Jukka, The process that you describe for the AL threads sticking to the Stainless steel bolt is called "galling" AKA cold-welding. It can be reduced by lightly lubricating the threads, since it is not the threads that carry the current (the top surface of the terminal where the copper strap is bolted down onto is the current carrying surface, the stainless steel bolt is too high resistance and its only function is to hold the copper tightly clamped to the Alu terminal surface, so lubricating the threads will not alter the electrical behavior but reduce the galling.
NOTE that I have also seen corrosoin on Alu terminals where the case apparently was not completely tight and this or the neighbor cell leaked electrolyte, eating into the smooth Alu surface. That is my reason to apply cheap Petroleum Jelly to all surfaces that are supposed to make good electrical contact or under mechanical stress, so the corroding fluids can't reach it and they stay clean. Any electrical contact is *not* affected by the grease, because even mild pressure will squeeze it out from between mating surfaces and the electrical connection is made clean and tight. I have now also used it on my lead-acid terminals, since the brass clamps were eaten away by any leaked acid, forming beautiful (and poisonous) blue "flowers". Keep your terminals clean & tight! BTW, the earlier recommendation to look up the recommended torque force of steel bolts might be *very* misleading, because even for small 1/4" bolts the torque is at least 10 Ft-pound. That will very likely destroy your Alu terminal in short order, you need to use torque force for *Alu* if you are torqueing something into an Alu post. Add the effect of galling which increases with higher force and temp and you are in for disaster if you over-torque your bolts. If the surfaces are clean then it takes wery little force and you probably are best off using strong split washers and applying only as little force as to take the split washer down until it is flat, no more and you are good. That is not much torque at all, but I have measured the resistance of even finger-tightened nuts and there is no difference with heavy torqueing, so keeping them clean is more important in my humble opinion and experience. Regards, Cor van de Water Chief Scientist Proxim Wireless Corporation http://www.proxim.com Email: cwa...@proxim.com Private: http://www.cvandewater.info Skype: cor_van_de_water Tel: +1 408 383 7626 -----Original Message----- From: ev-boun...@lists.evdl.org [mailto:ev-boun...@lists.evdl.org] On Behalf Of Jukka Järvinen Sent: Tuesday, January 07, 2014 3:02 PM To: Electric Vehicle Discussion List Subject: Re: [EVDL] Keeping LiFePO4 battery terminals tight every 10k miles Well.. The whole pole assembly (in these certain prismatic cells) has been under doubt for a decade now. Some failures but not as much as one would think. Many of you know the foils inside the cell are compressed together and drilled with one or two holes. There are similar holes in the Al or Cu pole assembly. Foils are tightened with stainless steel bolt and nut against the pole blocks. Some washers included. And this is inside the cell where they may be soaked in electrolyte. If the connection gets loose it will fail. On some I've found just black goo where the bolt was supposed to be. The idea is that the current goes where it most easiest go. Through tight copper-copper or Al-Al connection. That steel bolt has no chance of transferring any significant amount of current. I would keep all lubricants and pastes away from the cell poles. As long as they are dry they will work nicely for a decade. Clean and dry makes it easier to find the loose connections. At every service I test drive the vehicles and after some driving I measure every pole bolt manually for heat. If the heating occurs (more than others) on any bolt that is already tight it indicates internal bolt failure. Cell has to be changed. Early indication is higher voltage drop with high discharge current but the capacity seems to be there. BMS detects this failure easily. As the depth of threads and length of bolts are not exact I tighten the bolts manually 'tight enough'. After few thousand poles and bolts it's in the wrist. The big problem is on cells that have been unbolted for a dozen times. The Al pole looses it's threads. It happens when unbolting. Al sticks to the stainless steel threads and we have nice hole on the pole with no threads left. Depending on the case I change the whole cell or just drill a bit bigger hole and rethread it for a bit larger bolt. Spring washers and lockable bolts will keep the connection perfect at any condition. -Jukka 2014/1/7 Lee Hart <leeah...@earthlink.net> > Michael Ross wrote: > >> It is necessary to have a properly sized torque wrench, the willingness to >> look up the proper torque setting for the materials involved, and a >> willingness to spend the time getting all of the bolts right. The >> alternative is you take a chance on shortening the life of an expensive >> pack, or running out of power in transit. It is a choice one makes not >> torquing well... http://bit.ly/1dsHSCw >> > > Good advice, Michael. Thanks for the reference! (That's a new way to look > it up.) > > It should be noted that sometimes the manufacturer's choice of materials > and recommended torque is woefully ignorant. For example, a stainless steel > screw in aluminum is a bad design choice. It's likely to be electrically > bad, and is also likely to gall and seize in time. > > -- > "Obsolete" means nothing more than "the salesmen would prefer you buy > something else." -- Dave McGuire > -- > Lee Hart -- See my Xmas projects at www.sunrise-ev.com/projects.htm > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/ > group/NEDRA) > > -------------- next part -------------- An HTML attachment was scrubbed... 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