Bus bar contact resistance

[JamesB7]

I've just purchased 4 EVE LF280K cells from 18650 Battery Store. These have two screws on each of the positive and negative terminals. They included bus bars.

At any rate, I am finding a voltage drop between two of the cell pairs, each 0.1V at 20A charging. That adds up to 10 mOhm. As you can imagine, I am a bit concerned how this will perform at 100A -- instead of the 4 watt loss, I'm going to be looking at 100W. Not usable.

I've measured between the sides of the bus bar, and that's not where the voltage drop is coming from. I've measured cell terminal to bus bar, and it's actually 4 mV *higher* than cell terminal to screw (why would going through the screw and nut, to the very same bus bar, be a better current path?). This is very odd, because the nuts are compressing the bus bar to the cell terminal quite well, so it clearly makes contact.

I am thinking the problem has to be simple contact resistance. Any other possibilities, or is that about right?

Thanks

 

[sunshine_eggo]

Remove bus bars and scuff the terminal surfaces. Aluminum forms an oxide layer that impacts resistance.

5/12/2023 EDIT: this should be done just prior to making the connection and for best results, something like NO-OX-ID A Special or similar conductive corrosion inhibiter should be used between the contact surfaces.

 

[Madco]

You might consider applying some antiox after scuffing the surfaces. If the bus bars are aluminum and copper wire is attached, there's going to be galvanic action that will yield some really high contact resistances though!

 

[chrisski]

Would you mind sharing pictures?

I have not measured voltage, but I regularly measure each busbar with an it gun and see no noticeable temp difference on any connections.

I have measured my car battery like you mentioned troubleshooting, and no voltage difference there.

 

[Tanishq]

Clean the terminals with very fine sand paper and apply an anti oxide compound or if you don't have it you can use vaseline. And use only tinned copper bus bars on aluminium and use split lock washers.

 

[audiobill5]

I use a rudder pencil eraser to not chew up the surface and T9 spray lube to protect them.

 

[sunshine_eggo]

I use a rudder pencil eraser to not chew up the surface and T9 spray lube to protect them.

Pencil eraser will not break through the oxide.

T-9 is non-conductive and has no business in an electrical connection. It's fine to use it over the top of a completed electrical connection, but it should not be on contact surfaces.

I'm sure "this has worked fine for me" is your position, but it's not at all best practice, and you shouldn't be making these recommendations.

Thank you though for allowing me to realize my own advice was incomplete.

 

[Cal]

I'm curious how much contact resistance are folks getting. I get about 0.5 mOhm resistance.

 

[chrisski]

I'm curious how much contact resistance are folks getting. I get about 0.5 mOhm resistance.

And how do you measure this? Voltage loss?

 

[Cal]

And how do you measure this? Voltage loss?

Increase load from perhaps 10A to 100A. Measure voltage (at busbars on both sides of a cell or the entire battery) before and directly after load change. The best way to measure voltage is with a storage scope, but eyeballing meter voltage also works.

total resistance = delta V / delta I

contact resistance = total resistance - cell resistance

 

[Skypower]

I don’t even mess with the math, I just look for the rogue high resistance or high milivolt. You can use an internal resistance meter to measure between the terminal and buss/cable lug but the batteries can’t be in use (bms on) for any useful values. Another method method using a digital voltmeter with resolution down four places(.xxxx) and a constant load, charge or discharge at say 50amps for large prismatic cells works well. If memory serves me I was getting.0002 to .0008 mv between the terminal and buss or lug. I clean contact surface to remove any greases then hand remove oxides only ( no Dremel) from both contact surfaces with 3M cloth just prior to assembly and wipe well to remove grit. I use No-ox-id A special but I starting think that marine “Green grease” is the superior non-metallic antioxidant. The Kiaweets HT208D meter that Will recommend on his channel a while back has really delivered.

 

[Hedges]

Rather than between + and - connections of a cell, if you can probe cell terminal and busbar then you can measure voltage drop across contact directly, and use lower range of meter.
That is particularly easy in top-terminal automotive applications because cell terminal sticks through cable ends. For these lithium cells, maybe DMM probe can get past busbar and reach cell terminal. The 2-bolt style, if welded on top of cell terminal, should be easy to reach.

 

[Skypower]

Rather than between + and - connections of a cell, if you can probe cell terminal and busbar then you can measure voltage drop across contact directly, and use lower range of meter.
That is particularly easy in top-terminal automotive applications because cell terminal sticks through cable ends. For these lithium cells, maybe DMM probe can get past busbar and reach cell terminal. The 2-bolt style, if welded on top of cell terminal, should be easy to reach.

On some of the older non welded terminals I use a feeler gauge with Kapton tape on one side to sneak under the buss for a contact.

 

[Cal]

Another method method using a digital voltmeter with resolution down four places(.xxxx) and a constant load, charge or discharge at say 50amps for large prismatic cells works well. If memory serves me I was getting.0002 to .0008 mv between the terminal and buss or lug.

0.0002 V / 50A = 4 micro ohms

That's 125 times smaller than what I measured! I'll revisit my measurements in a couple of weeks.

BTW, I use 847 Carbon Conductive Assembly Paste.

 

[Skypower]

0.0002 V / 50A = 4 micro ohms

That's 125 times smaller than what I measured! I'll revisit my measurements in a couple of weeks.

BTW, I use 847 Carbon Conductive Assembly Paste.

I’ve found carbon paste highly reactive with aluminum. An absolute nightmare between aluminum and copper(all discolored or oxidized). I’ve even tried it in crimped lugs and measured no appreciable improvement. The only thing it’s good at is making a mess. It’s is now known as the black crap from hell. I also stopped using Noalox (dark grey). While it may be great for older aluminum wiring in panels, I’ve found minuscule pitting on the on the original no welded terminals and the nickel plated copper buss when disassembled 6 months later. Silicone dielectric(translucent) is very good but it’s hard to see where it’s put and flows away with time and warmth. No-ox-id A special(brownish semi translucent) is non-metallic doesn’t migrate but is pasty and thick in cold climates. Use caution not to contaminate contact surfaces with strands of the applicator brush.

 

[sunshine_eggo]

I’ve found carbon paste highly reactive with aluminum. An absolute nightmare between aluminum and copper(all discolored or oxidized). I’ve even tried it in crimped lugs and measured no appreciable improvement. The only thing it’s good at is making a mess. It’s is now known as the black crap from hell. I also stopped using Noalox (dark grey). While it may be great for older aluminum wiring in panels, I’ve found minuscule pitting on the on the original no welded terminals and the nickel plated copper buss when disassembled 6 months later. Silicone dielectric(translucent) is very good but it’s hard to see where it’s put and flows away with time and warmth. No-ox-id A special(brownish semi translucent) is non-metallic doesn’t migrate but is pasty and thick in cold climates. Use caution not to contaminate contact surfaces with strands of the applicator brush.

You NEVER put dielectric grease between contact faces. It is non conductive. Using it to "seal" an existing connection is fine, but do not put in on the contact faces.

 

[Hedges]

Is your preferred corrosion inhibitor compound NO-OX-ID actually conductive as the link says?

 

[Skypower]

You NEVER put dielectric grease between contact faces. It is non conductive. Using it to "seal" an existing connection is fine, but do not put in on the contact faces.

We have a huge insurmountable differences of opinion. I have proven it to myself. I won’t use any metal or carbon carrying substances on terminals and I will continue fill the contact surfaces (not inside crimped lugs) with a quality dielectric because of my excellent low resistance, high current, no galvanic corrosion results. The biggest problem is failure to remove oxidation before assembly.

Some confirmation I found later;

Dielectric Grease;

Dielectric means that it does not conduct electricity, so it would seem counterintuitive for use in an electrical connection. In fact, a dielectric grease is perfectly acceptable for most electrical connections. You want something that will seal out water and air, preventing both galvanic and general corrosion. In crimps, screw-on connections, and even most plug connections, the clamping force pushes the grease out of the way, settling in micro-crevices and around the outside of the connection where it displaces and keeps out air and moisture.
Excerpt from;
https://www.practical-sailor.com/boat-maintenance/conductive-greases-vs-corrosion or

 

[Skypower]

Is your preferred corrosion inhibitor compound NO-OX-ID actually conductive as the link says?

NO-OX-ID A Special is not conducive. Noalox has zinc and is somewhat conducive and not what I use anymore.

 

[sunshine_eggo]

NO-OX-ID A Special is not conducive.

You sure about that?

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