Let's Solve the Lifepo4 Welded Stud Problem!! Should We Worry About the Small Contact Area? 

A very Big thank you for all your hard work. You and your wife show much love in the time and effort that you put into making these videos. We all appreciate it. Making our lives happier and our battery building more successful. Cheers from Stephen in Australia.

Just for giggles, and science, you should make a few bus bars with holes that fit the stud and test those against the ones supplied. You have stated elsewhere that expansion/contraction doesn’t worry you and I agree.

You tension everything with the same torque: With the washer, you have less preassure distributed over the contact area, but you have more contact area. Without the washer, you have more preassure distributed over the small contact area.

thank you for making this video. it really makes me think of all the bonehead things i have done and how i should approuch my dyi projects but most of all safety is number one! and your video shows us how to be safe or what could happen. thank you for sharing keep the videos coming.

Good considerations here. Thank you. I began using Bellville(spring) washers on top, under the nut. Night and day better than lock washers. They spread the load out and keep the tension or torqure quite well.

Try inspecting the battery terminals under load with a non-contact thermometer. Lossy connections get hot. There can be lots of reasons they get lossy. Built an electric car in the mid '90's with 16 golfcart batteries. Made lots of mistakes. Drove it for five years and had lots of fun.

I bought the same batteries and I thought exactly same like you. So thanks for this video man.

Hi Ray, Watching through your video a made a few observations. 1) You highlighted an issue with the curvature of the busbars that is probably a result of the stamping process I assume they are using in the manufacture and is easy to straighten as you indicate. In addiction to the curvature of the busbar the flatness of the connection will also be affected by any height difference of the studs and how square the shoulder of the stud is with respect to the busbar. 2) Adding an aluminum washer adds two more contact surfaces for the current to go through at each connection. Also having different materials copper to Al could cause a corrosion problem over time so I would not think this would be a good idea. You could try using copper washers. 3) The current flow is not only between the stud shoulder to the busbar but also between the stud and the nut (across the threads) and then to the top of the busbar. 4) I was unsure about what you were using to measure the resistance but as you were able to detect the difference in resistance you were able to get the required comparisons. I have a 4 wire resistance resistance meter that can measure low resistance values and may do some tests to see what I get. 5) Personally I would drill the holes through the busbar and use a bolt rather than tapping the holes. It will be quicker and easier. 6) I would expect the use of Carbon Paste to be a good idea. 7) One thing that this has highlighted are the benefits of making a good connections. Great work Mike

Your resistacne tester seem to be a model that measire Impedense ( AC at 1kHZ) not DC.. and so the value might be different a bit due to inductance of the busbar circuit. To measure resisatcne i suggest using a simple power supply set to 10A and to cary a current thru your busbar and measure the voltage drop in mV and calculate R=V/I... That way I am getting stable measurement of true DC resistance. I get 0.0325miliohms for my short 52mm long busmars with cross section of 35mm2 for my EVE 105Ah cells. Which is a great value. I then get 0.0397 miliohm measured between the cell tabs under the tightened busbar. That mean the total contact resistance of two tabs to busbar is (0.0397 - 0.0325 ) = 0.0072 miliohms... or 0.0036 miliohms per contact . Doing measurement with proper method and equipement is key! Also since you are usinga 4 wire probe setup that have an orientation, pay attention to which tip of the probe is sensing the voltage and the tip that are carying the current. on close distance measurement, this might affect the value! ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Ci2XGohIFxU.html

Brilliant video. I've read and seen in battery manuals pattery post, busbar, flat washer, springwasher and then nut. The big round washer, spring washer and nut pushes down onto the busbar, which is pushed down onto the battery terminal.

Love the testing you are doing. Great work I used aluminium flanged studs(non welded cells) and noalox between the mated surfaces, (after i had cleaned everything with alcohol). I have never rubbed or polished any surfaces, only cleaned. The whole aluminium stud, not just the 'mated surfaces', conducts better, hence the nut on top of the connection, also increases potential conducting contact area. The Flexible tinned 'braided copper' connections means no stress on terminals, less corrosion potential due to dissimilar metals and maximised surface area on the contacts(they can stay perfectly flat when torqued, as no strain on the connections). This is for a RV setup, with lots of movement and vibration to contend with...not sure it would be necessary on a fixed building set up.

Dude avin floating bus bars in the middle pack makes me wince. It’s just one slip. An molten mettle is flying You will then soon realise what little resistance. There is with. Just a small touch

If you worry about changing the terminal shape while sanding, then you could use a small wire brush to remove the oxide layer.

Sounds like you should be stacking copper crush washers to make the surfaces of the termanil and busbar really conform to each other. Having the busbars lasercut or wsterjet cut from thicker more uniform plate would probaby make a huge difference from the stamped busbars and washers you've got. More expensive, lots less labor.

Aluminum Oxide is one of the hardest substances known to man, just softer than diamonds. Zinc is not able to cut or dissolve aluminum oxide either. The aluminum oxide that typically forms on an aluminum connection is only 50-120 angstroms thick. Sanchem's NO-OX-ID penetrates the oxide film by the chemical action of our corrosion inhibitor system.

@31:00 minutes I kicked it into 1.5 speed once I saw you had coffee! 1.25 seems about right for the first half though. ;)

Very informative video. Thank you

Thanks for your hard work. Would be much appreciated if you could put links to your tools and hardware.

Aft5er watching this video, man , I FEEL YOUR FRUSTRATION !

1. Yes, you should have been wearing safety goggled or face shield. 2. dont leave bus bars lying on the cells! I have 8x Catl 310Ah cells, the larger capacity seem more likely to get bloated. 4 of my cells were already bloated when supplied as New; and most of the supplied 1mm thin copper bus bars did not fit. I do not like those welded in studs, which look like stainless steel, and have 10mm diameter base, obscuring most of the terminal. Those studs were originally used on lower capacity cells. Eve cells have a 6mm threaded hole, which are better, using set screws. I was advised by supplier not to try to remove the studs. I bought 5x second hand North Star brand longer bus bars, ex lead acid. had to clean them up. Then filed out the slots, for exact fit for each connection, tight against the stud base, onto the terminal proper. The North Star are copper plate at the terminal ends, and flexible flat stranded wire sandwiched between. 2 of the original bus bars fitted, but i tripled them. Having dismantled the battery, recently, i found those to be too short also. I made flexible 35mm2 wire bus bars, (not marine grade, so more flexible and hold shape, better than Digital Mermaids) with 200A crimped lugs having a 10mm diameter hole, required a little filing, fitted onto the terminal. Since then my battery pack has performed better; using for small aircon, overnight, via SNADI low frequency toroidal inverter. I now use less than 20 percent capacity, instead of 30 to 40 percent.

anyway you can raise the volume in your videos, they are way to low

You're scaring the death out of me with that lose buss bar on top of those cells. LOL

Hi Ray thanks for your efforts and the wonderful videographers. What you are doing here makes a lot of sense, especially the adoption of carbon paste in the smaller circuits, I would love to know how well Oxygen Free Copper works, that is if you can get it thick enough as well as tight fitted and elongated hole copper multilayer busbars. Stay safe

the cells I got has double (non welded studs) which are offcenter so you dont have to worry about puncturing the cell anymore. I quite like it, gives a large flat area for good contact with the busbar

The groove in a 3.2 battery busbar is larger than post due to the design of the busbar. The groove provides a larger surface area for the***** battery terminals******** to make contact with, ensuring a more efficient and secure connection. This is important for high-power applications where the battery is subject to high currents and vibrations not the bolt screwed into the battery for securing the bars. You have to allow for heat expansion the chemical reaction is a larger concern of time. A chemical package like 847 - Carbon Conductive Assembly Paste would be a clear winner but are you really building life support equipment to be used on combat ship or plane..We used gold and silver when you had be to near perfect with monthly maintenance.

Nevermind the glove comments Ray, do your thing! Great project

you can also try for the terminals the black hard plastic "sponge" that is used to clean copper pipes before soldering. It cleans the surface but does not make scratches on it

I wonder if antioxidant paste for aluminum wiring would help? I always use it for AC disconnects...

Maybe someone else addressed this already but 2 dissimilar metals such as Aluminum and copper or any other metal will electrolys when electrons flow. No Ox should be used to prevent corrosion as this is part of the NEC when using aluminum wire/cable to make connections in wiring systems. This is definitely a place where a milli ohm meter would a must. Also using Ohms law to figure how much voltage drop/current lose will be present at all the contacts. The smallest amount of resistance in a circuit with rather high currents can generate an amazing amount of heat which will affect copper as when copper is heated on any surface, it actually becomes a copper oxide diode. We don't want or need diodes herer!

Thanks for all your passion, and wanting to do it right. I am building an 8s system and enjoy your learnings( or not :). I agree simplicity is the key. Adding extra, complicating, can make varied results. A couple of thoughts while doing this. 1. Cleaning of everything. Sanding the corrosion off the washers, and terminals makes sense, however cleaning of these contact surfaces was not done.. makes me thing there is sand,grit/oxides remaining when you bolt things down. would like to see rubbing used as cleaner prior to bolting down on all contact surfaces( whether carbon paste makes a difference is to be seen still).2. putting an aluminum washer in place to increase the contact surface area would make sense, but increasing the thickness of aluminum between that and the copper bus bar may not be the best idea... actually I thought of perhaps drilling a copper penny as a better washer/conductor.. but then corrosion issue etc.. are issues... LOL... arrrgh... simplicity... keep up the great work. Love your boxes, and design.. Thanks for all you do!

Best way to test is just charge the pack as fast as you are willing, or discharge. After say 20 min of high amp input in or out, look at the terminals with a thermal camera and identify any hot spots. This is good for connectors, wiring ect. It is a resistance test with in situ.

One thing you should think about, aluminum will oxidize in the presence of air, unless its protected. Either with dielectric grease (messy) or something like NO-OX-ID (expensive).

I have successfully flattened copper pipe to make busbars and have always wondered if the contact with the battery terminals was as good as it should be. Do you think it would be advisable to coat the ends of the busbars which come in contact with the battery with molten lead or solder? I would guess? that the softer material would make better contact at the slight expense of the conductivity loss of the lead while perhaps also lessen any future oxidation of the copper. What do you think?

I wonder if you could have glued sand paper to the bottom of a nut and turned the nut back and forth at the point of contact for a more precise removal of the Alum oxide?

on TOYOTA PRIUS HYBRID Battery BUS-BARS the "go-to" wet/dry SAND PAPER is 'WET" 1500 / to / 2500 grit Sand Paper to "POLISH" the bare copper BUS BARS just an FYI COOP ...

Aluminum Oxide has burned down a lot of mobile homes over the years my friend. I think the use of stainless or copper washers with anti oxidation compound might be a better choice in this case. The process of surfacing and cleaning the bases of the bus bar seats is a good idea. Also as I mentioned in the previous post the oxidation inhibitor compound would be a great idea. Sanding can also leave tiny amounts of residue behind. Maybe you considered that already. Washing with Isopropahl alcohol and a little bit of an air blast but nothing with any moisture, use an air drier such as one used in body shops when painting a car. Either way anything and everything that can be done to keep humidity away from the connections is needed as you already have mentioned, Al O2 forms real fast and if the air in the work areas has much humidity to it will make a difference.

Thanks

When I put my 4s 280ah pack together - with welded studs - I too assumed the small contact area would be an issue. So I put SS washers on both sides of the bus bars ( top & bottom ) before adding the nuts. After assembly was complete I tested the pack at 30 amps ( charge & load ). I measured the voltage drop between the tops of the studs and noticed around 10mv drop. I removed the washers and ran the test again. No more voltage drop! So my recommendation is to simply use the hardware that the battery supplier provides. In my case the bus bars & nuts were the same as pictured in your video.

The readings you are seeing frequently are in the range normally used in shunt resistors and current limiting stuff. I do a fair amount of electronic circuit construction and use stuff like this a lot. It doesn't take much to mess with you with this kind of stuff. As for the question in the video title the answer is a resounding yes worry much about it! This is the same issue as with sizing wires for the amount of amps needed to transfer from point A to B. Larger wire less resistance, more amps moving with less heat. I'm curious why you mentioned carbon paste because cardon is used as a resistance material in carbon film resistors. I used to do lots of industrial welding where we used carbon arc rods coated with copper to burn out faulty weld joints. Another thing is the difference in hard drawn and annealed copper. If the copper is hard drawn, it won't conform easily to the contact points which can make some difference. I also just looked up a conductivity rating of grades of copper and there is a noticable difference in grades. The fact these are tin plated should have negated the issue. My last comment here will be this. Have you tried copper anti sieze compound? I lied, investigate the differences in compressed as crimped connections verses soldered connections. No difference here with a bolt down connection as to a crimped, obviously there is no way a soldered joint can be done here but the same concept is present.

Thank you for sharing. Have same kind of battery terminals and same issue. I use stainless steel washer, aluminum washers, copper tinned washers and any added component add resistance. Actually i did not measure the resistance but voltage drop between 2 battery terminals connected with a busbar at 20A constant load. I got better result by making my own semi flexible busbar out of copper pipe with smaller holes. I expect that my good results from now will change over time due to copper oxides.

Grinding lenses uses laps to form a curve even with infinite radius (flat). Lapping two surface together will get them to mate. Lap the washer to the terminal with a fine grit of emory. Lap the other side of the washer to the buss bar. Use x pattern lapping the buss side of the washer. Use cerium oxide or fine polish for final lap. The tinned copper and zinc nuts factor in the resistance. Love the video and your approach.

copper washers and testing from the top (in the circle of the nut), it has the same potential as the bottom of the terminal.

I really like the analytical approach you take to all of this stuff, but when it comes to polishing the contact point on top of the aluminum studs, just take a piece of Scotchbrite pad, twist back n' forth a couple of times and be done with it. That dowel rod you cut is really just making it worse, unless you are painfully careful with how you use it. A longer piece of block material would work far better, but the Scotchbrite will get the job done and not round over the edges enough to worry about. If you really wanted to make a substantial improvement, buy or design a bushing made out of copper with the same upper ID as the slot in the bus bar and a wider base to contact the aluminum stud. That would actually solve your problem, while still allowing for expansion and contraction.

The washers might conduct higher amperages with less loss. i.E. heat. I would use No Ox on the terminals to prevent corrosion over time.

use studs in the female terminals, once installed the studs need not be rotating while tightening the connection, just tension from the stud. Vs a bolt that is both rotating (friction) & pulling, and it's rotating every time the connection is redone.

All very interesting. An observation and a thought I can offer on contact resistance; If you torque the nuts down to 6Nm, then you are applying approximately 120lbs of force on the bus bar/terminal over the contact area. The smaller the contact area then, the higher the psi between the aluminium and plated copper bus bar. I imagine a higher pressure will decrease the degradation of the contact resistance over time and I think we all agree greater area will increase the current capacity - if true, there must be a sweet spot for terminal area...the battery people have probably worked this out already given the bus bar and terminal materials. The other thought that occurs is what happens to these connections over time? My guess is that contact resistance will increase with time. With the aluminium washers added, since there is now double the surface area that can corrode, the resistance between the terminals will increase even more from degradation over time. The carbon grease should decrease the rate of degradation I suspect.

You might check the terminals for full welded contact to the batteries. I received ten out of sixteen where the threaded terminal was contact welded to only one end of the battery. Looked great when received but in the process of removing from package was the flaw discovered.

Hello Ray and lovely assistant.Carbon is more conductive than copper or brass. Run the paste down the whole buzz bar Can we buy carbon buzz bars.? Thanks for your skills. Kronky

O.28 mR that is good !. I have Varicore 280Ah with 66mR internal resistance. Some of the 8 cells can only be discharged at 5A or else they drop below 2.5V and the EBC-A20 tester cuts out.

Hi Ray, Re: My last comment on tapping the busbar holes. I see on one of your other videos you were connecting the wires from the BMS to the busbars and this would be more difficult if using bolts to connect the lugs to the busbars. So I would agree with you to drill and tap the holes for those with the diy skills to tap the holes will be better.

Ray, have you considered using belleville washers which are slightly cup-shaped and when force-loaded deform as a spring, since they are springs? They perform well in maintaining stack compression within a design load range. I'm also concerned about how casually you combine dissimilar metal conductors in an electrical path. Especially where humidity sometimes becomes high, those dissimilar metal electrically conductive surfaces are micro batteries which cause metal ion transfers.

Ray nicely done, What kind of anderson conector do you use 100amp? Would be nice if they make bigger contacts

Hi Ray. Thanks for your video. The area of ​​the outer ring of the cell contact pad is significantly larger than the welded contact area. Perhaps using a thick washer with a large inner hole to get a larger contact area makes sense?

I thought aluminium oxide forms in minutes. I used wd40 on the aluminium as I polished. Would be interested to see a test based on that method. I don't have a low ohms Meter.

If I ever got cells to hook up I was going to test resistance using Aluminum anticize. Nickle anticize. (used for SS hardware), copper anticize (used for sparkplugs grounding). I also would Artic Silver 5 that is used as a head sink past for computer processers. Silver is a great conductor.. I don't know if you have used any in the past.

Hi Ray, Before researching why connecting strips have such a high resistance, You just need to remember - why single-core cables have a higher resistance, than multi-core cables, that are used e.g. in audio systems etc. When You understand, then You need to apply the same effect to the copper bars, i.e. those bars must be multi-layered, they must be thinner and stacked on top of each other and screwed to reduce resistance, but if You will use soft aluminum, such metal will make better contact, because it is softer, fits better and the contact becomes more reliable. It is necessary to make and use multi-layered thin bars. Or if such a solution to the problem takes a lot of time and work, then you can do it differently, use very thick aluminum bars. Just try it . :) :) And also, I noticed,that electricians somehow do not pay attention and put copper bars on aluminum contacts, this is a straight path to metal corrosion. When connecting wires, they always warn, that you - can't twist aluminum and copper wires together, and when using bars of different metal for connections, most people forget this rule. Strange... This is just my opinion. I don't want to teach anyone... . But when I observe such commutations, I always have elementary questions. Maybe there is no problem in that, but it seems to me that if you want to have good contacts, you need to pay attention to it.

Having gone through all the procedures of testing all 16 batteries for capacity per your magic videos, (all well above the declared 304ah) looking back at this video I've tested the connection between my flexible bus bar and the contact area for resistance just like you did and found it to be a mere 0.07mΩ. Considering that the terminal itself from side to side measures 0.05 I would be happy with that. However, my question is what happens when a higher current/voltage than the one provided by the 9V battery of the tester goes through the contact? For all my PV wiring I have used a Magger which can shoot 250, 500 and 1000 volts through the cables in order to test for resistance and leakage but I don't feel safe applying such a voltage to the terminal of our batteries even though it would not go through the battery itself.

How about using stainless ("star") shakeproof washers under the nuts? Their points' penetration should guarantee consistent connections at the same torque. Looks like you have sufficient thread length to use one above and one below the bus bar if necessary.Thanks for your great videos BTW.

For the total contact area you have to add the area of the thread. Of course this is nickel plated steel, not copper, but I believe it adds a substantial amount of contact area to the whole setup. -

What brand and model of torque wrench are you using?

I would have a slightly different approach to this problem since the stud surface remains the same no matter how much bigger the penny washer is, even though it increases slightly the contact with the bus bar, I would instead take advantage of the stud itself and doubled up on the nuts using them as conductors increasing the area of contact that way perhaps adding a lager washer at the top thus taking full advantage of the bus bar with. Please correct me if I'm wrong 🙂

We do the samething on our packs only above 125AM draw did we see a difference between the washer and buss bar.

Okay Ray, why not copper washers? I understand that the graphite grease should conduct electricity but what about dielectric grease?

Where do YOU stand on " NO-OX " Paste on aluminum Terminal pads ? COOP ...

Hey Ray, if you have enough stud length try running a nut upside down with its flange facing upward to the bottom of the stud then the busbar, and a nut on top torqued down, then check the resistance? This way each busbar will be sandwiched between the flanges of both nuts!

Polishing is a good idea, however if you don't use oxidation paste the oxide will return. For aluminum transmitting antennas I use oxide inhibiting paste such as Penetrox. However, for dissimilar metals I am not sure of the chemistry. Therefore, polishing alone will only last so long before the resistance goes up.

I feel the contact of the testing probe with the terminal might contribute more to the measured resiststance then the contacts on the terminal and the bus bar as such.

Um... are the threaded studs not conductive? I'm sure they must be and so there will be a current flow into the bussbsr from them via the nut and any washer arrangement on the top. Or am I wrong?

All that don’t mix cooper with aluminum …ain’t supposed cooper got a niquel film to avoid corrosion?

Speaking about compression, i have two 48v lifepo4 in parallel with no compression and i see no difference at all. I dont know how compression will help at all so i dont follow that trend. Its been a month since your last upload, where you been sir

Use MG chem silver paste. Works great.

You sanded and then put terminal on? Without cleaning of the sand?

grit needs to be cleaned off after sanding, perhaps higher grit 600-1000

Just my $0.02 : But you need to know what is your end goal, how many watts loss are you trying to save across the junctions, or voltage drop ? Assuming you're running at 100A with a 16s battery or about 5kW( unlikely ?), with 16busbars at 0.1mOhm, that would be I^2 * R or about 16W total loss across the busbars, out of 5kW........0.3% Is it worth it chasing those few watts ? You could gain that with a few $ by changing a couple of light bulbs to LED ? Have a more efficient inverter ? Without an end in mind , it feels like this could be a endless exercise. Could the effort may be better spent elsewhere ? Thanks for the video - I think the resistance numbers are interesting and worth knowing.

I would like to add, I bought rept cells with "welded" rectangular "studs" and upon assembling them snapped one clean off.... the round one I don't see as an issue but I will never buy the ones with rectangular "studs" again.... j.m.o.

I check mine under lode with a voltmeter probing the terminals just under the bus bars. Looking for the oddball higher voltage.

I understand resistance is never good, I'm soon to build a pack so I aim best resistance. My question, if the BMS supports auto-balance, does the resistance matter as much?

Perhaps use copper washers? Seems that They offer better conductivity than aluminum. Just my two cents

if you used a copper cable bus bar, wouldn't it a line better? Lay flatter and have less resistance?

Aluminium and copper have a bigger reaction of oxidation betwen them that have aluminiun on aluminium. Ideal is copper on copper. When you have an aluminium terminal, you can use a different whasher. Other material than copper or aluminium. You can use something like nickel, for exemple. And the copper and aluminium don't oxidize anymore. 😉☺️ And you rezolve that problem. No oxidation, and good connextion. Try these, and make a comment here that i saw olso. Thanks!

I'd get a brass or copper bolt, drill a m6 hole and tap it, being about 10mm x 30mm, using a M6 grub screw to jam it on the post, then make the bus fit the new post, or what ever bolt you prefer!

I realize this is a year old however, would copper flat washers be a better choice ?

If it was a copper washer, I would agree. Placing aluminum between two copper points of contact raises the resistance because the inherent reduction in conductivity of the aluminum.

that loose bus sitting on the battery freak me out!

Aluminum is about twice as resistancent as copper. So you should really double the area in contact. I used to work on telephone systems the old landline stuff that ran on DC at many amps the old buss bars were copper but that got expensive so they started using aluminum but we had to double the size of the bars to be the same as copper.

Hi there Ray. I like your channel and what you do. But your safety is not improved enough. Especially @10:23 Time, you place a busbar near the terminals of negative and positive terminal of 2 batteries in series connection. I was not able to focus on topic anylonger, now thinking only how big a spark and melted brass that could end up with, just by an uncoodinated move??? 😲

Forgive the entry level question’s but, would it not be more efficient to insulate the bus bar’s? Also, why not copper? I have access to a lot of raw copper and can make my own if cost is the issue, I was just wondering. Edit: This episode is gold btw. Edit: When you sand down washers etc, blow the aluminium dust off before you use it.

Two 6 mm studs on each terminal on my new 280 Ah EVE cells, making my own flexible cord bus "bars".

Also polish them clean of sand and oil with micro fiber towel or coffee filter and alcohol

Would flexible busbars solve the uneven surface between battery terminals?

The current through the washer still has to come through the small shoulder on the stud so no advantage. A washer and long (connector) nut above the bus bar makes contact all the way up the threaded stud giving a bigger path from the cell.

My brand new cells don't have any holes or studs at all

No sanding of the buss bars? My bars were speckled with plating that wasn't smooth. Sanding them and terminals helped a lot. Something to be said about lead posts....soft so it forms a good contact and doesn't form a hard oxide layer. Copper lugs and wire is probably better yet. Would flex to get a good contact and have better material for a conductor.

Was the original busbar brass and the second one copper?

I had to pause and comment because watching you lay that busbar on those cells is making me queasy.

Got a part number for those aluminum washers

1000 series aluminum is the most conductive type aluminum. I'm not sure if this type of aluminum was considered. Was Ray using 1000 series aluminum washers? Oh, and wouldn't copper washers be a better solution for being softer and more forming to the battery aluminum contact?

Polishing is useless since the oxide just gets back in seconds to minutes. Maybe using copper washer might help ? But the real test is just pull as much curent from the battery as you would normally need and see if it heats up. If the busbars and screws stay cold then all is ok.

Why do you not consider the contact area between the bottom of the nut and the bus bar ?

have you checked for galvanic corrosion between aluminum and copper. I would think using a copper washer given better contact being softer than the aluminum?