Prismatic cell terminals with crooked threads - is this typical?
- Thread starter uberpixel
- Start date
ArthurEld
Solar Wizard
As Steve said, I would try to use the crooked studs before resorting to threaded inserts. They probably will work fine.
The torque probably won't be accurate but good enough. The bottom of the busbar will be down flat.
The torque probably won't be accurate but good enough. The bottom of the busbar will be down flat.
ArthurEld
Solar Wizard
Cheap labor probably get paid by number of cells per hour. And inexperienced driller decides what is good enough.
I've been to Shenzen. And there are ghettos of people working in dirt streets doing all sorts of "side" work that isn't worth going to one of the big shiny factories that required 100,000 a day minimum lots to be worth their while to even book the work in, or in some cases even overflow from the factory to re-work stuff that isn't worth their time to re-work on the main lines.
It is actually really depressing that you have such opposite ends of manufacturing sophistication almost working side by side. It is a bit like the opening scene in Alita Battle Angel over the scavengers looking for electronic scraps to use, or the orphanage scene in the latest Blade Runner.
I'm going to send the reseller some more detailed pictures today so I can discuss my concerns with them tonight. In their current state, I consider these cells unusable for my application and therefore would expect some sort of refund or credit. Not sure what a reasonable outcome looks like, but these would take at least a couple hours of work by a skilled machinist to repair (or a whole day if I do it), with the cost of labor and materials (flat bottom drill bit, bottoming tap, or helicoils) I'm afraid I'm stuck throwing good money after bad to make them work.
Custom bending grub screws sounds viable, but it also sounds like a fiddly process and will still result in asymmetric loading on the threaded aluminum. With the off-axis tapped hole, there's no way to get a proper perpendicular clamp on the threads which are the weak link in this joint. I agree that having the bus bars flat will ensure a proper path for the electrons, but the mechanical joint is what concerns me the most. Can you tell I'm a mechanical engineer and not an electrical engineer? Once those threads yield and you lose clamp, the quality of the connection will degrade and could leave me stuck with a dead battery in the middle of nowhere.
-uberpixel
Custom bending grub screws sounds viable, but it also sounds like a fiddly process and will still result in asymmetric loading on the threaded aluminum. With the off-axis tapped hole, there's no way to get a proper perpendicular clamp on the threads which are the weak link in this joint. I agree that having the bus bars flat will ensure a proper path for the electrons, but the mechanical joint is what concerns me the most. Can you tell I'm a mechanical engineer and not an electrical engineer? Once those threads yield and you lose clamp, the quality of the connection will degrade and could leave me stuck with a dead battery in the middle of nowhere.
-uberpixel
With that mindset consider that even the non-crooked ones are likely tapped quite poorly and likely to strip out and also potentially leave you with a dead battery.....
So the two options:
1) Fix it yourself so you know it's done right.
2) Pay someone you trust to fix it so it's done right.
I took option #1 and helicoiled all mine right after doing a inspection and random capacity test to make sure the cells were ok.
Even though the quality of the threads were very good (I went with a more expensive reseller who 'balanced' the cells I received) I wanted M8 studs to properly fit my batteymon M8's from Batrium and the ability to get some reasonable torque on the hardware. I do a lot of marine work and know that torque and corrosion seal are key for many years of trouble free operation of stud-type connections.
The tools and helicoils were under $60 to do this correctly which isn't much allocated across 16 cells, and possibly more in the future.
So, you installed M8 helicoils? I could see the benefit since they would be a bit more standardized with other terminals in the system and typical lug sizes. I was debating whether it's beneficial to increase the bolt size since the bolt really isn't the weak link, it's the aluminum, and an M6 stainless bolt can supply far more clamp load than the threaded aluminum could ever support. Plus with a lower thread pitch, you're getting fewer threads of engagement for a given depth. That said, with how off-axis my holes are, drilling for an M8 helicoil tap will ensure that I have a clean hole to tap with no remnants of the previously drilled/tapped hole.
Curious what kind of cells you had and how deep you drilled the holes? Did you have to trim the helicoils to fit. Also, how did you break the tang at the bottom of the hole?
-uberpixel
An M6 bolt is very close to 1/4". There are plenty of 1/4" lugs available. I used 1/4" lugs and they fit just fine without any modifications on my M6 screws.
Ring Terminals (Tinned Copper)
temcoindustrial.com
Understood. The M6 or 1/4" lugs are certainly available, but I was thinking it could be nice to use M8 or 5/16" lugs to match things like power distribution blocks, battery cutoff switches, bulkhead connectors, circuit breakers, etc. Just less differentiation in lug sizes to order or keep on hand.An M6 bolt is very close to 1/4". There are plenty of 1/4" lugs available. I used 1/4" lugs and they fit just fine without any modifications on my M6 screws.
Ring Terminals (Tinned Copper)
-uberpixel
I ended up with a large collection of lugs. If you have M8 studs/screws then certainly use a lug for that size. The closer the size of the lug aligns to the stud/screw size the better the fit so it will be properly centered. Not a big deal to be off a bit. I'm looking to have as much surface contact as possible. I have plenty of cables with lugs on each end that are different sizes. I make my own cables, so it wasn't a big deal.
Yes, I installed M8 helicoils. If you are installing helicoils the work is the same regardless of the size so I went M8 threaded for the reasons I point out above.
I also believe that the bigger diameter of the larger helicoil gives more thread contact to the aluminum and therefore less likely to pull out. As we're limited for depth to get more thread area, we can only go larger diameter to get more thread area.
- I used the M8 tang-free 8mm in length, and cut off one thread to get ~6.75mm length.
- I drilled in a quality drill press with negliable wobble with the cell secured well.
- I used a standard bit to 5mm depth, then cleaned the cruft out of the hole with a blast of compressed air, and then a flat tip machine bit to 7mm depth, and blew it out again.
- Then tapped with a standard tap to 4mm with a drop of machine oil on the end of the tap, removed, blow out hole, and finish with a flat end tap to the bottom.
- I did it this way as the tapered tap was easier to get started cleanly versus the flat end.
- You use a M10 1.25mm tap for the M8 ID thread HC's
- I then finished by blowing it out, cleaning with a q-tip with brake clean on it's end, then helicoil in.
- Into the helicoil I installed a M8*25mm stud and torqued to 80 in-lb
Here’s a couple close up images of the impact of the misalignment of the threaded terminal. With the nut run all the way down on the stud but not fully tightened. This gap is apparent on most of the terminals in this batch of cells.
You can clearly see that the nut is contacting the washer on one side and not on the other. I assume if I torque this down it will reduce the gap, but it will also introduce uneven loading into the already delicate threads and potentially lead to a joint failure.
I’ve shared these pictures with the vendor and we are discussing next steps.
-uberpixel
You can clearly see that the nut is contacting the washer on one side and not on the other. I assume if I torque this down it will reduce the gap, but it will also introduce uneven loading into the already delicate threads and potentially lead to a joint failure.
I’ve shared these pictures with the vendor and we are discussing next steps.
-uberpixel
I would put a couple washers under the head and torque the nut to the recommended amount for aluminum threads. The stud may straighten on its own.
If it doesn't, get a piece of steel tubing with a 6mm hole in the end. Another option is a 6mm threaded rod coupler (leave 1 thread exposed at the bottom). Then gently bend the stud square. I suggest using loctite on the stud once straightened to keep it from spinning out of plane later.
If it doesn't, get a piece of steel tubing with a 6mm hole in the end. Another option is a 6mm threaded rod coupler (leave 1 thread exposed at the bottom). Then gently bend the stud square. I suggest using loctite on the stud once straightened to keep it from spinning out of plane later.
The stud is stainless steel and the aluminum terminal is very soft. I would be concerned about damaging the terminal. If it was me I would try self leveling washers with maybe 3nm of torque. If that works then use a thread locker and torque to 4 to 4.5nm.
Yeah. No thanks on the first option. Loading up one half of notoriously weak threads sounds like a recipe for disaster. Maybe some self aligning spherical washers could help, but the limit on those is only 3-4 degrees of correction. These studs are more than double that amount of misalignment.
The second option with the custom bent studs may help by reducing the rotational moment on the fastener due to the misalignment, but it would still lead to asymmetric loading of the aluminum threads. Probably the better of the 2 options but still not great...
I'm thinking my preferred route would be to use 1/4-20 x .280 long stainless TIME-SERT inserts. Although the material and tool cost alone would be over $100USD - plus a weekend of my life. Not cool for a battery purchase that should have been free from any of these issues if they had only implemented basic quality control on a critical part of the process (drilling and tapping the holes).
Amazon.com: TIME-SERT Inch Stainless Steel Insert 1/4-20 X .280 Part # 01406 : Automotive
Buy TIME-SERT Inch Stainless Steel Insert 1/4-20 X .280 Part # 01406: Inch Inserts & Kits - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
-uberpixel
Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
Two items that pop out.
1) The bolt/nut are not the contact surface you have to worry about. It is a matter of having the BusBars make smooth full contact with the contact pad on the cell which is where the electricity is supposed to pass through. THAT is the essential part, is that contact between busbar & pad.
2) The BMS Lead with Ring Terminal has to make Good Solid contact on the TOP of the busbar and also be secure. The slight bit of angle shown there is "not that bad". You can use a flat-washer then the Ring Terminal then serrated locking washer & nut on that. Do NOT tighten more than 30PSI.
BTW: A Tip that can be useful in this type of instance.
If you use a flat washer over the the busbar & BMS Ring terminal, there are "Lead" washers which can make up for the slight angle as they are soft and will "squish". These can only be used ONCE ! If you ever loosen them to make a change, you will more than likely have to use a new one to replace it. Lead Washers are hard to come by now because of the "lead" the alternatives out there exist but are quite costly.
NOTE: There are many grades of Aluminium, what is used on Battery Cells is very soft. This in NO WAY compares to Aluminium used for making Motors or Automotive engine components. I've rebuilt more car engines than I count. My Last engine build was a Chevy 402 BBC that Dynoed up to 10,000 RPM and made monster power complete with Roller Valve train and all the best goodies which was far from cheap.
Cinergy also just posted this resource (a Video), which may answer some questions and provide an option to those who are willing to give it a shot...
diysolarforum.com
1) The bolt/nut are not the contact surface you have to worry about. It is a matter of having the BusBars make smooth full contact with the contact pad on the cell which is where the electricity is supposed to pass through. THAT is the essential part, is that contact between busbar & pad.
2) The BMS Lead with Ring Terminal has to make Good Solid contact on the TOP of the busbar and also be secure. The slight bit of angle shown there is "not that bad". You can use a flat-washer then the Ring Terminal then serrated locking washer & nut on that. Do NOT tighten more than 30PSI.
BTW: A Tip that can be useful in this type of instance.
If you use a flat washer over the the busbar & BMS Ring terminal, there are "Lead" washers which can make up for the slight angle as they are soft and will "squish". These can only be used ONCE ! If you ever loosen them to make a change, you will more than likely have to use a new one to replace it. Lead Washers are hard to come by now because of the "lead" the alternatives out there exist but are quite costly.
NOTE: There are many grades of Aluminium, what is used on Battery Cells is very soft. This in NO WAY compares to Aluminium used for making Motors or Automotive engine components. I've rebuilt more car engines than I count. My Last engine build was a Chevy 402 BBC that Dynoed up to 10,000 RPM and made monster power complete with Roller Valve train and all the best goodies which was far from cheap.
Cinergy also just posted this resource (a Video), which may answer some questions and provide an option to those who are willing to give it a shot...
Repairing an aluminum terminal on a LiFePO4 cell
I made a video documenting my process:
diysolarforum.com
Following on Steve's comment that the busbar and contact pad is the surface of concern I also want to refer back to your proposed usage and reliance upon the bolts and nuts as a mechanical connection. These terminals should not be relied upon as the (sole or primary) mechanical connection between these cells. The terminals will fail regardless of their original state. There are discussions on this board about how to compress the cells and use vibration reduction. This will need to be done if you are going to put them on the tongue of a trailer.Two items that pop out.
1) The bolt/nut are not the contact surface you have to worry about. It is a matter of having the BusBars make smooth full contact with the contact pad on the cell which is where the electricity is supposed to pass through. THAT is the essential part, is that contact between busbar & pad.
2) The BMS Lead with Ring Terminal has to make Good Solid contact on the TOP of the busbar and also be secure. The slight bit of angle shown there is "not that bad". You can use a flat-washer then the Ring Terminal then serrated locking washer & nut on that. Do NOT tighten more than 30PSI.
BTW: A Tip that can be useful in this type of instance.
If you use a flat washer over the the busbar & BMS Ring terminal, there are "Lead" washers which can make up for the slight angle as they are soft and will "squish". These can only be used ONCE ! If you ever loosen them to make a change, you will more than likely have to use a new one to replace it. Lead Washers are hard to come by now because of the "lead" the alternatives out there exist but are quite costly.
NOTE: There are many grades of Aluminium, what is used on Battery Cells is very soft. This in NO WAY compares to Aluminium used for making Motors or Automotive engine components. I've rebuilt more car engines than I count. My Last engine build was a Chevy 402 BBC that Dynoed up to 10,000 RPM and made monster power complete with Roller Valve train and all the best goodies which was far from cheap.
Cinergy also just posted this resource (a Video), which may answer some questions and provide an option to those who are willing to give it a shot...
Repairing an aluminum terminal on a LiFePO4 cell
I made a video documenting my process:
Good point, and thanks for the reminder/clarification. I don't intend to rely on the mechanical connection of the terminals to hold the cells together. I will use some method of strapping or compression (still debating my approach), but even with strapping or compression there could still be some mechanical force input into the terminals due to small expansion or contraction of the cells and also general vibration of the entire assembled pack.
-uberpixel
Hopefully you used a credit card and can do a charge back. If the vendor expects me to fix his screw ups, they would quickly find out how much I make an hour. This is not acceptable.
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