Firefly Electric Conversion

[jakobnev]

You'll want some sort of thread locking solution on a vehicle battery.

(Vibrations and bumps and all that.)

[RedDevil]

It probably wasn't tight all along. OTOH, if the cells in the pack have any wriggling space at all their movement may undo the bolts, even with thread lock.

Bolt should be fine for current, but I'd swap it just so any future inspection of the pack won't bug you and if it ever dares loosen and burn again you'd notice.

I thought about using spring washers on my PHEV buddy pack, but I wanted minimal resistance and worried the washers would increase it.

[Piotrsko]

the bolts SHOULDN’T be current carriers, that is for the bus bars. the only thing they should do is hold straps tightly to the battery to prevent fretting corrosion. They will however carry up to about 400 amps for a bit depending on how hot you allow them to get. measure bolt resistance then use Ohm's law for watts to be dissipated.

lock washers have no effect on terminal resistance on a properly torqued bolt.

[RedDevil]

Right, but a tiny part of the current will flow through them anyway - even if they are tightened well enough.
When not, they get enough current to burn them, or so it appears.

I agree about the washers. But if the cells cannot move the bolts, once tightened, should not release by themselves anyway.
I duck taped my 12 Ah cells tightly together; heat is not an issue for my buddy packs and the tape provide yet another layer of isolation from the iron boxes that house the packs.

[Stubby79]

I'm not sure "burn" is quite the right word. They're stainless. The rest of the bolt, except the head, is still pristine. So...it's oxidation from the high-heat. Like the blue ring you get around a weld. Even stainless does this, apparently.



The temperatures are a bit different for stainless, but this is close enough. My bolt head is mostly brown, with a spot of purple on it. It got pretty toasty. Hopefully localized and brief enough not to have spread and caused any damage. (I haven't noticed anything amiss with the cell).

I'll keep a close eye on it.

Meanwhile, I've been working on the second half of the battery. I have the cells strapped with end plates, decided where they're going (for now, at least), and will be working on a tray for them next.

[WyrTwister]

You can get a " paste / grease " that contains tiny particles of copper .

I think this is one example ;
http://ecat.burndy.com/Comergent/en/...%23%08false%08

It looks very much the color of copper . When applied to copper bolt connections , it is said to lower the resistance of the surface contact juncture .

You can also get copper bolts , nuts & washers . Those might also lower resistance . Of course , they will not have the tensile strength as steel .

God bless
Wyr

[Piotrsko]

The minute acceleration and deceleration are what causes the loosening in vibrating things.don't matter if its solid or tin plate. Copper bolts are expensive you dont need 90+ inch lbs TORQUE to hold bus bars on imho.

[WyrTwister]

There are the hex nuts I call aircraft nuts . With the plastic insert in the top ?

https://www.google.com/search?q=airc...ntu&channel=fs

Do not know how much they cost .

In doing power for Telco , almost all of out terminations use crimp 2 hole lugs . When I asked why 2 hole , I was told it almost eliminates the loosening due to vibration . I shook my head up & down .

I realize you can not use 2 hole lugs , but evidently vibration is a recognized problem .

God bless
Wyr

[Stubby79]

I'll keep an eye on them to see that it doesn't reoccur.

I have a theory as to what happened with this one. The bus bars aren't completely flat. They're like an inverted V, although only slightly, with the result that if you put them down on something that is perfectly flat, they would only touch said surface at either end with the center raised a bit. Torquing them down should force them to be flat.

Should. If it gets caught in the grove between the terminal and the nut around it, it won't spread flat. Or the torque on it was insufficient to make it spread out fully. Either way, it wasn't really in contact with the terminal itself. All the power had to flow through the aluminum nut around the terminal or through the bolt. And stainless steel is a lousy conductor compared to aluminum or copper.

I've re-torqued all the bolts. And, with the second half of my battery, I've bent the slight V out of the bus bars by hand. They sit flat even before the bolts are on, and don't try to get stuck in the gap between the terminal and it's nut.

I don't think it' something we need to figure out how to re-engineer. I don't expect it to be an issue again...unless a bolt come loose. In which case I'll deal with it then.

Thanks for the ideas. If I need them, I'll use them.

[thingstodo]

The conductive grease that was mentioned by @WyrTwister, or something close to that, is what our electricians use to bolt together buss bars in electrical gear. The idea appears to be that even when it looks flat there are small variances in the surface of the copper. Torquing down the bolts tightly bends/squishes the soft copper and gets more surface contact. The grease is intended to fill the voids that are left and provide a path through the 'low' areas that would not normally carry any current.

It is applied with a razor-scraper type tool. Very thin, so that when the bolts are torqued the grease flows to the 'low' spots and fills the voids, but does not hold the 'high' spots apart.

It is tough to tell if it helps. The buss bar is between 3 and 6 inches wide, there are 4 bolts torquing down the connection. It does keep moisture out. And there is no obvious discoloration on the copper buss when you take the connection apart. But the grease has to be cleaned off so some surface discoloration may have been present.

EVTV advises to keep the battery connections clean and dry so that they can be checked ... one a year? Something like that. They also mention that high currents will expand the metal slightly then it contracts after cooling ... as a source for the bolts loosening off. They are big on nord-loc washers, that dig into the softer aluminum or copper and keep the bolts from working themselves loose. There is also a spring component to the washer that keeps pressure on the joint and allows for minute amounts of expansion and contraction without distorting the bolt.

I can't find the EVTV reference on Nord-loc. they have a couple of episodes where they talk about them. For the battery connections they have discussed 'clean and dry' many times.

I use Nord-locs on my battery pack, but it is not in a running car as yet - so no real experience to share. The thin layer of conductive grease makes sense to me, but again I have no testing of do versus don't to share.