question

tyania avatar image
tyania asked

Help! I stripped my lithium battery threads!

I disconnected my battery to install battery hold downs and upon reconnecting my victron smart lithium battery’s negative cable, the bolt seems to have gone in at a slight angle and stripped/offset the thread in the batteries female post connection. I stopped tightening with the ratchet when I could feel it slipping.

Basically what I’m left with Is that the bolt bypasses the first 1/8 inch of threads then engages for two or three turns and then gets extremely tight when reaching the orginal thread spacing. What are my best options here? I could/and will eventually need to use a slightly longer bolt anyhow but I don’t wanna force it into the remaining good thread and risk ruining it. If worse came to worse, will the entire female receiver be replaceable? This is a brand new 160 amp hour victron smart lithium battery.

I very much appreciate any advise.

Lithium Battery
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6 Answers
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

Hi @tyania

Ouch - a very good tip for life is to twist the bolt in the wrong direction before tightening to feel for the 'click' when the threads have properly seated. Then tighten. That's for next time though.


Official advice - contact your dealer with what has happened and ask for their advice.

Not-official advice, very carefully use a tap and die set to re-thread?

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falco avatar image falco commented ·

Are you sure it is 5/16"" I would have thought it would be metric.

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ falco commented ·

It is definitely metric, it is M8.

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tyania avatar image tyania falco commented ·

Thank you. I think I just assumed since the ring terminals I used were 5/16.

If I want to get a slightly longer bolts, I would be looking for a M8... In stainless steel or another particular metal alloy?

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tyania avatar image tyania commented ·

thank you @Guy Stewart (Victron Community Manager)
2 questions:

1) Would I try to re-thread at the same 5/16 inch? I believe I can have my stainless steel welder do this since he has tools and experience making threaded holes.

And

2) am I to assume that the piece itself is not replaceable?

And yes, a very rookie mistake. The Negative terminal was in a hard to reach awkward location and I made the mistake of starting the bolt with the ratchet.

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Guy Stewart (Victron Community Manager) avatar image Guy Stewart (Victron Community Manager) ♦♦ tyania commented ·

I would use the original threading, you just want to clean it up, should be absolutely minimal work for the tool

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alan avatar image alan Guy Stewart (Victron Community Manager) ♦♦ commented ·

Data sheet shows the threads are metric

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ commented ·

It happens to me also 2 or 3 times over the past few years and it is a pain in the a**.

It would be great if Victron could change the material of the terminals to something tougher, maybe brass or another copper alloy.

And also the screws a to short in my opinion. If you have to connect more than one wire the screw has only a few mm to hold in the thread.

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lehrling avatar image
lehrling answered ·

Suggest putting in a stud in place of the bolt.

If is a blind hole, the stud can be made or sourced long enough to be bottomed in the hole nice and snug.

With a stud, these compromised threads never need to be disturbed again (and further weakened) when disconnecting/reconnecting cables.

Maybe also use a suitable grease on the stud when installing to keep moisture out and prevent corrosion. Fluid Film is convenient and good, have never had an issue using it on low voltage electrical connections.

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seb71 avatar image seb71 commented ·

Even better, those terminals could me made to be replaceable. They are already somewhat replaceable, since those are not the cell terminals, but you have to cut the battery box/case to get access.

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lehrling avatar image
lehrling answered ·

"They are already somewhat replaceable, since those are not the cell terminals, but you have to cut the battery box/case to get access."

How is cutting the battery box a better solution?

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seb71 avatar image seb71 commented ·

Read again what I wrote, and don't cut a part of my post:

Even better, those terminals could me made to be replaceable. They are already somewhat replaceable, since those are not the cell terminals, but you have to cut the battery box/case to get access.

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blutow avatar image
blutow answered ·

I'd also recommend running a tap down to trying cleaning up the threads and seeing how they look. You'll want to use a tap made to take threads all the way to the bottom of the hole. It's a special "bottom" tap, the normal ones are tapered at the end and won't run the threads all the way down. It may not matter if you only damaged threads near the top, but I'd try to clean them up all the way to the bottom to maximize your chances of success. If the original threads are too far gone, you should be able to add a helicoil or time-sert re-thread. I prefer time-sert, but they are a little more $. I'd also recommend going to a stud as mentioned previously and also use some loctite. A stud is a lot easier on the threads because it minimizes shear force on the terminal threads as the nut is tightened.

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tyania avatar image tyania commented ·

Thank you @blutow and @Lehrling

In the case if using a stud, I would still like to know exactly what metal alloy I would use. Is this just a stainless steel or do I need some thing in particular for the conductivity.

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blutow avatar image blutow tyania commented ·

People generally use stainless, sometimes brass. Stainless actually isn't a great conductor (relatively speaking), but that's not really what the bolt/stud are there for. For most battery terminals, your primary point of conduction is where the lug meets the terminal. The conduction through the bolt is just bonus, but shouldn't be critical if the connection is made properly. All that said, I don't know the specifics of the the battery you are dealing with, so I'm just going off of "typical" battery terminal and lug connections.

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lehrling avatar image
lehrling answered ·

I would use the same material for the stud and nut as the original bolt.

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seb71 avatar image
seb71 answered ·

If you are going to use a tap, practice on something else first. Drill some holes in a thick metal plate and thread them, for instance.


You can really mess up the threads if you use the tap wrong.


Make sure the thread type, size and pitch are correct.

Make very sure you start the re-threading correctly, with the tap aligned properly.


Also, just in case you do not know, unlike when screwing a bolt (when you only rotate the bolt in one direction), you need to rotate back the tap from time to time when you cut the threads with it.

And preferably use some oil/lubricant (then clean the threaded hole, when done).

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