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CATL 271Ah 'Grade A' prismatic cells from Shenzhen Basen - Cell terminals without threading

Influencer

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Mar 6, 2021
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Hey all,

This is my first solar/battery project, and I suppose I'll document my experiences here. I ordered 4 280Ah EVE cells from Basen in march. They got back to me about a week later and said that they were actually out of EVE cells but could replace them with CATL 271Ah cells. I accepted this, mostly because I had taken about a week myself between her drafting the order and me paying for it (I had another opportunity arise and then disappear..), and the prices were suddenly going up and things were getting harder to find, so it still seemed like the best deal I could find.

I received the package today. The packaging was awesome - basically pure padding around each cell - and they included the extra bus bars I requested as well. The cells look good with no dents or bulging that I can tell. There are some scratches on the terminals, but not enough to concern me.

Cell voltages were 3.274, 3.26, 3.275, and 3.272. Should I be concerned about the 3.26?
I still have to capacity test, which I know will be the next big proof whether this was a good deal or not. Unfortunately, I won't have a full day in town to do this until the week after next, but that is apparently within the 'conflict resolution'/refund period, which is 15 days for Basen and 30 days for Alibaba.

The biggest issue thus far is that one of the cell's terminals has holes drilled, but the holes completely lack threading. With another cell, one if its terminal's threadings is wonky and I can only get the bolt in probably a mm or so. The rest of the terminals appear to be straight and fine.

I'm curious how you all think I should resolve this, what I should expect from Basen as compensation, and how I should communicate with them about it. Should I ask for at least a replacement cell? or a partial refund?
In terms of fixes, I don't have any connections equipment, shop, or person wise who could add the threading. Should I just get copper rods that fit the unthreaded terminals as tightly as possible, and secure them the best I can? And on the other cell's poorly threaded terminal, just secure the stud as well as possible?

I attached a photo of all four cells, and photos of the threadless terminals and the height of a stud in the poorly threaded terminal.
 

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I warned strongly about Shenzen Basen months ago...

 
Yeah, I weighed the sentiment about the company (perhaps what was most visible at the time when I made the decision was particularly positive), and the price, and did the risk/return assessment. Especially with the fact that I had to downgrade the Ah of my battery to maintain my order, I will definitely never order from them again unless they are very generous in resolving the issue.
 
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I don't know how thick the inside of the plate is, but if you have access to a tap-and-die you could tap the threads. Worst case I think you re-tap the holes to M8 instead of M6.
 
Basen offered to pay for me to get the terminals threaded. I'd prefer that they just send me a new cell, but at least they offered that much. What kind of shops tend to be willing to do this kind of thing? I'm going to call around, but if anyone knows about how much it would cost me that'd be good to know (I'm in the Portland, OR area, if someone knows a good place).
Even if I do it myself, at least I'll know what to quote Basen at for my trouble.

From skimming through that forum post and video, tap-and-die looks to be pretty straightforward for someone who hasn't done it before? It seems there are kits online for about $50, are these good enough without additional investment? I guess I can use it once and then return it, lol.

In the mean time, I have the cells charging in parallel. Ideally I'd wait for the fix, but I want to be comfortably within the Basen and Alibaba return periods after I capacity test the cells. On the threadless cell, I have the bus bars taped on with electrical tape in the meantime... The power supply reads 3.5v as of this morning, and the lower voltage cell is up in the 3.4v range. The voltage reading has dropped a few times, but in general has trended upward.

I notice when I use the multimeter on the cell with the lower voltage, the voltage reading doesn't settle quickly like on the other cells' terminals. Does this mean anything? I have been taking the reading from the stud/nut and I know those are less conductive than the bus bar.
 
Any auto mechanic will have a tap and die set on hand. Electronics repair shops probably have them handy too (and might be more willing to do it to a battery)

Honestly I'd say spend $40-50 and buy one off Amazon and do it yourself. It's pretty easy to do, particularly a soft material like aluminum. If you screw up tapping it with an M6 tap you can retry with M8 (and eventually M10. M12, etc) so long as the metal plate/bar/whatever the battery uses is thick enough. Just be sure to cover the unused terminal while you're working on it just in case you drop the tap so you don't short across the plates.
 
Cool, maybe I'll try it myself. I don't need to drill if I have to move to a bigger size? And I don't need a Helicoil? I'm still trying to understand when those are used.

I'm wondering how much compensation I should ask for from Basen, for the threadless terminals and the single messed up terminal on another cell.
 
If they were drilled with a ~5mm bit, then a 6mm (M6) tap will work fine. You basically just twist the tap into the hole a bit and then back it out over and over until you create a set of threads for your screws. If the hole is too big (like they used a 6 or 7mm drill bit, or in the case of one of your holes which is munged up you can't get a nice clean set of 6mm threads) you can step up to the next size (i.e. 8mm) and the tap just makes a hole that works with M8 bolts instead of M6. Obviously if you're using a larger tap like M8 you'll need some larger bolts, washers, etc to match. This is pretty much the process:


You don't need a helicoil. They're mostly used in automotive applications where you can't create good threads with a tap (either due to size, or thickness of material, or position), so you basically enlarge the hole and then insert those. I wouldn't want to use one in this application because you'll have dissimilar metals (since your battery tops are likely aluminum but the helicoil will be steel) which can end up creating corrosion. The only reason I'd consider one is if you CANNOT tap a larger hole because the battery material is too thin, in which case you'd use a helicoil with a smaller 4mm (M4) thread and then you'd have to downside your bolts and washers.

As far as compensation, I'd ask for the cost of the tools if you don't have them (~$50), or alternately I'd ask for the cost of a replacement + shipping that they'd have to supply to you if you sent this one back ($100?).
 
If you’ve never taped a hole before, you should know that while aluminum is easy to tap, there are several important precautions to take.

First is to assure that the hole is drilled straight and perpendicular to the surface. This is not something that I would ever attempt with a hand-held drill motor.

Second, is that the tap must be held very parallel to the hole. No wobbling allowed. No crooked alignment allowed. Precisely because aluminum is so soft, it is very easy to get an oversized thread that is not straight. This will make your threads even weaker than they are with the most perfect hole and tap. I would use a tap alignment tool chucked in the drill press.

If you don’t at least have access to a drill press, pay a machine shop a few dozen dollars to do the job. If you decide to do it yourself, practice first on some scrap aluminum.

There is disagreement on this forum about this but, once done, don’t install with screws. Install bus bars with threaded studs, washers, and nuts. My procedure is to carefully clean the hole and stud with alcohol or acetone, and carefully affix the steel stud in the aluminum threads with epoxy adhesive. The stud doesn’t need to be stainless steel unless you are taking the battery to sea. Black oxide coated studs are just fine.

There are hundreds of posts on this forum discussing these exact issues. Do a search and read a bunch of them.
 
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I think if I had to tap a terminal anyway .... I would just go ahead and install a helicoil while I was at it .... but everyone will have a different opinion about that.
There are some good threads on here about tapping and installing a helicoil.
 
If you do this in a drill press, make sure you insulate the battery case from the vice. There might be sparks since the case can be live.
 
I thought I didn't need to drill holes because they are already drilled, just missing the threads? I definitely don't have access to a drill press.

Also, if I want to secure the studs really well, is there a good glue/epoxy to use?
 
I thought I didn't need to drill holes because they are already drilled, just missing the threads? I definitely don't have access to a drill press.

Also, if I want to secure the studs really well, is there a good glue/epoxy to use?
I didn’t understand that all holes were already drilled. But the tap still needs to go in straight. Search Amazon for the words “tapping guide block” and get one of those, at least. You also need both a starting tap, and a “bottoming” tap to reach the bottom of the hole. Or two starting Taos with the taper ground off the end of one of those .

Any name brand of epoxy will work fine. They all have more than enough holding strength. Many like black JB Weld, so why not? (JB Weld is touted because it is filled with fine metal particles that reduces shrinkage of the epoxy as it cures. But who knows if that helps with the tiny amount that will coat the threads?). Carefully clean the uncurled epoxy from around the stud with a little alcohol so it does not cure and interfere with the terminal contact.

You want tight contact at the terminals, but don’t rip out the threads. If you can, tap a practice hole to the same depth in scrap aluminum, epoxy a stud into it, let it cure a day, then see how much torque it takes to rip out the stud with a nut. Then, don’t use THAT much.

Without the epoxy, it is easy to rip the studs out with a little too much torque. Some on this forum prefer to use LokTite instead of epoxy.

Are you really an influencer? I try to be, but my wife and kids won’t usually be influenced.
 
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Not to thread jack but another basen story… ordered 4x 280ah like you in early April, they marked it shipped a month ago, then in the last couple days messaging back and forth about status, I’ve found out they’re still in China, supposedly just left their location (“delayed”), and supposed to go on a ship that is still 2 weeks away from even docking in China to pick them up. Hopefully I get decent cells some day…
 
Thanks for the advice. I'm just making sure any type of epoxy won't interfere with conductivity, but as I understand, the surface of the terminal where it meets the bus bar is the import part, so glue is fine within the hole.

That sucks that it's taking so long, Thorium. I imagine recent global shipping and supply issues have not helped...
 
Any reason not to use threadlocker for this? just one drop spread over the thread and it's not ever coming off, I think it requires something like 300 ft lbs of torque to break loose...

This question is much discussed in many threads around this forum. Msny users here do choose thread locker. After trying both, and finding that epoxy is much stronger, that’s just what I chose. Many have found that LokTite does not bond well to the aluminum without using an “accelerant“.

It is very important to use one, or the other. These aluminum terminals a very soft, and the threads are shallow. It is scary to feel one of your studs pulling out as you gently tighten a nut. I was able to salvage that terminal by fixing the stud with epoxy. That was what tipped me to now using epoxy with the very first install of every stud.

it is certainly possible to repair a damaged terminal thread with a heli-coil insert. But I have 48 cells (96 terminals) in service, and I don’t want to mess with a bunch of heli-coil repairs if I can avoid it easily and cheaply with the epoxy.
 
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Thanks for the advice. I'm just making sure any type of epoxy won't interfere with conductivity, but as I understand, the surface of the terminal where it meets the bus bar is the import part, so glue is fine within the hole.
You are exactly correct. The stud is to impart downward clamping pressure on the terminal. The stud absolutely does NOT need to be conductive. But carefully clean the terminals with a scotch bright pad, and wipe with alcohol, or acetone, before clamping a lug down onto it.

These small studs, in soft aluminum, cannot impart enough down pressure to overcome the way that even a cat hair, or a dust particle, can interfere with the contact surface area. The more amps you intend to draw from the battery, the more important this clamped contact area becomes.

And always pay the tiny extra cost for tinned-copper battery terminal lugs ... do not use the pretty, shiny, bare copper ones.
 
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