picked based on the max amperage per cell you will move.
a 0.15mm nickle plate is fine for maybe 5-7 A (per cell) but will get hot above 10A, for example.
you don't say what you are going to use the pack for, and what the max load per cell might be.
Go to one of the E-bike /elect skate board forums and do a search, small cell packs are common over there.

 

[iClick]

It's a general purpose 12.8V 135Ah battery. Pics explain it better here I have a 1200W inverter that will be used occasionally but it will be mostly use to run a portable camping fridge and lighting.

The back story is that the battery was refunded under warranty due to a short that killed the BMS. I bought back the battery from the seller for $50 with the goal to get it going again. Unfortunately when removing it from the battery enclosure it tore off the bottom nickel plate from the terminals as I was unaware that the lower nickel plate was directly glued to the enclosure. (bad design)

I can't determine exactly the capacity per cell however it would need to be at least 15Ah each.

 

[OffGridForGood]

Ah is not what you need to know,
you want to know the max current per cell (Amps), so you can pick the nickel thickness that can carry that current without over-heating.
You will need a spot welder that can work with the metal you select.

 

[RCinFLA]

25-45 um is enough nickel plating to provide the necessary inter-metallic, dissimilar metal interaction prevention.

The base metal is the primary conductor.

Second requirement is the spot-welding ability. Here there is opposing requirements. A good base metal conductor like copper has less resistance (good for electrical) but the low resistance reduces the heating necessary for spot welding. Copper also does not provide a strong spot welding bond, as you have discovered.

Typical spot-welded 18650 battery straps are either pure nickel or nickel-plated steel. Steel base metal is used as a cheaper, poorer resistance alternative to pure nickel straps.

Resistance comparison:
Copper 1x reference
Aluminum 1.6x
Nickel 4.1x
Brass 4.7x
Tin 6.8x
Stainless steel 12x
Steel 8x to 20x depending on makeup of steel.

For example, clamped down bus bars for typical prismatic cells with a dimension of 2mm thick, 20 mm wide, by 70 mm length have a pure copper resistance of about 0.04 milliohms that increases to about 0.05 milliohms with 40 um thick nickel plating necessary to provide the barrier to prevent intermetallic corrosion of aluminum by bare copper contact.

Same dimension nickel plated bus bar with brass substituted for copper raises bus bar resistance to 0.17 milliohms.

 

[iClick]

Ah is not what you need to know,
you want to know the max current per cell (Amps), so you can pick the nickel thickness that can carry that current without over-heating.

Thanks, I’m learning as I go so good thing this post is in beginners corner
These are the closest battery specs that I can estimate the cells to be:

Nominal capacity: 15Ah
Continuous discharge current: 75A
Maximum current discharge: 5C

 

[iClick]

25-45 um is enough nickel plating to provide the necessary inter-metallic, dissimilar metal interaction prevention.

Thanks, that is a very educational reply.
Does surface area play a role in the capacity to handle greater loads? I’m not sure if the design of the nickel plated sheets having a large surface area compensates for thin material.

 

[OffGridForGood]

On the E-bike batteries I have built, I only used Nickel plate - I have not used copper or steel core.
Are you going to change out the plate on the other side of the pack, or just replace the one that got pulled off?

 

[RCinFLA]

Thanks, that is a very educational reply.
Does surface area play a role in the capacity to handle greater loads? I’m not sure if the design of the nickel plated sheets having a large surface area compensates for thin material.

When spot welding there is no surface to surface clamped area, only the spot welds which add to series resistance.

You did not specify the width of the straps but assuming about 12.5 mm width with 0.1 mm thickness, if strap is pure nickel, the resistance is about 1.4 milliohms per inch. Thickness of 0.15 mm is common strap thickness.

To this you add resistance of spot welds. Each sq mm of spot weld area adds about 0.02 milliohms, so most of resistance is in strap length between cells.

 

[iClick]

On the E-bike batteries I have built, I only used Nickel plate - I have not used copper or steel core.
Are you going to change out the plate on the other side of the pack, or just replace the one that got pulled off?

For now, I was just planning on replacing just the side that was pulled off.

 

[OffGridForGood]

Then just match the same thickness, 0.15 is common

 

[iClick]

When spot welding there is no surface to surface clamped area, only the spot welds which add to series resistance.

You did not specify the width of the straps but assuming about 12.5 mm width with 0.1 mm thickness, if strap is pure nickel, the resistance is about 1.4 milliohms per inch. Thickness of 0.15 mm is common strap thickness.

To this you add resistance of spot welds. Each sq mm of spot weld area adds about 0.02 milliohms, so most of resistance is in strap length between cells

The width of the existing straps are 50mm. however I’m having trouble sourcing this size in Australia. I thought the safest bet would be too reinstate the same materials as came from the factory. One concern is however would be that the 50mm wide strap would hide the terminals, making it difficult to determine where to spot weld. My only thought is if I press down hard enough, I may make an indent outlying where the terminals are.

I would appreciate any recommendation for nickel strips/straps that would handle the current passing through this battery. From my guesstimation, it would appear that the battery can output approximately 1C which would place it at around 15 A

Looking closely at the profile, I can see a copper substrate surrounded by nickel so from my Internet sleeping I have determined that C7701 Copper-nickel alloy strip is the closest candidate.

I’ve just taken delivery of a spot Welder, so waiting several weeks for the nickel sheets will be a test of my patience.