The minus terminal of one of my LiFePO4 batteries lost its screw thread today. I removed the screw and then i screwed it back in and found that it turns forever. Here you can see 4 of them. It seems like the terminal is softer than the screw.

How can I connect a bus bar to it now?

  • With a larger diameter screw that you carefully self-tap into the hole (and don't over-tighten)? May 30 '21 at 18:18
  • Does the metal disk into which the screw threads also turn with the screw? If it does, the internal connection has sheared away and you have an expensive paperweight.
    – fred_dot_u
    May 30 '21 at 18:48
  • @fred_dot_u nope, the terminal does not move. just the screw rotates forever. May 30 '21 at 18:59
  • @WeatherVane ok. it was a M6 screw hole. I would need a M7 or M8 screw, right? what about Loctite or JB Weld? I just saw that on Youtube.com. Which exact version of these glues would you recommend? May 30 '21 at 19:02
  • Would not recommend glues - as they are not conductive.
    – Rory Alsop
    Jun 4 '21 at 12:53

Considering that the boss does not rotate, one can use a helicoil, aka threaded repair insert to drill out the hole, re-tap for the insert and continue to use the existing bolt and thread. This provides for a hole as strong or stronger than the original. Using an M7 or M8 re-tap is not going to be as strong.

Using adhesive risks the chance of not being able to later remove the bolt.

  • I’m not sure I’d agree that tapping out to M7 (good luck finding an M7 screw) will be weaker than a helicoil, which also requires tapping out the hole. Also, helicoils are made from stainless steel, which is not a great conductor of electricity. It might seem like a small matter but I’ve certainly found that using stainless screws on battery terminals gives noticeably poorer performance.
    – Frog
    May 31 '21 at 7:15
  • I agree though, don’t use adhesive, this will result in a terrible electrical connection, if any.
    – Frog
    May 31 '21 at 7:17
  • My LiFePO4 cells are similar to those in the linked image. The thread is cut into a relatively large boss on which the cabling end rests/contacts, with a much greater area than that presented by the threads of the fastener.
    – fred_dot_u
    May 31 '21 at 9:20
  • The large diameter part may be a thin disc to provide a larger area of contact, I wouldn’t assume that it’s that big all the way down. Obviously be careful not to drill deeper than the original hole, that could end spectacularly.
    – Frog
    May 31 '21 at 19:35
  • @Frog I tried JB Weld 8265 and i didnt break it, although i tightended the nut quite much. But: The threaded rod isnt completely perpendicular to the "boss". Luckily the rod has electrical contact to the hole despite of the glue. Because: I do not have the tools and skills to drill a proper hole and cut a thread. :) Jun 8 '21 at 11:26

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